ALBERT

Notes: The south-east Reynolds Range, central Australia, is cut by steep north-west-trending Alice Springs age (c. 334 Ma) shear zones that are up to hundreds of metres wide and several kilometres long with reverse senses of movement. Amphibolite facies (550–600 °C, 500–600 MPa) shear zones cut metapelites, while greenschist facies shear zones (420–535 °C, 400–650 MPa) cut metagranites. The sheared rocks commonly underwent metasomatism implying that the shear zones were the pathways of significant fluid flow. Altered granites within greenschist facies shear zones have gained Si and K but lost Ca and Na relative to their unsheared counterparts, suggesting that the fluid flowed down-temperature (and hence probably upward) through the shear zones. Time-integrated fluid fluxes calculated from silica addition are up to 2.1×1010 mol m−2 (c. 4.2×105 m3 m−2). Similar time-integrated fluid fluxes are also estimated from changes in K and Na. The sheared granitic rocks locally have δ18O values as low as 0 which is much lower than the δ18O values of the adjacent unsheared granites (7 to 9), implying that the fluid which flowed through these shear zones was derived from the surface. For the estimated time-integrated fluid fluxes, the fluids would be able to retain their isotopic signature for many tens to hundreds of kilometres. The flow of surface-derived fluids into the ductile middle crust, with subsequent expulsion upwards through the shear zones, may have been driven by seismic activity accompanying the Alice Springs deformation.

Notes: The eclogite facies assemblage K-feldspar–jadeite–quartz in metagranites and metapelites from the Sesia-Lanzo Zone (Western Alps, Italy) records the equilibration pressure by dilution of the reaction jadeite+quartz=albite. The metapelites show partial transformation from a pre-Alpine assemblage of garnet (Alm63Prp26Grs10)–K-feldspar–plagioclase–biotite±sillimanite to the Eo-Alpine high-pressure assemblage garnet (Alm50Prp14Grs35)–jadeite (Jd80–97Di0–4Hd0–8Acm0–7)–zoisite–phengite. Plagioclase is replaced by jadeite–zoisite–kyanite–K-feldspar–quartz, and biotite is replaced by garnet–phengite or omphacite–kyanite–phengite. Equilibrium was attained only in local domains in the metapelites and therefore the K-feldspar–jadeite–quartz (KJQ) barometer was applied only to the plagioclase pseudomorphs and K-feldspar domains. The albite content of K-feldspar ranges from 4 to 11 mol% in less equilibrated assemblages from Val Savenca and from 4 to 7 mol% in the partially equilibrated samples from Monte Mucrone and the equilibrated samples from Montestrutto and Tavagnasco. Thermodynamic calculations on the stability of the assemblage K-feldspar–jadeite–quartz using available mixing data for K-feldspar and pyroxene indicate pressures of 15–21 kbar (±1.6–1.9 kbar) at 550±50 °C. This barometer yields direct pressure estimates in high-pressure rocks where pressures are seldom otherwise fixed, although it is sensitive to analytical precision and the choice of thermodynamic mixing model for K-feldspar. Moreover, the KJQ barometer is independent of the ratio PH2O/PT. The inferred limiting a(H2O) for the assemblage jadeite–kyanite in the metapelites from Val Savenca is low and varies from 0.2 to 0.6.

Notes: Petrographic analysis is a useful, but underused tool to aid in distinguishing between subsolidus and anatetic-related textures in migmatites. This study focuses on assessing the relative contributions of these two processes in the development of migmatitic orthogneiss textures in the Velay Massif, French Massif Central. The results of this study show that subsolidus processes are more important in the development of migmatitic textures in the orthogneiss than anatectic leucosome development. Four textural stages are identified from the mylonitic non-anatectic orthogneiss, annealed, migmatitic orthogneiss to diatexite. The monomineralic K-feldspar and plagioclase–muscovite banding was transformed with increasing temperature to polymineralic plagioclase–quartz–muscovite and K-feldspar–quartz–muscovite layers by the wetting of feldspar boundaries during heterogeneous nucleation of quartz from a fluid phase at high surface energy triple points. A further increase of temperature led to the growth of K-feldspar probably related to production of small amounts of melt in plagioclase rich aggregates, controlled by muscovite abundance. Solid state annealing processes in conjunction with incipient anatexis resulted in the formation of apparent granitic-like textures in plagioclase dominated aggregates. By contrast, in K-feldspar dominated aggregates exclusively subsolidus processes prevail, leading to the development of coarse grained leucosome. With the onset of biotite dehydration melting the plagioclase-dominated aggregates are destroyed by the melt whereas the K-feldspar aggregates may be preserved.

Notes: The 〉1800 km long Coast Mountains–North Cascades orogen of the Canadian Cordillera and north-western US developed as a continental magmatic arc. Metamorphic rocks in the orogen contain widespread evidence for burial of supracrustal rocks to depths of c. 40 km, followed by nearly isothermal decompression to depths of 〈10 km. Near many shallowly-emplaced, mid-Cretaceous plutons, low-pressure contact metamorphic effects were overprinted by high-pressure regional metamorphic minerals and textures, as evidenced by kyanite±staurolite pseudomorphs after andalusite in metapelitic rocks. Therefore, near-pluton rocks record the loading history of the orogen. Metapelitic rocks not associated with plutons only preserve evidence for high-pressure conditions and/or high-temperature decompression, as indicated, for example, by sillimanite and cordierite after kyanite and garnet, respectively. Petrological evidence for burial and decompression is therefore recorded in different rocks. Various regions of the orogen differ in timing of metamorphism, the overall shape of P–T  paths and the relative timing and regional extent of the high-pressure event, but most of these data and observations are consistent with thrusting and/or pure shear thickening as primary loading mechanisms throughout the orogen, as opposed to magma-dominated loading. This interpretation is further supported by comparison with thermal models, which demonstrate that the P–T  paths are consistent with simultaneous thrusting and folding at a high initial geothermal gradient (35–40 °C km−1) in much of the orogen. A high geothermal gradient supports tectonic models invoking intra-arc contraction and suggests that magmatism played an important role in regional temperature-time paths. This tectonic-thermal history may be typical of other contractional orogens and illustrates the importance of large vertical displacement of crust in magmatic arcs.

Notes: The gneisses of the Makuti Group in north-west Zimbabwe are characterized by complex geometries that resulted from intense non-coaxial deformation in a crustal scale high-strain zone that accommodated extensional deformation along the axis of the Zambezi Belt at c. 800 Ma. Within low-strain domains in the Makuti gneisses, undeformed metagabbroic lenses preserve eclogite and granulite facies assemblages, which record a part of the metamorphic history that predates Pan-African events. Eclogitic rocks can be subdivided into: (1) corona-textured metagabbros that preserve igneous textures, and (2) garnet–omphacite rocks in which primary textures are destroyed. The lenses of eclogitic rocks are enveloped in a mantle of garnet–clinopyroxene–hornblende gneiss, which is a common rock type in the Makuti gneisses. The eclogites preserve multi-staged, domainal, symplectic reaction textures that developed progressively as the rocks experienced loading followed by decompression–heating. In the metagabbros, the original clinopyroxene, plagioclase and olivine domains acted separately during the peak of metamorphism, with plagioclase being replaced by garnet and kyanite, and olivine being replaced by orthopyroxene and possibly omphacite. The peak assemblage was overprinted by: (1) the multi-mineralic corona assemblage pargasite–orthopyroxene–spinel–plagioclase replacing garnet–kyanite–clinopyroxene (possibly at c. 19 kbar, 760±25 °C); (2) orthopyroxene–pargasite–plagioclase–scapolite coronas replacing orthopyroxene (15±1.5 kbar, 750±50 °C); and (3) moats of orthopyroxene–plagioclase replacing garnet (10±1 kbar, 760±50 °C). The garnet–omphacite rocks record similar peak conditions (15±1.1 kbar, 760±60 °C). Garnet–clinopyroxene–hornblende–plagioclase gneisses envelop the eclogites and record matrix conditions of 11±1.5 kbar at 730±50 °C using assemblages that are oriented in the regional fabric. These rocks are characterized by decompression-heating textures, reflecting temperature increases during exhumation of the Makuti gneisses.The eclogite facies rocks formed during a collisional event prior to 850 Ma. Their formation could be related to a suture zone that developed along the axis of the Zambezi Belt during the formation of Rodinia (between 1400 and 850 Ma). The main deformation-metamorphism in the Makuti gneisses occurred around 800 Ma and involved extension and exhumation of the high-P rocks (break-up of Rodinia), which experienced a high-T metamorphic overprint. Around 550–500 Ma, a collisional event associated with the formation of Gondwana resulted in renewed burial and metamorphic recrystallization of the Makuti gneisses.

Notes: A suite of metapelitic, basic and quartzofeldspathic rocks intruded by enderbitic gneiss from the southernmost tip of the Eastern Ghats Belt, India, and metamorphosed at c. 750–800 °C, 6 kbar, were subjected to repeated ductile shear deformation, hydration, cooling and accompanying alkali metasomatism along narrow shear zones. Gedrite-bearing assemblages developed in the shear zones traversing metapelitic rocks. Interpretation of the reaction textures in an appropriate P–T  grid in the system FMASH, an isothermal–isobaric μH2O–μNa2O grid in the system NFMASH, and geothermobarometric data suggest a complex evolutionary history for the gedrite-bearing parageneses. Initially, gedrite-bearing assemblages were produced due to increase in μNa2O at nearly constant but high μH2O accompanying cooling. Gedrite was partially destabilized to orthopyroxene+albite due to progressively increasing μNa2O. During further cooling and at increased μH2O a second generation of gedrite appeared in the rocks.

Notes: Alternative assignment of invariant point stabilities in a possible P–T  phase diagram is given by a family of grids that derives from a form of the Euler equation. Invariant points are represented by great circles that divide the surface of a sphere (the Euler sphere) into polygonal regions that correspond to the number of potential solutions or grids in n-component systems with n+3 non-degenerate phases. A particular invariant point is stable in all grids on one side of the great circle and metastable on the other. The advantage of this representation is the ease and efficiency by which all grids consistent with experimental and theoretical constraints can be identified. The method is well suited for systems of n+3 phases in which the thermochemical data necessary for direct calculation of the phase diagram is either uncertain or non-existent for one or more of the phases. The mass balance equations among the n+3 phases of interest define the Euler sphere for any particular system. There is a unique Euler sphere for unary systems, and another for binary systems. Ternary and quaternary systems have four and 11 different types of Euler spheres, respectively. In the ternary case with six phases, the 16 non-degenerate chemographies belong to four groups that are associated with the four Euler spheres. An analysis of those groups shows a close relationship between the topologies of the chemographies and the topologies of the grids represented on the Euler sphere. Euler spheres for degenerate chemographies are characterized by a smaller number of spherical polygons. A useful application of the Euler sphere concept is the systematic derivation of possible FMAS petrogenetic grids from subsystem constraints. Assumption of just one stable invariant point in each of MAS and FAS systems is consistent with seven FMAS grids involving cordierite, garnet, hypersthene, quartz, sapphirine, sillimanite and spinel.

Notes: Meta-peridotites outcropping at different structural levels within the Alpine metamorphic complex of the Cycladic island of Naxos were studied to re-examine their metamorphic evolution and possible tectonic mechanisms for emplacement of mantle material into the continental crust. The continental margin section exposed on Naxos, consisting of pre-Alpine basement and c. 7 km thick Mesozoic platform cover, has undergone intense metamorphism of Alpine age, comprising an Eocene (M1) blueschist event strongly overprinted by a Miocene Barrovian-type event (M2). Structural concordance with the country rocks and metasomatic zonation at the contact with the felsic host rocks indicate that the meta-peridotites have experienced the M2 metamorphism. This conclusion is supported by the similarity between metamorphic temperatures of the ultrabasic rocks and those of the host rocks. Maximum temperatures of 730–760 °C were calculated for the upper-amphibolite facies meta-peridotites (Fo–En–Hbl–Chl–Spl), associated with sillimanite gneisses and migmatites. Relict phases in ultrabasics of different structural levels indicate two distinct pre-M2 histories: whereas the cover-associated horizons have been affected by low-grade serpentinization prior to metamorphism, the basement- associated meta-peridotites show no signs of serpentinization and instead preserve some of their original mantle assemblage. The geochemical affinities of the two groups are also different. The basement-associated meta-peridotites retain their original composition indicating derivation by fractional partial melting of primitive lherzolite, whereas serpentinization has led to almost complete Ca-loss in the second group. The cover-associated ultrabasics are interpreted as remnants of an ophiolite sequence obducted on the adjacent continental shelf early in the Alpine orogenesis. In contrast, the basement-associated meta-peridotites were tectonically interleaved with the Naxos section at great depth during the Alpine collision and high P/T  metamorphism. Their emplacement at the base of the orogenic wedge is inferred to have involved isobaric cooling from temperatures of c. 1050 °C within the spinel lherzolite field to eclogite facies temperatures of c. 600 °C.

Notes: Fluid inclusion salinities from quartz veins in the Otago Schist, New Zealand, range from 1.0 to 7.3 wt% NaCl eq. in the Torlesse terrane, and from 0.4 to 3.1 wt% NaCl eq. in the Caples terrane. Homogenization temperatures from these inclusions range from 124 to 350 °C, with modal values for individual samples ranging from 163 to 229 °C, but coexisting, low-salinity inclusions exhibiting metastable ice melting show a narrower range of T h from 86 to 170 °C with modes from 116 to 141 °C. These data have been used in conjunction with chlorite chemistry to suggest trapping conditions of ≈350–400 °C and 4.1–6.0 kbar for inclusions showing metastable melting from lower greenschist facies rocks, with the densities of many other inclusions reset at lower pressures during exhumation of the schist. The fluid inclusion salinities and Br/Cl ratios from veins from the Torlesse terrane are comparable to those of modern sea-water, and this suggests direct derivation of the vein fluid from the original sedimentary pore fluid. Some modification of the fluid may have taken place as a result of interaction with halogen-bearing minerals and dehydration and hydration reactions. The salinity of fluids in the Caples terrane is uniformly lower than that of modern sea-water, and this is interpreted as a result of the dilution of the pore fluid by dehydration of clays and zeolites. The contrast between the two terranes may be a result of the original sedimentary provenance, as the Torlesse terrane consists mainly of quartzofeldspathic sediments, whilst the Caples terrane consists of andesitic volcanogenic sediments and metabasites which are more prone to hydration during diagenesis, and hence may provide more fluid via dehydration at higher grades.

Notes: Petrological and thermochronological data provide our best record of the thermal structure of deeply eroded orogens, and, in principle, might be used to relate the metamorphic structure of an orogen to its deformational history. In this paper, we present a two-dimensional thermal model of collisional orogens that includes the processes of accretion and erosion to examine the P–T  evolution of rocks advected through the orogen. Calculated metamorphic patterns are similar to those observed in the field; metamorphic temperatures, depths and ages generally increase with distance from the toe of the orogen; P–T  paths are anti-clockwise, with rocks heating during burial and early stages of unroofing, followed by cooling during late-stage unroofing. The results indicate that peak metamorphic temperatures within the core of a collisional orogen and the distance from the toe of an orogen to the metamorphic core can be related to the relative rates of accretion, erosion and plate convergence. Model orogens displaying high metamorphic temperatures (〉600 °C) are associated with low ratios of accretion rate to plate convergence velocity and with high heat flow through the foreland. Model orogens with metamorphic cores far from the toe of the orogen are associated with high ratios of accretion rate to erosion rate. Calculated metamorphic gradients mimic steady-state geotherms, and inverted thermal gradients can be preserved in the metamorphic record, suggesting reconsideration of the concept that the metamorphic record does not closely reflect geothermal gradients within an orogen.

Notes: We discuss upper-amphibolite to granulite facies, early Palaeozoic metamorphism and partial melting of aluminous greywackes from the Sierra de Comechingones, SE Sierras Pampeanas of Central Argentina. Consistent P–T  estimates, obtained from equilibria involving Al and Ti exchange components in biotite and from more traditional thermobarometric equilibria, suggest that peak metamorphism of the exposed section took place at an essentially constant pressure of 7–8 kbar, and at temperatures ranging from 650 to 950 °C. Mineral compositions record an initial decompression, after peak metamorphism, of c. 1.5 kbar, which was accompanied by a cooling of c. 100 °C. Upper-amphibolite facies gneisses consist of the assemblage Qtz+Pl+Bt+Grt+Rt/Ilm. The transition to the granulite facies is marked by the simultaneous appearance of the assemblage Kfs+Sil and of migmatitic structures, suggesting that the amphibolite to granulite transition in the Sierra de Comechingones corresponds to the beginning of melting. Rocks with structural and/or chemical manifestations of partial melting range from metatexites, to diatexites, to melt-depleted granulites, consisting of the assemblage Grt+Crd+Pl+Qtz+Ilm±Ath. The melting stage overlapped at least partially with decompression, as suggested by the occurrence of cordierite, in both the migmatites and the residual granulites, of two distinct textural types: idiomorphic porphyroblasts (probably representing peritectic cordierite) and garnet-rimming coronas. Metapelitic rocks are unknown in the Sierra de Comechingones. Therefore, it appears most likely that the Al-rich residual assemblages found in the migmatites and residual granulites were formed by partial melting of muscovite- and sillimanite-undersaturated metagreywackes. We propose a mechanism for this that relies on the sub-solidus stabilization of garnet and the ensuing changes in the octahedral Al content of biotite with pressure and temperature.

Notes: The Chinese western Tianshan high-pressure/low-temperature (HP–LT) metamorphic belt, which extends for about 200 km along the South Central Tianshan suture zone, is composed of mainly metabasic blueschists, eclogites and greenschist facies rocks. The metabasic blueschists occur as small discrete blocks, lenses, bands, laminae or thick beds in meta-sedimentary greenschist facies country rocks. Eclogites are intercalated within blueschist layers as lenses, laminae, thick beds or large massive blocks (up to 2 km2 in plan view). Metabasic blueschists consist of mainly garnet, sodic amphibole, phengite, paragonite, clinozoisite, epidote, chlorite, albite, accessory titanite and ilmenite. Eclogites are predominantly composed of garnet, omphacite, sodic–calcic amphibole, clinozoisite, phengite, paragonite, quartz with accessory minerals such as rutile, titanite, ilmenite, calcite and apatite. Garnet in eclogite has a composition of 53–79 mol% almandine, 8.5–30 mol% grossular, 5–24 mol% pyrope and 0.6–13 mol% spessartine. Garnet in blueschists shows similar composition. Sodic amphiboles include glaucophane, ferro-glaucophane and crossite, whereas the sodic–calcic amphiboles mainly comprise barroisite and winchite. The jadeite content of omphacite varies from 35–54 mol%. Peak eclogite facies temperatures are estimated as 480–580 °C for a pressure range of 14–21 kbar. The conditions of pre-peak, epidote–blueschist facies metamorphism are estimated to be 350–450 °C and 8–12 kbar. All rock types have experienced a clockwise P–T  path through pre-peak lawsonite/epidote-blueschist to eclogite facies conditions. The retrograde part of the P–T  path is represented by the transition of epidote-blueschist to greenschist facies conditions. The P–T  path indicates that the high-pressure rocks formed in a B-type subduction zone along the northern margin of the Palaeozoic South Tianshan ocean between the Tarim and Yili-central Tianshan plates.

Notes: Metapelites containing muscovite, cordierite, staurolite and biotite (Ms+Crd+St+Bt) are relatively rare but have been reported from a number of low-pressure (andalusite–sillimanite) regional metamorphic terranes. Paradoxically, they do not occur in contact aureoles formed at the same low pressures, raising the question as to whether they represent a stable association. A stable Ms+Crd+St+Bt assemblage implies a stable Ms+Bt+Qtz+Crd+St+Al2SiO5+Chl+H2O invariant point (IP1), the latter which has precluded construction of a petrogenetic grid for metapelites that reconciles natural phase relations at high and low pressure. Petrogenetic grids calculated from internally consistent thermodynamic databases do not provide a reliable means to evaluate the problem because the grid topology is sensitive to small changes in the thermodynamic data. Topological analysis of invariant point IP1 places strict limits on possible phase equilibria and mineral compositions for metamorphic field gradients at higher and lower pressure than the invariant point. These constraints are then compared with natural data from contact aureoles and reported Ms+Crd+St+Bt occurrences. We find that there are numerous topological, textural and compositional incongruities in reported natural assemblages that lead us to argue that Ms+Crd+St+Bt is either not a stable association or is restricted to such low pressures and Fe-rich compositions that it is rarely if ever developed in natural rocks. Instead, we argue that reported Ms+Crd+St+Bt assemblages are products of polymetamorphism, and, from their textures, are useful indicators of P–T  paths and tectonothermal processes at low pressure. A number of well-known Ms+Crd+St+Bt occurrences are discussed within this framework, including south-central Maine, the Pyrenees and especially SW Nova Scotia.

Notes: Shrimp U–Pb zircon dating of structurally constrained felsic orthogneiss samples in the western Musgrave Block has been used to delineate discrete magmatic and metamorphic events at c.1300 and c.1200 Ma. The dating of pre-D1 and post-D1 felsic orthogneiss constrains D1 to have occurred at 1312±16 to 1324±4 Ma. This is the first geochronological study to identify such a metamorphic and deformation event in the Musgrave Block. D1 was accompanied by a major magmatic event involving the emplacement of voluminous felsic orthogneiss between 1296 and 1324 Ma. Zircon overgrowths on numerous igneous zircon cores give a consistent age of c.1200 Ma, reflecting zircon growth during a second high-grade metamorphic event (D2). This c.1200 Ma metamorphic event was followed by the intrusion of a c.1190 Ma megacrystic granite. The c.1300 and c.1200 Ma events in the Musgrave Block can be tentatively correlated with metamorphic events in the Albany-Fraser Orogen, and the Windmill Islands and Bunger Hills in east Antarctica. A major continuous Grenville-age orogenic belt joining these areas may have represented a plate boundary between the pre-Rodinian proto-Australian continent and proto-Antarctica during the formation of Rodinia in the Mesoproterozoic.

Notes: The Petermann Orogeny is a late Neoproterozoic to Cambrian (c. 560–520 Ma) intracratonic event that affected the Musgrave Block and south-western Amadeus Basin in central Australia. In the Mann Ranges, within the central Musgrave Block, Mesoproterozoic granulite facies gneisses, granites and mafic dykes have been substantially reworked by deep crustal non-coaxial strain of late Neoproterozoic to early Cambrian age. Dolerite dykes have recrystallized to garnet granulite facies assemblages, associated with the development of a mylonitic fabric at P=12–13 kbar and T =700–750 °C. Migmatization is restricted to discrete shear zones, which represent conduits for hydrous fluids during metamorphism. Peak metamorphism was followed by decompression to c. 7 kbar, reflecting exhumation of the terrane along the south-dipping Woodroffe Thrust. In scattered outcrops north of the Mann Ranges, peak metamorphism occurred at P=9–10 kbar and T =c. 700 °C. The Woodroffe Thrust separates these deep crustal mylonites from granites that were metamorphosed during the Petermann Orogeny at P=c. 6–7 kbar and T =c. 650 °C. The similarity in peak temperatures at different crustal levels implies an unusual thermal regime during this event. The existence of a relatively elevated geotherm corresponding with Th- and K-enriched granites that were in the mid-crust during the Petermann Orogeny suggests that radiogenic heat production may have substantially contributed to the thermal regime during metamorphism. This potentially has implications for the mechanisms by which intra-plate strain was localized during this event.

Notes: The dominant foliation (S2) in the metapelites of the Southalpine basement, near the western side of the Tertiary Adamello intrusive stock, is a Variscan greenschist facies planar fabric, slightly reworked during thick-skin Alpine tectonics. S2 is defined by muscovite and chlorite and was formed by decrenulation of pre-existing foliations, which are confined to metre-size, less-deformed domains and defined by biotite and white mica. The pre-S2 fabric is composite (D1a & D1b) and defined by contrasting amphibolite facies metamorphic assemblages in different residual sites. Cld+BtI+Grt+MsI+Pl+Qtz and St+BtII+Grt+MsII+Pl+Qtz assemblages mark D1a and D1b fabrics respectively; these developed during successive steps of a single, temperature-prograde polyphase event, rather than during separate tectonometamorphic imprints affecting different tectonic units, later coupled during a D2 greenschist facies stage. Thermobarometric estimates of assemblages formed during D1a, D1b and D2 show a transition from T =480–540 °C (during D1a) to T =570–660 °C (during D1b), corresponding to a slight pressure-increase from 0.75–0.95 GPa to 0.85–1.15 GPa. D2 greenschist retrogression corresponds to a pressure and temperature decrease (T 〈400–550 °C and P〈0.3–0.4 GPa). This P–T–deformation–time path is inferred to be the result of uplift from a depth of c. 35 km, after Palaeozoic subduction and continental collision; it is consistent with models postulated for other metamorphic units of the Variscan Belt in Europe. This is the first documented example in the Southern Alps of temperature-prograde metamorphism before Palaeozoic collision.

Notes: Restricted occurrences of early, syn- and late-kinematic kyanite adjacent to large domal batholiths in the Archean granite–greenstone terrane of the east Pilbara craton, Australia, are considered to result from partial convective overturn of the crust. The analogue models of Dixon & Summers (1983) and thermo-mechanical models of Mareschal & West (1980), involving gravitionational overturn of dense greenstone crust that initially overlay sialic basement, successfully explain the geometry, dimension, kinematics and strain patterns of the batholiths and greenstone rims. Application of these models suggests that andalusite and sillimanite are the stable aluminosilicate polymorphs in domal crests and rims, where prograde clockwise P–T–t paths, with small pressure changes, should be recorded. Both aluminosilicates are predicted to overprint kyanite, which is observed locally around the east Pilbara domes. Kyanite is the predicted aluminosilicate polymorph in the deeper parts of domal rims and within sinking greenstone keels, reflecting rapid, near-isothermal burial. The narrow zones of kyanite-bearing schists adjacent to some batholiths in the Pilbara craton are metamorphosed, highly strained equivalents of altered felsic volcanic rocks in the low-grade greenstone succession, dragged to mid-crustal depths (6 kbar) during greenstone sinking. The schists rebounded as an arcuate tectonic wedge along the southern Mount Edgar batholith rim, during the later stages of doming, and were juxtaposed against regional, greenschist facies, low-strain greenstones. Thus, kyanite was preserved: if the walls had remained at depth, it would have been overprinted by the higher-temperature aluminosilicate polymorphs during thermal recovery.Kyanite growth in the Pilbara craton is unlikely to have resulted from ballooning of plutons, mantled gneiss doming, metamorphic core complex formation, or early crustal overthickening. The typical subvertical foliations and lineations of the tectonic wedge suggest that subvertical fabrics extended to mid-crustal depths (c. 20 km) before rebound, providing a three-dimensional glimpse of Archean dome-and-keel structures. The general occurrence of large granitoid domes in Archean granite–greenstone terranes, restriction of rare kyanite to the adjacent, high-strain batholith margins, and its absence from the batholiths, suggest that partial convective overturn of the crust may have been a common process at this early stage of Earth history.

Notes: Chlorite and associated minerals from the volcanogenic Taveyanne metasediment of the western Helvetic nappes, Switzerland, were investigated by electron microprobe (EMP) and transmission electron microscopy (TEM) in order to determine their textural and chemical evolution during low-temperature metamorphism. EMP analyses of chloritic material from sub-greenschist facies outcrops show a decrease of Si and Σ(Ca, Na, K) with increasing metamorphic grade. A number of conclusions may be drawn from combined TEM images and analytical electron microscopy (AEM) data.1 In diagenetic-grade samples, chlorite crystals (observed maximum defect-free distance=80 nm) always contain some 1 nm layers (with a maximum of 29% of all layers) and less frequently some 0.7 nm berthierine-like layers. With increasing grade, the amounts of 1 and 0.7 nm layers decrease, and most chlorite from the epizone is structurally pure or contains less than 2% of 1 nm layers.2 A positive correlation was found between the amount of 1 nm layers and the Ca+K+Na content, indicating that the 1 nm layers are saponite.3 Observations and calculations suggest that the transformation reaction of saponite to chlorite takes place by the replacement of the interlayer cations in saponite by brucite-like layers resulting in a local volume decrease. In contrast, the destruction of berthierine has only minor influence on the local bulk volume.These results confirm recent studies which show that the change in composition measured by EMP of diagenetic-grade chloritic material are mainly the result of mixtures of chlorite and saponite. The use of chlorite ‘geothermometry’ in such systems is greatly influenced by the presence of saponite and hence is not based on reaction equilibria, even though temperatures calculated in this study agree with temperatures derived from other methods. Therefore, chlorite evolution should be treated as a kinetically controlled grade indicator and developed as a qualitative scale similar to the illite crystallinity index.

Notes: The high-P, medium-T  Pouébo terrane of the Pam Peninsula, northern New Caledonia includes barroisite- and glaucophane-bearing eclogite and variably rehydrated equivalents. The metamorphic evolution of the Pouébo terrane is inferred from calculated P–T  and P–T –XH2O pseudosections for bulk compositions appropriate to these rocks in the model system CaO–Na2O–FeO–MgO–Al2O3–SiO2–H2O. The eclogites experienced a clockwise P–T  path that reached P≈19 kbar and T ≈600 °C. The eclogitic mineral assemblages are preserved because reaction consequent upon decompression consumed the rocks’ fluid. Extensive reaction occurred only in rocks with fluid influx during decompression of the Pouébo terrane.

Notes: Bimodal metavolcanic rocks, granitic gneisses and metasediments are associated in the Frankenberg massif, Germany. These rocks are faulted against underlying very low-grade Palaeozoic sequences and adjacent metamorphic complexes of the Variscan basement. The granitic gneisses record an Rb–Sr whole-rock isochron age of 461±20 Ma that is taken as at least a minimum protolith age. The bimodal meta-igneous suites are interpreted to have formed during rifting of the Gondwana continental margin in the Cambro-Ordovician. The various metamorphic units have all experienced a common P–T  history. The peak-pressure stage is constrained to around 490–520 °C and 10–14 kbar (10–12 kbar being most realistic). The metamorphism proceeded along a clockwise P–T path towards conditions of around 580–610 °C and 7–8.5 kbar at the thermal peak followed by a final low-pressure overprint which spanned amphibolite facies to prehnite–actinolite facies temperatures. Owing to a secondary Rb–Sr whole-rock isochron age of 381±24 Ma, interpreted to date the retrograde stage, the whole metamorphic cycle in the Frankenberg massif is ascribed to the late Silurian–early Devonian high-pressure event widely recorded in the European Variscides. The antiformal complexes bordering the Frankenberg massif underwent a well-documented early Carboniferous metamorphism, suggesting that the Frankenberg massif constitutes a klippe which was overthrust towards the end of this second metamorphic cycle.

Notes: Two anhydrous equilibria can be written among the components of stilpnomelane, chlorite, white mica and quartz, namely 89 daphnite+131 Fe-celadonite+190 quartz=96 stilpnomelane+71 muscovite, and amesite+Mg-celadonite=muscovite+clinochlore. We assume that the free energy change of reaction, ΔG=ΔGo+ΣRT lnaij, is approximated by ΔG=A−BT +C(P−1)+ΣRT lnaij, where ΔGo is the free energy change of the end-member components at temperature T  and pressure P, ai is the activity of component i whose coefficient in the equilibrium is j, and A, B and C are constants to be determined. Values of C can be approximated by the change in volume on reaction, namely C=406.517 J/bar for the first reaction and C=0.613 J/bar for the second reaction. Constants A and B were determined by using six occurrences of the assemblage stilpnomelane–chlorite–white mica for which P–T  have been otherwise estimated. Using solution models from the literature, linear regression gives for the first equation A=−6118.269 kJ, B=−4584.09 J/K, and for the second equation A=19.397 kJ, B=66.72 J/K. These values predict P–T  within 0.5 kbar and 25 K for all occurrences, and appear reasonably robust relative to probable analytical errors. P–T  are determined by intersection of the curves generated by given compositions in P–T  space. Fine-grained and/or zoned chlorite and white mica make application of the geothermobarometer difficult in some instances, but our work in the Bathurst region of New Brunswick suggests that, with patience and care, useful analyses can be obtained, and the database for the geothermobarometer greatly expanded.

Notes: Sm–Nd ages from the Harts Range in the south-eastern Arunta Inlier in central Australia indicate that regional metamorphism up to granulite facies occurred in the Early Ordovician (c. 475 Ma). This represents a radical departure from previous tectonic models for the region and identifies a previously unrecognized intraplate event in central Australia. Peak metamorphic assemblages (800 °C and 10.5 kbar) formed at around 476±14 Ma and underwent approximately 4 kbar of near-isothermal decompression at 475±4 Ma. A coarse-grained unfoliated garnet–clinopyroxene-bearing marble inferred to have recrystallized late in the decompressional evolution, gives an age of 469±7 Ma. Two lines of evidence suggest the Early Ordovician tectonism occurred in an extensional setting. First, the timing of the high-grade lower crustal deformation coincides with a period of marine sedimentation in the Amadeus and Georgina basins that was associated with a seaway that developed across central Australia. Second, isothermal decompression of lower crustal rocks was associated with the formation of a regional, sub-horizontal mid-crustal foliation. In the Entia Gneiss Complex, which forms the structurally lowest part of the Harts Range, upper-amphibolite facies metamorphism (c. 700 °C, 8–9 kbar) occurred at 479±15 Ma. There is no evidence that P–T conditions in the Entia Gneiss Complex were as high as in the overlying units. This implies that the extensional system was reworked during a later compressional event. Sm–Nd data from the mid-amphibolite facies (c. 650 °C and 6 kbar) detachment zone that separates the Irindina Supracrustal Assemblage and Entia Gneiss Complex give an age of 449±10 Ma. This age corresponds to the timing of a change in the pattern and style of sedimentation in the Amadeus and Georgina basins, and indicates that the change in basin dynamics was associated with mid-crustal deformation. It also suggests that compressional deformation culminating in the Devonian to Carboniferous (400–300 Ma) Alice Springs Orogeny may have begun as early as c. 450 Ma. At present, the extent of Early Ordovician tectonism in central Australia is unknown. However, granulite facies metamorphism and associated intense deformation imply an event of regional extent. An implication of this work is that high-grade lower crustal metamorphism and intense deformation occurred during the development of a broad, shallow, slowly subsiding intraplate basin.

Notes: In low-pressure environments, precipitation of graphite is hindered at low to moderate temperatures by the high solubility of carbon in C-O-H fluids and by kinetic barriers to nucleation. Those low-temperature fluids that do attain saturation tend to precipitate graphite continuously during flow and cooling, thereby producing widely dispersed films of low-crystallinity graphite. In contrast, at high temperatures, particularly when combined with high pressures, the precipitation of graphite is enhanced by decreased solubility of carbon in C-O-H fluids and by improved nucleation under those conditions. The longevity of fluid systems in high-temperature, high-pressure terranes permits efficient, long-term scavenging of dispersed carbon from the crust. The latter may be redistributed in a much more concentrated form as fluids rise, cool and decompress, and as the carbon is finally precipitated as highly crystalline graphite in fracture systems. The combined effects of the thermochemical controls on carbon solubility and the geological controls on fluid generation, movement and P–T  pathways are the reason that large, epigenetic graphite deposits form dominantly at high temperatures and pressures. Those high-temperature, high-pressure conditions, in turn, account for the uniformly high crystallinity of the fluid-deposited graphite.

Notes: New geochemical and sulphur isotopic data are presented for a number of pyrite deposits from the Late Jurassic–Early Cretaceous Cameros Basin, Spain. The deposits were formed at, or close to, the peak of metamorphism and are always related to sandstone units in the mainly metapelite sequence. Iron remained immobile and conservative, pyrite iron being derived by sulphidation of chlorite in the host metapelites. Reduced sulphur, however, was supplied from two external sources: thermochemical reduction of sulphate and release of S during metamorphism of sedimentary sulphides. These sources provided isotopically heavy and light S, respectively, with variation in pyrite isotopic composition between different deposits resulting from differences in their relative importance at each site. During metamorphism, the sandstone units acted as aquifers, carrying the sulphidic pore waters to locations where permeability provided by syn-depositional fractures on a scale of 0.5–5 m allowed its interaction with the metapelites. Transport distances for sulphide during metamorphism were of the order of hundreds of metres.

Notes: Continental collision results in deep burial of crustal rocks and their subsequent partial melting. Field relations of melt along zones of intense deformation suggest that partially molten rocks may play an important role in regional tectonics. However, subsequent deformation may erase the microstructures produced by the earlier deformation mechanisms, inhibiting our understanding of the rheology of partially molten crustal rocks. Thus, in this paper, we report the results of an experimental study of the distribution of 2–5 vol% melt in quartzo-feldspathic aggregates of various grain sizes: 2–5, 5–10, 10–16 and 26–31 μm. Three types of samples were examined, all with the composition of 60 wt% albite, 25 wt% potassium feldspar, 10 wt% quartz and 5 wt% biotite. The first group included mineral powders annealed at 1000 °C, 1.0 GPa, for c. 100 h. The second group included commercially hot-pressed mineral powders which yielded c. 25 vol% glass; cores of this material were also annealed at 1000 °C, 1.0 GPa, for c. 100 h. The third group included cores of hot-pressed material that were annealed at 1000 °C, 1.0 GPa, for c. 45 h, then deformed. All samples were quenched rapidly in order to examine the melt distribution. Wetting angles are very similar in both the hydrostatically annealed and the deformed samples. Analysis of melt pool orientations reveal that melt migrates away from grain boundaries normal to the maximum compressive stress direction in response to the applied non-hydrostatic stress. This response is easily seen in the coarser-grained samples in which melt pools elongated parallel to the maximum compressive stress direction formed during deformation. If these results extrapolate to naturally deformed rocks, it will be important to consider the orientation of the state of stress in a region during syn-magmatic deformation because of its effect on the melt distribution.

Notes: Metapelites from the southern aureole of the Vedrette di Ries tonalite (eastern Alps) were variably overprinted by contact and earlier regional metamorphic events during pre-Alpine and Alpine metamorphic cycles. In these rocks, starting from a primary garnet mica-schist (garnet stage), a complex sequence of transformations, affecting the site of the garnet, has been recognized. In the outermost part of the aureole, the primary garnet sites are occupied by nodules of kyanite (kyanite stage). Closer to the tonalite, kyanite is replaced by staurolite (staurolite stage), which in turn is pseudomorphed by muscovite (muscovite stage). The aggregates of kyanite do not overgrow garnet directly; they post-date a stage (fibrolite stage) represented by the pseudomorphic alteration of garnet into fibrolitic sillimanite plus biotite. A further sericite stage is likely to have occurred between the fibrolite and kyanite stages. Preservation of the sub-spherical garnet shape during all these transformations and persistence of mineralogical and textural relicts from earlier stages were favoured by the very low strain experienced by the rocks since the garnet stage. The textural sequence is in agreement with the metamorphic history of this part of the Austroalpine basement of the Eastern Alps: the garnet and fibrolite stages, and the coeval main foliation of the samples, are referred to the high-grade Hercynian metamorphism; the kyanite stage to the Eo-Alpine metamorphism; the staurolite and muscovite stages to the Oligocene contact metamorphism. It is suggested that kyanite growth as microgranular aggregates took place in polymetamorphic rocks where static, high-P/low-T  metamorphism overprinted high-T  assemblages that contained sillimanite or andalusite.

Notes: U–Pb isotopic data from the northern Monashee complex, one of the deepest structural exposures in the southern Canadian Cordillera, indicate that the age of metamorphism varies according to structural position in a 6 km thick section. This metamorphism resulted in an unusual sequence in which rocks with the lowest-grade mineral assemblage (kyanite–sillimanite–staurolite–muscovite) are underlain and overlain by higher-grade rocks. Xenotime and monazite U–Pb dates vary progressively from 64 Ma in the structurally highest rocks to 49 Ma in the deepest rocks. Discordant U–Pb ages from Proterozoic and Cretaceous monazite and titanite are used to interpret the thermal significance of the early Tertiary dates. The discordant analyses define linear arrays with lower intercepts that broadly overlap with early Tertiary, and the amount of discordance varies with structural level; it is least in the deeper rocks and greatest in higher rocks. Electron microprobe work showed that the monazite discordance in the deeper rocks resulted from Tertiary mineral overgrowth and recrystallization rather than Pb diffusion. We use previous studies of Pb diffusion and the fact that Proterozoic monazite and titanite suffered only negligible to moderate amounts of diffusive Pb loss to contend that elevated temperatures (c. 600–650 °C are inferred from pelitic mineral assemblages) existed in the deeper rocks for a short duration, perhaps a few million years. The downwards younging 64–49 Ma U–Pb dates are interpreted as closely reflecting xenotime and monazite growth ages rather than cooling ages or substantially reset ages based on the lack of Pb diffusion in monazite and the previously obtained 40Ar/39Ar data which suggest that rapid cooling occurred immediately after the U–Pb dates. In addition, growth ages are interpreted as thermal peak ages based on U–Pb dates from coeval kyanite-bearing leucosomes, the consistent nature of the U–Pb dates throughout the study area, and petrographic relationships which suggest that monazite grew before or during development of the syn-metamorphic foliation. These interpretations lead us to conclude that metamorphism was diachronous according to structural level, with higher rocks attaining peak temperatures and cooling rapidly while deeper rocks were heating towards a thermal peak that was attained a few million years later. This thermal scenario requires that higher rocks cannot have been the heat source for the deeper metamorphism, as was previously proposed.

Notes: Schists from the foothills of the Central Sierra Nevada contain one dominant matrix foliation and yet four phases of growth of both cordierite and andalusite porphyroblasts can be distinguished. These occurred early during four separate deformation events that formed successive steep and shallow foliations. A fifth deformation event pre-dates the growth of all porphyroblasts studied. The multiple phases of porphyroblast growth allow correlation of structures across and along the region. A repeated pattern of deformation, in terms of the curvature of earlier foliations against the overprinting one, allows samples containing porphyroblasts with simpler inclusion trail geometries to be interpreted with confidence. The large-scale fold structures in this region formed before or during the second of the five deformation events recorded by the porphyroblasts. However, the matrix foliation is predominantly a product of the fourth deformation, which has commonly reactivated or re-used older foliations, and is dominated by east-side-up shear. The intervening third deformation produced locally intense foliations and was accompanied by top-to-the-east shear. The very weak fifth deformation produced weak crenulations with subhorizontal axial planes and was coaxial. Multiple phases of episodic but synchronous growth of cordierite and andalusite were produced by the KFMASH univariant equilibrium Ms+Chl+Qtz=And+Crd+Bt+H2O. The rocks crossed this reaction at a pressure just below the intersection with the KFMASH divariant equilibrium Ms+Chl+Qtz=Crd+Bt+H2O; the latter being overstepped in favour of the former as there is no evidence for cordierite growth prior to andalusite in these rocks. Subsequent multiple episodes of synchronous growth of cordierite and andalusite indicate that the possible variation in P–T  during subsequent deformations was not large. This requires the high-amplitude macroscopic fold to form prior to porphyroblast growth and then be simply tightened and modified by the younger deformations.

Notes: Contacts between rocks recording large differences in metamorphic grade are indicative of major tectonic displacements. Low-P upon high-P contacts are commonly interpreted as extensional (i.e. material points on either side of the contact moved apart relative to the palaeo-horizontal), but dating of deformation and metamorphism is essential in testing such models. In the Western Alps, the Piemonte Ophiolite consists of eclogites (T ≈550–600 °C and P≈18–20 kbar) structurally beneath greenschist facies rocks (T ≈400 °C and P≈9 kbar). Mapping shows that the latter form a kilometre-wide shear zone (the Gressoney Shear Zone, GSZ) dominated by top-SE movement related to crustal extension. Rb–Sr data from micas within different GSZ fabrics, which dynamically recrystallized below their blocking temperature, are interpreted as deformation ages. Ages from different samples within the same fabric are reproducible and are consistent with the relative chronology derived from mapping. They show that the GSZ had an extensional deformation history over a period of c. 9 Myr between c. 45–36 Ma. This overlaps in time with the eclogite facies metamorphism. The GSZ operated over the entire period during which the footwall evolved from eclogite to greenschist facies and was therefore responsible for eclogite exhumation. The discrete contact zone between eclogite and greenschist facies rocks is the last active part of the GSZ and truncates greenschist facies folds in the footwall. These final movements were therefore not a major component of eclogite exhumation. Pressure estimates associated with old and young fabrics within the GSZ are comparable, indicating that during extensional deformation there was no significant unroofing of the hangingwall. Since there are no known extensional structures younger than 36 Ma at higher levels in this part of the Alps, exhumation since the final juxtaposition of the two units (at 36 Ma) seems to have been dominated by erosion.Key words: deformation age, eclogite, exhumation, Rb–Sr dating, tectonic.

Notes: Partial melting and retrogression related to Variscan tectonic exhumation have been recognized in the high-grade metapelites of the Tatra Mountains, Western Carpathians. Staurolite and kyanite relics document an early stage of the prograde metamorphism at c. 600 °C and 9–10 kbar. An increase in temperature to 〉730 °C at 11–12 kbar resulted in partial melting and incipient migmatization in the stability field of kyanite. Further heating at decreasing pressure during the earliest stage of exhumation led to the dehydration-melting of muscovite and biotite at 〉750–800 °C and 6–10 kbar, producing garnet-bearing granite as leucosomes in migmatite. Subsequent cooling is documented by garnet resorption by biotite and sillimanite (a reversal of the prograde biotite dehydration-melting reaction). This was followed by nearly isothermal decompression to c. 4–5 kbar producing cordierite and some melt due to biotite decomposition. Later nearly isobaric cooling led to cordierite pinitization and formation of orthoamphibole, chlorite and carbonates. Densities of primary, monophase CO2–N2 inclusions (0.69–1.06 g cm−3) from the migmatite leucosome are consistent with the near-peak and retrograde conditions. Highly varying N2 contents (5–30 mol%) are thought to result from the nitrogen uptake in retrograde K-bearing minerals, or dilution by CO2 liberated during interaction of melt-derived water with metapelite graphite. The relatively high nitrogen content, not observed until now in migmatites, could have been inherited from the high-pressure metamorphism stage. It is assumed that the water-absent composition of fluid inclusions is not representative of the bulk water content (XH2O≤0.7), which was masked by mechanical separation of the CO2- and H2O-dominated immiscible phases, and/or by post-entrapment modifications of the fluid inclusions. Decompression and the final stage of exhumation were accomplished by top-to-the-south thrusting as well as west–east (orogen-parallel) extension. They were most probably related to regional uplift and gravitational collapse of thermally weakened Variscan crust.

Notes: Structures within the aureole of the Acadian (Devonian) Victory Pluton suggest that movement along the Monroe Fault occurred during pluton intrusion. Pre- or syn-tectonic garnet textures, including discordant internal and external foliations, sigmoidal inclusion trails and deflection of the matrix foliation around garnet, are found along the Monroe Fault. In sillimanite+K-feldspar grade rocks, the matrix foliation, defined by biotite and fibrolitic sillimanite, wraps around garnet and sillimanite porphyroblasts. Granite dykes and sills near the pluton contact are folded and boundinaged, and leucosome fills garnet pressure shadows and shear bands. Away from the fault and pluton contact, microstructures indicate much less deformation during metamorphism, suggesting that deformation was partitioned into the fault or the pluton. Deformation ceased before crystallization of the main body of the Victory Pluton was complete. It is possible that magmatism facilitated deformation along the Monroe Fault, and/or that the magma was transported to the mid- to upper crust along the Monroe Fault. This study suggests that Acadian deformation in north-eastern Vermont occurred as late as 390–370 Ma.

Notes: The Laramie Mountains of south-eastern Wyoming contain two metamorphic domains that are separated by the 1.76 Ga. Laramie Peak shear zone (LPSZ). South of the LPSZ lies the Palmer Canyon block, where apatite U–Pb ages are c. 1745 Ma and the rocks have undergone Proterozoic kyanite-grade Barrovian metamorphism. In contrast, in the Laramie Peak block, north of the shear zone, the U–Pb apatite ages are 2.4–2.1 Ga, the granitic rocks are unmetamorphosed and supracrustal rocks record only low-T amphibolite facies metamorphism that is Archean in age. Peak mineral assemblages in the Palmer Canyon block include (a) quartz–biotite–plagioclase–garnet–staurolite–kyanite in the pelitic schists; (b) quartz–biotite–plagioclase–low-Ca amphiboles–kyanite in Mg–Al-rich schists, and locally (c) hornblende–plagioclase–garnet in amphibolites. All rock types show abundant textural evidence of decompression and retrograde re-equilibration. Notable among the texturally late minerals are cordierite and sapphirine, which occur in coronas around kyanite in Mg–Al-rich schists. Thermobarometry from texturally early and late assemblages for samples from different areas within the Palmer Canyon block define decompression from 〉7 kbar to 〈3 kbar. The high-pressure regional metamorphism is interpreted to be a response to thrusting associated with the Medicine Bow orogeny at c. 1.78–1.76 Ga. At this time, the north-central Laramie Range was tectonically thickened by as much as 12 km. This crustal thickening extended for more than 60 km north of the Cheyenne belt in southern Wyoming. Late in the orogenic cycle, rocks of the Palmer Canyon block were uplifted and unroofed as the result of transpression along the Laramie Peak shear zone to produce the widespread decompression textures. The Proterozoic tectonic history of the central Laramie Range is similar to exhumation that accompanied late-orogenic oblique convergence in many Phanerozoic orogenic belts.

Notes: During Hercynian low-pressure/high-temperature metamorphism of Palaeozoic metasediments of the southern Aspromonte (Calabria), a sequence of metamorphic zones at chlorite, biotite, garnet, staurolite–andalusite and sillimanite–muscovite grade was developed. These metasediments represent the upper part of an exposed tilted cross-section through the Hercynian continental crust. P–T  information on their metamorphism supplements that already known for the granulite facies lower crust of the section and allows reconstruction of the thermal conditions in the Calabrian crust during the late Hercynian orogenic event.Three foliations formed during deformation of the metasediments. The peak metamorphic assemblages grew mainly syntectonically (S2) during regional metamorphism, but mineral growth outlasted the deformation. This is in accordance with the textural relationships found in the lower part of the same crustal section exposed in the northern Serre. Pressure conditions recorded for the base of the upper crustal metasediments are c. 2.5 kbar and estimated temperatures range from 〈350 °C in the chlorite zone, increasing to 500 °C in the lower garnet zone, and reaching 620 °C in the sillimanite–muscovite zone. Geothermal gradients for the peak of metamorphism indicate a much higher value for the upper crust (c. 60 °C km−1) than for the granulite facies lower crust (30–35 °C km−1). The small temperature difference between the base of the upper crust (620 °C at c. 2.5 kbar) and the top of the lower crust (690 °C at 5.5 kbar) can be explained by intrusions of granitoids into the middle crust, which, in this crustal section, took place synchronously with the regional metamorphism at c. 310– 295 Ma.It is concluded that the thermal structure of the Calabrian crust during the Hercynian orogeny – as it is reflected by peak metamorphic assemblages – was mainly controlled by advective heat input through magmatic intrusions into all levels of the crust.

Notes: A compilation of literature data on the Fe-Mg composition of coexisting chlorite and chloritoid from metapelites metamorphosed at various P-T  conditions shows that the logarithm of the Fe-Mg partitioning (lnKD) varies linearly with the inverse of temperature, from about 2.4 at 300 °C to about 1.3 at 600 °C. In contrast, no trend was observed with pressure, and the molar volumes of Mg- and Fe-chlorite end-members suggest that the pressure dependence of lnKD is not significant. Therefore, the chloritoid-chlorite Mg-Fe exchange reaction is a potential thermometer and has been empirically calibrated using the analyses of 112 chloritoid-chlorite pairs from 28 different localities. Temperatures estimated using the Chl-Cld thermometer were checked against independent estimates for 20 samples not involved in the calibration (Beni Mzala window, Morocco), and the results are in fair agreement with independent temperature estimates. However, the analytical uncertainties and errors are too large to obtain reliable temperature estimates for extremely Mg-rich or Fe-rich compositions. The Chl-Cld thermometer is unreliable at XMg-CLD〈0.2 and XMg-CLD〉0.8 at 700 °C, and XMg-CLD〈0.1 and XMg-CLD〉0.9 at 300 °C. Using the results of the empirical calibration, we calculated new thermodynamic data for daphnite. Implementing these data, it becomes possible to estimate T  and P conditions of metamorphism for the invariant chlorite-chloritoid-quartz-aluminosilicate assemblage that is widespread in low-grade metapelites. These estimates appear to be relevant only in the stability field of kyanite, whereas the uncertainties on the calculated pressure conditions are very large in the stability field of kaolinite and pyrophyllite.

Notes: This study investigates marbles and calcsilicates in Central Dronning Maud Land (CDML), East Antarctica. The paleogeographic positioning of CDML as part of Gondwana is still unclear; however, rock types, mineral assemblages, textures and P–T  conditions observed in this study are remarkably similar to the Kerala Khondalite Belt in India. The CDML marbles and calcsilicates experienced a Pan-African granulite facies metamorphism at c. 570 Ma and an amphibolite facies retrogression at c. 520 Ma. The highest grade assemblage in marbles is forsterite+spinel+calcite+dolomite, in calcsilicates the assemblages are diopside+spinel, diopside+garnet, scapolite+wollastonite+clinopyroxene±quartz, scapolite±anorthite±calcite+clinopyroxene+wollastonite. These assemblages constrain the peak metamorphic conditions to 830±20 °C, 6.8±0.5 kbar and XCO2〉0.46. During retrogression, highly fluoric humite-group minerals (humite, clinohumite, chondrodite) replaced forsterite, and garnet rims formed at the expense of scapolite during reactions with wollastonite, calcite or clinopyroxene but without involvement of anorthite. Metamorphic conditions were about 650 °C, 4.5±0.7 kbar, 0.2〈 XCO2fluid〈0.36, and the co-existence of garnet, clinopyroxene, wollastonite and quartz constrains fO2 to FMQ-1.5 log units. Mineral textures indicate a very limited influx of H2O-rich fluid during amphibolite facies retrogression and point to significant variations of fluid composition in mm-sized areas of the rock. Gypsum was observed in two samples; it probably replaced metamorphic anhydrite which appears to have formed under amphibolite facies conditions. The observed extensive anorogenic magmatism (anorthosites, A-type granitoids) and the character of metamorphism between 610 and 510 Ma suggest that the crustal thermal structure was characterized by a long-lived (50–100 Ma) rise of the crustal geotherm probably caused by magmatic underplating.

Notes: Presented textural and petrological data show that the deep to intermediate continental crust may fracture and that microfractures are the locus of fluid and mass transfer necessary for retrograde metamorphism. Kyanite eclogites from Ulsteinvik, Norway, underwent partial retrogression to granulite and amphibolite facies assemblages during near-isothermal exhumation from depths equivalent to more than 2.0 GPa at temperatures of 700–800 °C. Plagioclase-bearing assemblages, rich in hydrous phases, formed along margins of eclogite lenses and along mesoscopic fracture systems. Hydrated zones are from 1–50 cm thick, with adjacent wall-rock eclogite replaced by symplectites. At a low degree of reaction, the secondary minerals in the wall-rock are found along intra- and intergranular microfractures (typically 50–100 μm wide). Minerals filling the microfractures include orthopyroxene–plagioclase–spinel in garnet; plagioclase–sapphirine, plagioclase–corundum and plagioclase–spinel in kyanite; and diopside–plagioclase in omphacite. The microfractures are often arranged en echelon and are connected through microfaults. Releasing bends filled with amphibole and spinel form along microfaults in garnet. The faulting and fracturing caused localized chemical change in garnet: the damage zones close to faults are enriched in FeO and MnO with steep compositional gradients (8 wt% FeO over 〈20 μm). These FeO- and MnO-enriched zones form wedge-like structures around the tip of the faults (horsetail structures) and rose- or flame-like structures at sticking points along faults. They may represent examples of stress-induced chemical transport during fracture propagation. The change from dry to amphibole-bearing assemblages at the tip of the fracture, and fractures ending in splays of fluid inclusions trails, reflect the involvement of a fluid phase during fracture propagation. This suggests that the ‘dry’ granulite facies retrogression was also driven by fluid infiltration and that metamorphism at depth in collision zones may not be controlled by pressure and temperature alone.

Notes: Oxygen bulk diffusion rates were experimentally determined in a natural ultramylonite sample (c. 5 μm grain size; 15–20% biotite, 20% quartz, 60–65% feldspars, and minor Fe-oxides) from the Gerrish Island shear zone, SE Maine, USA. The diffusion experiments were performed at 250–550 °C and 100 MPa water pressure. Oxygen bulk diffusion rates were determined both parallel and perpendicular to the strong foliation of the sample. The Arrhenius parameters for transport parallel to the foliation are: Dbulk0=2.0×10−11 m2 s−1 and Q=30±6 kJ mol−1. The bulk diffusivity perpendicular to the foliation is about a factor of 3.5 less than that parallel to the foliation with the same activation energy. The values of bulk diffusivity and activation energy obtained are consistent with ionic diffusion through a static aqueous fluid, suggesting that an interconnected fluid exists in the ultramylonite even under hydrostatic conditions. The microstructure of the ultramylonite was characterized using transmission electron microscopy (TEM). The nature and distribution of the interconnected fluid cannot be completely resolved from the TEM analysis; however, the low percentage of three-grain channels and open grain/interphase boundaries suggests that the fluid resides as a thin film on the grain surfaces. The results of this study have direct applications in many important geological settings and provide valuable insights into the observed rapid diffusion rates, strong lithological control and pervasive nature of fluid transport in mica-bearing rocks.

Notes: New petrological and microstructural data from various metaperidotite ‘boudins’ within large ductile shear zones in the Cabo Ortegal allochthonous complex in NW Spain have important implications for the tectonic models of the area. The peridotites (mylonitic garnet harzburgite, Ti-clinohumite and magnesite–olivine orthopyroxenite) contain mineral assemblages that equilibrated at high- to ultra-high-pressure metamorphic conditions as well as microstructures of tectonic origin formed at temperatures well above 800 °C. Olivine and orthopyroxene fabrics resulted from flow at high temperature (〉1000 °C) and solid-state non-coaxial plastic flow at intermediate temperature (800–900 °C). Flow caused dynamic recrystallization and formation of moderate to strong lattice preferred orientations under low to moderate differential stresses and strain rates characteristic of upper mantle and deep crustal deformation. The microstructures and textural relationships suggest that the mylonitic garnet harzburgite represents mantle fragments with lithospheric and asthenospheric imprints, whereas the olivine orthopyroxenite resulted from serpentinite burial to depths where it acquired a characteristic high/ultra-high-pressure metamorphic signature. Both types of ultramafites converged to a common site in a subduction zone that was later incorporated during continental collision to the NW Iberian Massif as exotic, allochthonous complexes that record structural and metamorphic evidence of the earliest phases of the Hercynian orogeny.

Notes: Chemical relationships among four metapelites have been studied by investigation of mineral and bulk chemistry data and by singular value decomposition analysis of single and composite assemblage matrices. Bulk rock compositions cluster close together in an AFM diagram, all within the intersection space defined by the four sample assemblages. The similarity of bulk compositions normalized on a silica-free, anhydrous basis indicates that sample chemistries differ mainly as a result of inhomogeneous distribution of quartz layers. The existence of mass balance relationships among samples indicates that assemblages also overlap in the Si–Ti–Al–Fe–Mg–Mn–Ca–Na–K multisystem. Analysis of single and composite matrices helps in defining possible mass balances linking sample mineral facies to one another during progressive contact metamorphism. The assemblage in sample A can form as the result of the model reaction 5.000 Ky+0.269 Grt+0.965 Bt+0.314 Pl=0.049 Ilm+1.115 Ms+0.849 Chl+0.306 St and react to assemblage B via reaction 0.97 Chl+0.52 Grt+0.66 Ms+0.14 Ilm+1.26 Ky=0.42 St+0.63 Bt+0.22 Pl coupled with the Ky→And transition. Assemblage B can transform into C by initial progress of Ky+Ilm+Chl+Grt+Ms=And+St+Bt+Pl followed by 0.142 Ilm+0.175 St+1.089 Chl+1.533 Ms+0.003 Grt=5.000 And+1.266 Bt+0.551 Pl Matrix analysis cannot satisfactorily model the C–D transition, because it predicts a net production of staurolite, which is in disagreement with petrographic evidence. All mass balances in the C–D composite matrix indicate net consumption of muscovite; this is integrated with the contrasting evidence of prograde pseudomorphs of muscovite after staurolite, observed in the nodules of sample D, within a model involving the progress of ionic reaction cycles.

Notes: Fe–Ti gabbros from the Baie du Nord Segment of the Manicouagan Imbricate Zone, metamorphosed under high P–T  conditions during the Grenvillian orogeny, have been the focus of a detailed micropetrological study. Textures and mineral chemistry suggest that the mineral assemblages represent progressive stages of metamorphic transformation resulting in the formation of coronas, pseudomorphs after igneous phases (transitional) and true, granoblastic eclogites. The transitional and eclogitic samples also have coronas which are developed locally around igneous xenocrysts of plagioclase and olivine. The deformed margins of coronitic Fe–Ti gabbros are transformed to amphibolite and contain clinopyroxene-bearing leucosomes with garnet poikiloblasts that are indicative of high-P–T  dehydration melting. Interpretation of garnet zoning and thermobarometry suggest that the highest P–T conditions are recorded by coronas around xenocrysts (c. 720–800 °C at 14–17 kbar) and garnet–clinopyroxene cores in granoblastic assemblages (c. 740–820 °C at 13–17 kbar) in the eclogitic samples. Re-equilibration during the early stages of exhumation at high-T  conditions (〉700 °C) affected all samples, and is evidenced by the widespread development of pargasite-bearing plagioclase collars in the coronitic and transitional metagabbros and by widespread re-equilibration of the eclogites giving lower P–T  estimates at grain boundaries. However, the difference in calculated pressure conditions between coronite and eclogite samples is consistent with increasing pressure (depth) from the coronites (11–13 kbar) to the eclogites (13–17 kbar). The P–T  conditions recorded by these rocks define a metamorphic field gradient which suggests high heat flow through the lower crust during the Grenvillian orogeny.

Notes: The origin of snowball and sigmoidal inclusion patterns in porphyroblasts is discussed. Snowball garnets are peculiar to shear zones whereas sigmoidal patterns occur in porphyroblasts both in shear zones and on the limbs of folds. There are currently two models for the development of snowball garnets and these have been discussed extensively in the literature. We show that although the typical two-dimensional snowball pattern can be produced by either model, the three-dimensional inclusion patterns are model-specific thus providing a distinguishing criterion. We have applied this criterion to all the available data and find that the classical model, which is dependent on the rotation of garnet relative to a single foliation, is applicable in all cases. Syn-kinematic porphyroblasts on the limbs of horizontal normal folds generally show little rotation relative to geographical coordinates. What rotation they do show generally has the same sense as that of the host limb, but is less in magnitude. This has been used as evidence that the porphyroblasts have remained irrotational while the rocks deformed around them; the implication being that they were unaffected by vorticity associated with folding. This has been explained by claiming that the porphyroblasts are restricted in distribution to small domains of coaxial deformation path. We show that for reasonable deformation models of horizontal normal folds, porphyroblasts affected by vorticity will rotate little with respect to geographical coordinates and our results predict the commonly observed natural patterns. We conclude therefore that lack of rotation relative to geographical coordinates cannot be used to demonstrate that porphyroblasts have grown only in coaxially deforming domains; much less restrictive and more reasonable interpretations are possible. Consequently, the lack of rotation relative to geographical coordinates is more significant for fold modelling than it is for the garnet controversy.

Notes: Garnet from a kinzigite, a high-grade gneiss from the central Black Forest (Germany), displays a prominent and regular retrograde diffusion zoning in Fe, Mn and particularly Mg. The Mg diffusion profiles are suitable to derive cooling rates using recent datasets for cation diffusion in garnet. This information, together with textural relationships, thermobarometry and thermochronology, is used to constrain the pressure–temperature–time history of the high-grade gneisses. The garnet–biotite thermometer indicates peak metamorphic temperatures for the garnet cores of 730–810 °C. The temperatures for the outer rims are 600–650 °C. Garnet–Al2SiO5–plagioclase–quartz (GASP) barometry, garnet–rutile–Al2SiO5–ilmenite (GRAIL) and garnet–rutile–ilmenite–plagioclase–quartz (GRIPS) barometry yield pressures from 6–9 kbar. U–Pb ages of monazite of 341±2 Ma date the low-P high-T metamorphism in the central Black Forest. A Rb/Sr biotite–whole rock pair defines a cooling age of 321±2 Ma. The two mineral ages yield a cooling rate of about 15±2 °C Ma−1. The petrologic cooling rates, with particular consideration of the fO2 conditions for modelling retrograde diffusion profiles, agree with the geochronological cooling rate. The oldest sediments overlying the crystalline basement indicate a minimum cooling rate of 10 °C Ma−1.

Notes: Cr-rich magnesiochloritoid in the eclogitized ophiolites of the Monviso massif occurs in the least differentiated rocks of the gabbroic sequence (troctolites to melatroctolites). Chloritoid (XMg=0.63–0.85; Cr≤0.55, atoms) co-exists with omphacite, talc and garnet. Minor, syn-eclogitic minerals are chromite, rutile and sometimes magnesite and Cr–Ti oxides.Coronitic textures, indicative of a static recrystallization, characterize the analysed samples. Layers of variable mineral composition develop among igneous plagioclase, olivine, clinopyroxene and spinel. The minerals in the coronitic layers display sharp compositional zonings. The igneous minerals are commonly not preserved; their presence in the original assemblage is inferred from the mineralogical composition of the pseudomorphs.Syn-eclogitic volatile components are indicated by the development of OH-bearing minerals (e.g. chloritoid & talc) and carbonates (e.g. magnesite), and supported by the presence of coarse-grained and fibrous mineral growths. The complex pseudomorphic replacements of igneous minerals suggest that these rocks changed their mineralogical composition prior to the eclogite facies recrystallization, most likely during ocean-floor metamorphism. It is suggested that syn-eclogitic fluids formed by breakdown reactions of pre-eclogitic volatile-bearing minerals.Geothermobarometry indicates that the investigated rocks recrystallized at a depth corresponding to 2.4 GPa and temperatures of 620±50 °C. The attainment of high-pressure conditions is supported by the presence of magnesiochloritoid, magnesite and garnet with high pyrope content (up to 58 mol%). P–T  estimates point to a very low thermal gradient (about 9 °C km−1), comparable to that deduced in the adjacent Dora-Maira ultra-high pressure unit.

Notes: Archean supracrustal sequences of pelitic, quartzitic, calcareous and mafic compositions in the central Laramie Mountains, Wyoming, have been affected by two metamorphic events: a 1.78 Ga amphibolite-grade regional metamorphism, and a 1.43 Ga contact metamorphism resulting from the intrusion of the Laramie Anorthosite Complex (LAC). Rb–Sr whole-rock isotopic data from both outside and within the LAC contact aureole define a linear array that lies along a 1.78 Ga isochron. This date has been independently established as the time of amphibolite facies regional metamorphism associated with collision of the Archean Wyoming province and the Proterozoic Colorado province along the Cheyenne belt. The Rb–Sr isotopic data require that Sr was redistributed during regional metamorphism on a scale of at least tens of metres. Although within the 2 km-wide aureole of LAC the pelitic rocks were thermally metamorphosed at temperatures greater than 800 °C, none of the whole-rock Rb–Sr data from samples within the LAC aureole show evidence of resetting at 1.43 Ga. It is interpreted that the regional metamorphism involved fluid transport which facilitated Sr isotopic resetting, whereas the contact metamorphism occurred in a relatively dry environment in which isotopic mobility was restricted to centimetre-scale or less. Rb–Sr data for biotite, feldspar and whole rock from a regional metamorphosed pelitic schist give an isochron age of 1450±40 Ma, which is interpreted as a cooling age resulting from crustal uplift. Rb–Sr data for biotite, quartz+feldspar and whole rock from a pelitic schist affected by contact metamorphism give an isochron age of 1420±43 Ma, the time of isotopic re-equilibration in response either to crustal uplift or to both contact metamorphism and crustal uplift. This study demonstrates that although the response of isotopic systems to metamorphism is complex, isotopic data provide insight into metamorphic processes that is difficult to obtain by other means.

Notes: Apatite occurs in the zeolite to greenschist facies metamorphic rocks of the Otago Schist, South Island, New Zealand, as both a groundmass constituent and as a hydrothermal phase hosted in metamorphic quartz veins. Groundmass apatite from low-grade rocks, ranging from the zeolite facies to the pumpellyite–actinolite zone, has chloride contents ranging from 0–1.4 wt%, and fluoride contents ranging from 2.2–4.2 wt%, whilst groundmass apatite from the greenschist facies (chlorite to biotite zone) is virtually pure fluorapatite. Vein apatite from all grades is also fluorapatite with little or no chloride. This difference in composition is interpreted as resulting from the preservation of the primary magmatic compositions of detrital Cl-apatite grains, out of equilibrium with the metamorphic fluid, at low grades, whilst higher-grade groundmass apatite and neoformed apatite in quartz veins have compositions in equilibrium with an aqueous metamorphic fluid. The presence of detrital Cl-bearing apatite during the early stages of metamorphism may constitute a significant reservoir of Cl, given the low porosities of compacted sediments undergoing prograde metamorphism. Calculations indicate that the release of Cl from detrital apatite in the Otago Schist, as a result of re-equilibration of apatite with the pore fluid, may have had a significant effect on the salinity of the metamorphic fluid.

Notes: The behaviour of spherical versus highly ellipsoidal rigid objects in folded rocks relative to one another or the Earth’s surface is of particular significance for metamorphic and structural geologists. Two common porphyroblastic minerals, garnet and staurolite, approximate spherical and highly ellipsoidal shapes respectively. The motion of both phases is analysed using the axes of inflexion or intersection of one or more foliations preserved as inclusion trails within them (we call these axes FIAs, for foliation inflexion/intersection axes). For staurolite, this motion can also be compared with the distribution of the long axes of the crystals.Schists from the regionally shallowly plunging Bolton syncline commonly contain garnet and staurolite porphyroblasts, whose FIAs have been measured in the same sample. Garnet porphyroblasts pre-date this fold as they have inclusion trails truncated by all matrix foliations that trend parallel to the strike of the axial plane. However, they have remarkably consistent FIA trends from limb to limb. The FIAs trend 175° and lie 25°NNW from the 020° strike of the axial trace of the Bolton syncline. The plunge of these FIAs was determined for six samples and all lie within 30° of the horizontal. Eleven of these samples also contain staurolite porphyroblasts, which grew before, during and after formation of the Bolton syncline as they contain inclusion trails continuous with matrix foliations that strike parallel to the axial trace of this fold. The staurolite FIAs have an average trend of 035°, 15°NE from the 020° strike of the axial plane of this fold. The total amount of inclusion trail curvature in staurolite porphyroblasts, about the axis of relative rotation between staurolite and the matrix (i.e. the FIA), is greater than the angular spread of garnet FIAs.Although staurolite porphyroblasts have ellipsoidal shapes, their long axes exhibit no tendency to be preferentially aligned with respect to the main matrix foliation or to the trend of their FIA. This indicates that the axis of relative rotation, between porphyroblast and matrix (the FIA), was not parallel to the long axis of the crystals. It also suggests that the porphyroblasts were not preferentially rotated towards a single stretch direction during progressive deformation.Five overprinting crenulation cleavages are preserved in the matrix of rocks from the Bolton syncline and many of these result from deformation events that post-date development of this fold. Staurolite porphyroblast growth occurred during the development of all of these deformations, most of which produced foliations. Staurolite has overgrown, and preserved as helicitic inclusions, crenulated and crenulation cleavages; i.e. some inclusion trail curvature pre-dates porphyroblast growth. The deformations accompanying staurolite growth involved reversals in shear sense and changing kinematic reference frames.These relationships cannot all be explained by current models of rotation of either, or both, the garnet and staurolite porphyroblasts. In contrast, we suggest that the relationships are consistent with models of deformation paths that involve non-rotation of porphyroblasts relative to some external reference frame. Further, we suggest there is no difference in the behaviour of spherical or ellipsoidal rigid objects during ductile deformation, and that neither garnet nor staurolite have rotated in schists from the Bolton syncline during the multiple deformation events that include and post-date the development of this fold.

Notes: X-ray fluorescence, instrumental neutron activation, and particle-induced X-ray emission methods were used to determine the distribution of numerous trace elements among garnet (Grt), Ca-pyroxene (Cpx), hornblende (Hbl), biotite (Bt), plagioclase (Pl) and K-feldspar (Kf) in a high-grade metamorphic terrane within the Grenville Province of the Canadian Shield. Results are presented as distribution formulae, e.g.Sr: Kf 1.1 Pl 16 Hbl 2.2 Cpx 1.0 Bt 1.2 GrtSc: Hbl 1.1 Cpx 1.0 Grt 7.8 Bt 22 Pl 2.6 KfV: Hbl 1.15 Bt 2.07 Cpx 6.0 Grt (1.4% CaO)〉1 (Pl, Kf)Zn: Bt 1.6 Hbl 1.62 Cpx 2.9 Grt 10 PlGa: Bt 1.2 Hbl 1.2 Pl 2.5 Cpx 1.3 Grtwhere numbers are distribution ratios, e.g. ppm Sr in Hbl/ppm Sr in Cpx=2.2. Examples of inter-element similarities and differences are (a) both Rb and Cs are concentrated in biotite relative to K-feldspar, but for Rb the ratio is 2.3 and for Cs it is 16, (b) the distribution formulae for seven lanthanides are similar except for the position of garnet, e.g.Ce: Hbl 2.7 Cpx 2.8 Pl 1.1 Bt 11 Kf 16 GrtYb: Grt 2.8 Hbl 2.7 Cpx 9 Pl 1.0 Bt 7 Kfand (c) all of Sr, eight lanthanides, Zr, V and Cr are concentrated in hornblende relative to Ca-pyroxene by a factor that lies in the narrow range of 2.2–3.1. There is a larger variation (departure from the mean) in some distribution ratios than in others. Thus the mean ratios (Hbl/Cpx) for each of six elements and in parentheses the percentage relative standard deviation are Zn 1.62 (8.6), V 2.38 (12), Cr 2.42 (18), Sr 2.7 (28), Ba 2.9 (36) and Ni 1.66 (38). We suggest that variation of this kind is the result of differences from place to place in the magnitude of deformation and recrystallization (which facilitated the rearrangement of atoms), combined with rates of lattice and crystal-boundary diffusion that are unique for the various elements, thus permitting some trace elements to approach equilibrium more closely than others.

Notes: In the Sveconorwegian granulite region of SW Sweden, sapphirine occurs in reaction coronas in Mg- and Al-rich kyanite eclogites which form parts of mafic complexes. Aluminous to peraluminous sapphirine forms symplectitic intergrowths with plagioclase±corundum±spinel after kyanite. Kyanite and omphacite were the main reactants in the formation of sapphirine. The sapphirine formed during decompression from the eclogite facies (P 〉15 kbar) through the high- to medium-pressure granulite and upper amphibolite facies at c. 750 °C. Preserved growth zoning in garnet, frozen-in reaction textures, and chemical disequilibrium suggest a rapid tectonic exhumation. Ductile deformation in the surrounding gneisses and parts of the mafic complex is characterized by foliation development, WNW–ESE stretching and dynamic recrystallization under granulite to upper amphibolite facies conditions, simultaneous with the sapphirine formation. This decompression, high-grade re-equilibration and associated deformation took place during the exhumation of the Sveconorwegian eclogites, bracketed between 969±14 and 956±7 Ma. Probable tectonic causes are late-orogenic gravitational collapse and/or plate divergence following the Sveconorwegian–Grenvillian continent–continent collision. There are no indications of metastability of aluminous and peraluminous sapphirine in the decompressed kyanite eclogites; sapphirine is stable in amphibole-poor and amphibolitized varieties, including rocks that have undergone dynamic recrystallization. Close similarities between rocks from different parts of the world with respect to reaction textures suggests that sapphirine+plagioclase-forming reactions are a universal feature in high-temperature decompressed kyanite eclogites.

Notes: In the W Hoggar (Algeria), the major transcurrent N–S East Ouzzal shear zone (EOSZ) hosts several world-class gold deposits over a 100-km length. The late Pan-African EOSZ separates two contrasting Precambrian domains: the Archaean In Ouzzal block to the west (orthogneisses with subordinate metasediments, reworked and granulitized in the c. 2 Ga Eburnean event) and a Middle Proterozoic block to the east (again orthogneisses and metasediments, involved in the c. 600 Ma Pan-African event).The EOSZ is a mylonite belt, 1–3 km wide, with a 50-m-wide ultramylonite belt hosting numerous quartz veins and lenses (giant hydrothermal quartz system) associated with a quartz-sericite-pyrite-carbonate (beresite) alteration. These hydrothermal events occurred under ductile (evolving towards brittle) conditions, between 500 and 300 MPa, at 500–300°C, with aqueous-carbonic fluids derived both from underlying devolatilized metamorphic rocks and a mantle source, as recorded by stable (C, O) isotope data. No gold mineralization was associated with these typical mesothermal events.Following a pressure drop (to 130 MPa), related to the inception of extensional tectonics, the EOSZ was later percolated by a new set of hydrothermal fluids, evolved from basinal waters that deeply penetrated into the In Ouzzal basement. These fluids were Ca-bearing brines (up to 25% wt. eq. NaCl), characterized by high δD (-9 to + 18‰ range), mobilized by the thermal energy released by the late Pan-African granite magmatism (Taourirt granites).As demonstrated by Pb isotope data, the brines leached Au from the In Ouzzal granulites (which contain 3 ppb Au). Fluid inclusion studies indicate that gold was deposited from these brines in the EOSZ at a depth of c. 5 km, due to mixing and cooling with descending diluted fluids.

Notes: Oppositely concave microfolds (OCMs) in and adjacent to porphyroblasts can be classified into five nongenetic types. Type 1 OCMs are found in sections through porphyroblasts with spiral-shaped inclusion trails cut parallel to the spiral axes, and commonly show closed foliation loops. Type 2 OCMs, commonly referred to as ‘millipede’ microstructure, are highly symmetrical, the foliation folded into OCMs being approximately perpendicular to the overprinting foliation. Type 3 OCMs are similar to Type 2, but are asymmetrical, the foliation folded into OCMs being variably oblique to the overprinting foliation. Type 4 OCMs are highly asymmetrical, only one foliation is present, and this foliation is parallel to the local shear plane. Type 5 OCMs result from porphyroblast growth over a microfold interference pattern.Types 1 and 2 are commonly interpreted as indicating highly noncoaxial and highly coaxial bulk deformation paths, respectively, during porphyroblast growth. However, theoretically they can form by any deformation path intermediate between bulk coaxial shortening and bulk simple shearing. Given particular initial foliation orientation and timing of porphyroblast growth, Type 3 OCMs can also form during these intermediate deformation paths, and are commonly found in the same rocks as Type 2 OCMs. Type 4 OCMs may indicate highly noncoaxial deformation during porphyroblast growth, but may be difficult to distinguish from Type 3 OCMs. Thus, Types 1–3 (and possibly 4) reflect the finite strain state, giving no information about the rotational component of the deformation(s) responsible for their formation. Furthermore, there is a lack of unequivocal independent evidence for the degree of noncoaxiality of deformation (s) during the growth of porphyroblasts containing OCMs. Type 2 OCMs that occur independently of porphyroblasts or other rigid objects might indicate highly coaxial bulk shortening, but there is a lack of supporting physical or computer modelling.It is possible that microstructures in the matrix around OCMs formed during highly noncoaxial and highly coaxial deformation histories might have specific characteristics that allow them to be distinguished from one another. However, determining degrees of noncoaxiality from rock fabrics is a major, longstanding problem in structural geology.

Notes: Ion probe traverses across garnets from peridotites of the Caledonides of Norway and the Variscides of Poland show zoning patterns for Y, V, Zr, Cr, Ti and the REE. The complexly zoned patterns of garnets from the Bystrzyca Górna peridotite, Poland, are interpreted in terms of a changing P–T history (isobaric cooling followed by decompression and cooling). Weak rimward gradients in REE concentrations in garnets from the Almklovdalen and Sandvika peridotites, Norway, may be relicts of the original growth history of the garnets, but the nearly flat Y, V, Zr, Cr and Ti profiles from the same garnets imply a later period of near-homogenization at uniform P–T. Crushed garnet separates from each body were separated into three or more fractions on the assumption that density and magnetic susceptibility vary with Fe/Mg ratio, and Fe/Mg ratios change from garnet core to rim. Sm-Nd garnet–clinopyroxene ‘ages’ were determined for each fraction to determine whether they are also zoned. Four garnet fractions from the Góry Sowie peridotite give nearly the same ages (397–412 Ma) that are believed to span the interval of garnet growth. Garnet fractions from the Norwegian peridotites define scattered ages (816–1350 Ma) that are suspect, but hint at a Sveconorwegian equilibration event. The data indicate the Variscan and Norwegian peridotites had different histories, despite superficial mineralogical and tectonic similarities. Norwegian garnet peridotites had a long pre-Caledonian history and were extracted from a relatively cold mantle whereas the Variscan garnet peridotites had a comparatively short pre- or Eo-Variscan history and were extracted from a hot mantle.

Notes: Paragonite-bearing amphibolites occur interbedded with a garbenschist-micaschist sequence in the Austroalpine Schneeberg Complex, southern Tyrol. The mineral assemblage mainly comprises paragonite + Mg-hornblende/tschermakite + quartz + plagioclase + biotite + ankerite + Ti-phase + garnet ± muscovite. Equilibrium P–T conditions for this assemblage are 550–600°C and 8–10 kbar estimated from garnet–amphibole–plagioclase–ilmenite–rutile and Si contents of phengitic muscovites. In the vicinity of amphibole, paragonite is replaced by symplectitic chlorite + plagioclase + margarite +± biotite assemblages. Muscovite in the vicinity of amphibole reacts to form plagioclase + biotite + margarite symplectites. The reaction of white mica + hornblende is the result of decompression during uplift of the Schneeberg Complex. The breakdown of paragonite + hornblende is a water-consuming reaction and therefore it is controlled by the availability of fluid on the retrogressive P–T path. Paragonite + hornblende is a high-temperature equivalent of the common blueschist-assemblage paragonite + glaucophane in Ca-bearing systems and represents restricted P–T conditions just below omphacite stability in a mafic bulk system. While paragonite + glaucophane breakdown to chlorite + albite marks the blueschist/greenschist transition, the paragonite + hornblende breakdown observed in Schneeberg Complex rocks is indicative of a transition from epidote-amphibolite facies to greenschist facies conditions at a flatter P–T gradient of the metamorphic path compared to subduction-zone environments. Ar/Ar dating of paragonite yields an age of 84.5 ± 1 Ma, corroborating an Eoalpine high-pressure metamorphic event within the Austroalpine unit west of the Tauern Window. Eclogites that occur in the Ötztal Crystalline Basement south of the Schneeberg Complex are thought to be associated with this Eoalpine metamorphic event.

Notes: The northern Dabie terrane consists of a variety of metamorphic rocks with minor mafic-ultramafic blocks, and abundant Jurassic-Cretaceous granitic plutons. The metamorphic rocks include orthogneisses, amphibolite, migmatitic gneiss with minor granulite and metasediments; no eclogite or other high-pressure metamorphic rocks have been found. Granulites of various compositions occur either as lenses, blocks or layers within clinopyroxene-bearing amphibolite or gneiss. The palaeosomes of most migmatitic gneisses contain clinopyroxene; melanosomes and leucosomes are intimately intermingled, tightly folded and may have formed in situ. The granulites formed at about 800–830 °C and 10–14 kbar and display near-isothermal decompression P–T paths that may have resulted from crust thickened by collision. Plagioclase-amphibole coronae around garnets and matrix PI + Hbl assemblages from mafic and ultramafic granulites formed at about 750–800 °C. Partial replacement of clinopyroxene by amphibole in gneiss marks amphibolite facies retrograde metamorphism. Amphibolite facies orthogneisses and interlayered amphibolites formed at 680–750 °C and c. 6 kbar. Formation of oligoclase + orthoclase antiperthite after plagioclase took place in migmatitic gneisses at T ≤ 490°C in response to a final stage of retrograde recrystallization. These P–T estimates indicate that the northern Dabie metamorphic granulite-amphibolite facies terrane formed in a metamorphic field gradient of 20–35 °C km-1 at intermediate to low pressures, and may represent the Sino-Korean hangingwall during Triassic subduction for formation of the ultrahigh- and high-P units to the south. Post-collisional intrusion of a mafic-ultramafic cumulate complex occurred due to breakoff of the subducting slab.

Notes: With increasing temperature during prograde metamorphism reactions will occur first at the lithological contacts of mixed pelite and calcsilicate terranes. At these interfaces, a fluid of lower chemical potential of H2O and CO2 than that required to produce a fluid in either layer can be produced whether reaction is caused by fluid infiltration or is initially fluid absent. If the interface region does not allow fluid transport then as temperature increases, a fluid pressure greater than lithostatic can develop. At some degree of over-pressure relative to rock pressure, the fluid hydraulically fractures the rock and a gradient in fluid composition away from the contact can be produced. These phenomena occur at the compositional interfaces whenever univariant reactions in the differing layers cross on a temperature vs. mole fraction of CO2 diagram with slopes of opposite sign. The first occurrence of these reaction products at lithological contacts delineates an isograd that defines temperature as well as the mole fraction of CO2 at constant pressure in systems open to fluid transport. These isograds can be contrasted with fluid-producing isograds in closed systems. As an illustration of possible effects, the reactions quartz + clinozoisite + muscovite = anorthite + K-feldspar + H2O and phlogopite + quartz + calcite = tremolite + K-feldspar + H2O + CO2 at 4 kbar are analysed and equations for fluid production and transport are developed.

Notes: The In Ouzzal terrane (IOT) or In Ouzzal granulite unit (IOGU) is an elongated Palaeoproterozoic block within the Neoproterozoic Pan-African belt of north-west Africa. The granulites derive from Archaean protoliths that include a large volume of metasediments which were deposited on a 3.2-Ga gneissic basement. Near-peak granulite facies conditions between 2.17 and 2 Ga were estimated at P=10 kbar and T rising from 800 to 1000°C. Premetamorphic orthogneisses were intruded at 2.5 Ga, and followed by the emplacement of syn- to late-kinematic charnockites, syenites and carbonatites at around 2 Ga. Cooling of the granulites occurred till 1800 Ma. Shortly after its exhumation coeval with crustal extension and related alkaline magmatism in adjacent areas, the IOT was buried beneath late Palaeoproterozoic and Neoproterozoic cover sequences, and then behaved as a rigid block. Both margins are lithospheric faults, as evidenced by the occurrence of shear-zone-related mafic and felsic plutons. Pan-African tectonothermal events were negligible in the north, but granulites in the south were significantly reworked under lower greenschist facies conditions during the northern motion of the block with respect to both the western and the eastern Pan-African terranes. The Cambrian molasse, associated with widespread alkaline volcanism and subvolcanic granites, is horizontal in the north. The IOT, which was part of a larger continental mass including its counterpart in northern Mali, is interpreted as an exotic terrane which may have docked during Pan-African plate convergence and lateral collision. The unchanged pediplain since c. 1.7 Ga in the north suggests that the IOT is underlain by thick Palaeoproterozoic lithospheric mantle, whereas its southern part is probably allochthonous and overlies Pan-African structural units.

Notes: Al-Mg granulites, with cordierite, garnet, sapphirine, orthopyroxene, sillimanite, spinel, phlogopite, K-feldspar, plagioclase and variable quartz from Ihouhaouene (In Ouzzal, Algeria), display a range of decompression textures involving the breakdown of orthopyroxene and sillimanite, and of garnet. The succession of parageneses suggests that the P–T–t evolution corresponds to decompression with cooling from peak conditions of about 950°C and 10 kbar. This decompression path is obtained from the paragenetic analysis in the FMAS system. However, according to current KFMASH grids, this P–T–t evolution should take place outside the stability field of phlogopite+quartz; yet this assemblage is probably stable during most of the P-T evolution, notably during peak metamorphism. This discrepancy is interpreted as the effect of the high content of F in phlogopite which should shift its stability limit towards higher temperature. The consequences of this shift on the phase relationships in the KFeMASH system are investigated and it is concluded that a topological inversion could exist in the F-bearing system.

Notes: Quartz Al–Mg granulites exposed at In Hihaou, In Ouzzal (NW Hoggar), preserve an unusual high-grade mineral association stable at temperatures up to 1050°C, involving the parageneses orthopyroxene–sillimanite–garnet–quartz, sapphirine–quartz and spinel–quartz. The phase relationships within the FMAS system show that a continuum exists between the earlier prograde reaction textures and those of the later decompressive event. The following mineral reactions involving sillimanite are deduced: (1) Grt+Qtz→Opx+Sil, (2) Opx+Sil→Grt+Spr+Qtz, (3) Grt+Sil+Qtz→Crd, (4) Grt+Sil→Crd+Spr, (5) Grt+Sil+Spr→Crd+Spl, (6) Grt+Sil→Crd+Spl, (7) Grt+Crd+Sil→Spl+Qtz and (8) Grt+Sil→Spl+Qtz. Minerals in quartz Al–Mg granulites display compositional variations consistent with the observed reactions. The Mg/(Mg+Fe2+) range of the main minerals is as follows: cordierite (0.81–0.97), sapphirine (0.77–0.88), orthopyroxene (0.65–0.81), garnet (0.33–0.64) and spinel (0.23–0.56). The reaction textures and the evolution of the mineral assemblages in the quartz Al–Mg granulites indicate a clockwise P–T trajectory characterized by peak conditions of at least 10 kbar and 1050°C, followed by decompression from 10 to 6 kbar at a temperature of at least 900°C.

Notes: Many thermodynamic calculations relating to metamorphic rocks hinge on the thermodynamic parameters of garnet. Though some models are widely used, it is not clear whether their underlying premise is correct: that a single set of equations can be written for the activities of the end-members of garnet covering the whole compositional range. A voluminous body of data can be used to constrain the thermodynamics of garnet, namely Fe–Mg exchange experimental data involving garnet and another mineral, particularly clinopyroxene, orthopyroxene and olivine. However, examination of these data reveals inconsistencies, apparently stemming from differences between the thermodynamics of low-Ca and high-Ca garnets, with a boundary of about XgCa= 0.15. In the two regions, for the high P–T of the experimental data, the thermodynamics follow the regular model, with values for the interaction parameters in the low Ca region of about wgFeMg= 50R and waFe–wgMgCa=– 1300R, in which R is the gas constant, and in the high Ca region of about wgFeMg= 1100R and wgCaFe–wgMgCa=– 2200R. Using the subregular, rather than the regular, model does not remove the discrepancy. The cause of the discrepancy needs to be identified if reliable calculations on rocks are to be made.

Notes: In the Hlinsko region (Variscan Bohemian Massif, Czech Republic) a major extensional shear zone separates low-grade metasedimentary series (Hlinsko schists) and high-grade rocks of the Moldanubian terrane (Svratka Crystalline Unit). During late-Variscan extension, a tonalite intruded syntectonically into the normal ductile shear zone, and caused contact metamorphism of the overlying schists. Concurrent syntectonic sedimentation of a flysch series took place at the top of the hangingwall schists. In order to decipher the detailed petrological evolution of the Hlinsko unit situated in the hangingwall of this tectonic contact, a phase diagram approach and petrogenetic grids, calculated with the thermocalc computer program, were used.The crystallization/deformation relationships and the paragenetic analysis of the Hlinsko schists define a P–T path with an initial minor increase in pressure followed by cooling. Calculated pseudosections constrain this anticlockwise P-T evolution to the upper part of the andalusite field between 0.36 and 0.40 GPa for temperatures ranging from 570 to 530°C. A low aH2O is required to explain the presence of andalusite-biotite-bearing assemblages, and could be related to the presence of abundant graphite.In contrast, the footwall rocks of the Svratka Crystalline Unit record decompression from around 0.8 GPa at a relatively constant temperature, followed by cooling. Thus, the footwall and the hangingwall units display opposite, but convergent P–T histories. Decompression in the footwall rocks is related to a rapid exhumation. We propose that the inverse, anticlockwise P–T path recorded in the hangingwall pelites is related to the rapid, extension-controlled sedimentation of the overlying flysch series.

Notes: Seventy-five spatially orientated, serial thin sections cut from a single rock containing ‘millipede’ porphyroblast microstructure from the Robertson River Metamorphics, Australia, reveal the three-dimensional (3-D) geometry of oppositely concave microfolds (OCMs) that define the microstructure. Electronic animations showing progressive serial sections of the 3-D microstructure are made available via the World Wide Web. The OCM amplitudes decrease regularly from a maximum in near-central sections to a minimum in near-marginal sections, whereas the OCM interlimb angles increase regularly from a minimum in near-central sections to a maximum in near-marginal sections. These observations illustrate that the OCMs are noncylindrical surfaces with culminations located in near-central sections. Because the porphyroblast cores appear to have been present before significant development of the syn-OCM foliation, all of the OCMs were formed by heterogeneous extension around these cores. The overall geometry of the OCMs described here reflects the strain state, and cannot be used to constrain deformation paths.

Notes: The high-grade migmatitic core to the southern Brittany metamorphic belt has mineralogical and textural features that suggest high-temperature decompression. The chronology of this decompression and subsequent cooling history have been constrained with 40Ar/39 Ar ages determined for multigrain concentrates of hornblende and muscovite prepared from amphibolite and late-orogenic granite sheets within the migmatitic core, and from amphibolite of the structurally overlying unit. Three hornblende concentrates yield plateau isotope correlation ages of c. 303–298 Ma. Two muscovite concentrates record well-defined plateau ages of c. 306–305 Ma. These ages are geologically significant and date the last cooling through temperatures required for intracrystalline retention of radiogenic argon. The concordancy of the hornblende and muscovite ages suggest rapid post-metamorphic cooling. Extant geochronology and the new 40Ar/39Ar data suggest a minimum time-integrated average cooling rate between c. 725 °C and c. 125 °C of c. 14 ± 4°C Ma-1, although below 600 °C the data permit an infinitely fast rate of cooling. Mineral assemblages and reaction textures in diatexite migmatites suggest c. 4 kbar decompression at 800–750 °C. This must have pre-dated the rapid cooling.Emplacement of two-mica granites into the metamorphic belt occurred between 345 and 300 Ma. The youngest plutons were emplaced synkinematically along shallow-dipping normal faults interpreted to be reactivated Eo-Variscan thrusts. A penetrative, west-plunging stretching lineation developed in these granites suggests that extension was orogen-parallel. Extension was probably related to regional uplift and gravitational collapse of thermally weakened crust during constrictional (escape) tectonics in this narrow part of the Variscan orogen. This followed slab breakoff during the terminal stages of convergence between Gondwana and Laurasia; detachment may have been consequent upon a change in kinematics leading to dextral displacement within the orogen. Dextral ductile strike-slip displacement was concentrated in granites emplaced synkinematically along the South Armorican Shear Zone. Rapid cooling is interpreted to have resulted from tectonic unroofing with emplacement of granite along decollement surfaces. The high-grade migmatitic core of the southern Brittany metamorphic belt represents a type of metamorphic core complex formed during orogen-parallel extensional unroofing and regional-scale ductile flow.

Notes: The metasedimentary sequence of the Deep Freeze Range (northern Victoria Land, Antarctica) experienced high-T/low-F metamorphism during the Cambro-Ordovician Ross orogeny. The reaction Bt + Sil + Qtz = Grt + Crd + Kfs + melt was responsible for the formation of migmatites. Peak conditions were c. 700–750° C, c. 3.5–5 kbar and xH2Oc. 0.5).Distribution of fluid inclusions is controlled by host rock type: (1) CO2-H2O fluid inclusions occur only in graphite-free leucosomes; (2) CO2–CH4± H2O fluid inclusions are the most common type in leucosomes, and in graphite-bearing mesosomes and gneiss; and (3) CO2–N2–CH4 fluid inclusions are observed only in the gneiss, and subordinately in mesosomes.CO2–H2O mixtures (41% CO2, 58% H2O, 1% Nad mol.%) are interpreted as remnants of a synmig-matization fluid; their composition and density are compatible P–T–aH2O conditions of migmatization (c. 750° C, c. 4 kbar, xH2Oc. 0.5). CO2-H2O fluid in graphite-free leucosomes cannot originate via partial melting of graphite-bearing mesosomes in a closed system; this would have produced a mixed CO2–CH4 fluid in the leucosomes by a reaction such as Bt + Sil + Qtz + C ± H2O = Grt + Crd + Kfs + L + CO2+ CH4. We conclude that an externally derived oxidizing CO2-H2O fluid was present in the middle crust and initiated anatexis.High-density CO2-rich fluid with traces of CH4 characterizes the retrograde evolution of these rocks at high temperatures and support isobaric cooling (P–T anticlockwise path). In unmigmatized gneiss, mixed CO2–N2–CH4 fluid yields isochores compatible with peak metamorphic conditions (c. 700–750° C, c. 4–4.5 kbar); they may represent a peak metamorphic fluid that pre-dated the migmatization.

Notes: Some granulites from the Amessmessa area (south In Ouzzal unit, Hoggar) contain the peak assemblage gedrite+garnet+sillimanite+quartz that was used to estimate the P–T conditions of metamorphism. The rocks developed symplectites and corona textures by the breakdown of the primary paragenesis to orthopyroxene, cordierite and spinel. The successive parageneses formed in separate microdomains according to a clockwise P–T path. Geothermometry, geobarometry and phase diagram calculations indicate that the textures formed by decompression and cooling from 7–9 kbar and 850–900°C to 3.5–4.5 kbar and 700–800°C. This P–T evolution is consistent with low to medium aH2O, between 0.4 and 0.7, and is similar to the metamorphic conditions deduced in Al–Mg granulites from the north of In Ouzzal.

Notes: The In Ouzzal Al–Mg granulites are found within sedimentary units deposited after 2.7 Ga, the whole association being metamorphosed under extreme temperature conditions (c. 1000 °C) at 2 Ga. The Al–Mg granulites are interlayered with other metasediments, including metapelites, quartzites and magnetite-bearing quartzites, forsterite-spinel marbles, and a few meta-igneous rocks (mainly pyroxenites). They do not occur at a specific position in the sedimentary suite, and they do not reflect any particular structural control.The major and trace element compositions of Al–Mg granulites (especially the high Cr, Ni, Co contents) show that their peculiar ‘refractory’ chemistry is more compatible with premetamorphic sedimentary characteristics rather than with metasomatic, metamorphic or partial melting processes. Sedimentary admixtures of a common mature detrital component coming from the weathering of the local acidic igneous crustal protoliths (normal pelitic component) with an extremely immature component derived from reworking of basic/ultrabasic lithologies (Al–Mg–Cr–Co–Ni–rich chloritic component) is consistent with the geochemistry of such rocks.As in other instances, the quartz-garnet oxygen isotopic thermometer here records an apparent temperature close to the peak metamorphism (c. 1000 °C). Although the persistence of pre-existing δ18O variations on a small scale during the metamorphism does not support a major pervasive fluid flow during metamorphism, it does not rule out the presence of syn- to post-metamorphic CO2. The low δ18O (c.+ 5 to + 6‰) of the most typical Al–Mg granulites indicate that the ‘chloritic component’ in these rocks was derived from hydrothermally altered mafic/ultramafic protoliths rather than dominantly from palaeosols. It is suggested that the presence of such Al–Mg–Cr–Co–Ni–rich sediments is indirect evidence for the presence of greenstone belts in the local crust of the In Ouzzal at 2.6–2.7 Ga.

Notes: One-dimensional fluid advection-dispersion models predict differences in the patterns of mineralogical and oxygen isotope resetting during up- and down-temperature metamorphic fluid flow that may, in theory, be used to determine the fluid flow direction with respect to the palaeotemperature gradient. Under equilibrium conditions, down-temperature fluid flow is predicted to produce sharp reaction fronts that separate rocks with isobarically divariant mineral assemblages. In contrast, up-temperature fluid flow may produce extensive zones of isobarically univariant mineral assemblages without sharp reaction fronts. However, during contact metamorphism, mineral reaction rates are probably relatively slow compared with fluid velocities and distended reaction fronts may also form during down-temperature fluid flow. In addition, uncertainties in the timing of fluid flow with respect to the thermal peak of metamorphism and the increase in the variance of mineral assemblages due to solid solutions introduce uncertainties in determining fluid flow directions. Equilibrium down-temperature flow of magmatic fluids in contact aureoles is also predicted to produce sharp δ18O fronts, whereas up-temperature flow of fluids derived by metamorphic devolatilization may produce gradational δ18O vs. distance profiles. However, if fluids are channelled, significant kinematic dispersion occurs, or isotopic equilibrium is not maintained, the patterns of isotopic resetting may be difficult to interpret. The one-dimensional models provide a framework in which to study fluid-rock interaction; however, when some of the complexities inherent in fluid flow systems are taken into account, they may not uniquely distinguish between up- and down-temperature fluid flow. It is probably not possible to determine the fluid flow direction using any single criterion and a range of data is required.

Notes: The high-grade rocks (metapelite, quartzite, metagabbro) of the Hisøy-Torungen area represent the south-westernmost exposures of granulites in the Proterozoic Bamble sector, south Norway. The area is isoclinally folded and a metamorphic P–T–t path through four successive stages (M1-M4) is recognized.Petrological evidence for a prograde metamorphic event (M1) is obtained from relict staurolite + chlorite + albite, staurolite + hercynite + ilmenite, cordierite + sillimanite, fine-grained felsic material + quartz and hercynite + biotite ± sillimanite within metapelitic garnet. The phase relations are consistent with a pressure of 3.6 ± 0.5 kbar and temperatures up to 750–850°C. M1 is connected to the thermal effect of the gabbroic intrusions prior to the main (M2) Sveconorwegian granulite facies metamorphism. The main M2 granulite facies mineral assemblages (quartz+ plagioclase + K-feldspar + garnet + biotite ± sillimanite) are best preserved in the several-metre-wide Al-rich metapelites, which represent conditions of 5.9–9.1 kbar and 790–884°C. These P–T conditions are consistent with a temperature increase of 80–100°C relative to the adjacent amphibolite facies terranes. No accompanying pressure variations are recorded. Up to 1-mm-wide fine-grained felsic veinlets appear in several units and represent remnants of a former melt formed by the reaction: Bt + Sil + Qtz→Grt + lq. This dehydration reaction, together with the absence of large-scale migmatites in the area, suggests a very reduced water activity in the rocks and XH2O = 0.25 in the C–O–H fluid system was calculated for a metapelitic unit. A low but variable water activity can best explain the presence or absence of fine-grained felsic material representing a former melt in the different granulitic metapelites. The strongly peraluminous composition of the felsic veinlets is due to the reaction: Grt +former melt ± Sil→Crd + Bt ± Qtz + H2O, which has given poorly crystalline cordierite aggregates intergrown with well-crystalline biotite. The cordierite- and biotite-producing reaction constrains a steep first-stage retrograde (relative to M2) uplift path. Decimetre- to metre-wide, strongly banded metapelites (quartz + plagioclase + biotite + garnet ± sillimanite) inter-layered with quartzites are retrograded to (M3) amphibolite facies assemblages. A P–T estimate of 1.7–5.6 kbar, 516–581°C is obtained from geothermobarometry based on rim-rim analyses of garnet–biotite–plagioclase–sillimanite–quartz assemblages, and can be related to the isoclinal folding of the rocks. M4 greenschist facies conditions are most extensively developed in millimetre-wide chlorite-rich, calcite-bearing veins cutting the foliation.

Notes: Key insights into the timing of tectonometamorphic events in a complex high-grade metamorphic terrane can be obtained by combining results from SHRIMP II ion microprobe studies of individual monazite grains with SHRIMP II studies and scanning electron microscope (SEM)-based cathodoluminescence (CL) imaging of zircons. Results from the Reynolds Range region, Arunta Block, Northern Territory, Australia, show that the final episode of regional metamorphism to high-T and low-P granulite facies conditions is most likely to have occurred at c. 1580 Ma, not at 1785–1775 Ma, as previously accepted. The previous interpretation was based on zircon studies of structurally controlled granitoids, without SEM-based CL imaging. Monazites in a 1806± 6 Ma megacrystic granitoid preserve rare cores that are interpreted to be inherited magmatic monazite, but record no evidence of another high-T event prior to 1580 Ma. Most monazites from the region record only a single high-T metamorphic event at c. 1580 Ma. Zircon inheritance is very common. Zircons or narrow overgrowths of zircon dated at c. 1580 Ma have only been found in two types of rocks: rocks produced by metasomatic fluid flow at high temperatures (≤750°C), and rocks that have undergone local partial melting. Previous explanations that attributed these 1580 Ma zircon ages to widespread hydrothermal fluid fluxing associated with post-tectonic pegmatite emplacement at amphibolite facies conditions are not supported by the available evidence including oxygen isotope data.The observed high regional metamorphic temperatures require the involvement of advective heating. However, contrary to a previous tectonic model for the formation of this and other low-P, high-T metamorphic belts, the granites that are exposed at the present structural level do not appear to be the source of that heat, unless some of the granites were emplaced at c. 1580 Ma.

Notes: Granulite facies quartzites from the Ihouhaouene region, in the northern part of In Ouzzal, contain the assemblage corundum+quartz+magnetite together with hercynitic spinel+quartz+magnetite, sillimanite+quartz+magnetite and almandine-rich garnet+quartz+magnetite. Two types of corundum have been recognized: the first is primary and is found with quartz and magnetite only; the second type is found together with magnetite and chlorite rimming spinel as a fine-grained corona. The textures show that spinel-rich magnetite probably exsolved primary corundum, sillimanite, spinel and garnet during the cooling history. The secondary corundum formed later from the spinel already exsolved from magnetite. The secondary corundum is certainly metastable with respect to quartz. This may also apply for the primary corundum. However, given the high-temperature setting of this rock, it cannot be excluded that the stable contacts observed between primary corundum and quartz indicate equilibrium between the two phases. Taking into account the uncertainties in the thermodynamic data, the stability of this assemblage would imply that this part of In Ouzzal has recorded very high P–T conditions, above 1100°C at 12 kbar.

Notes: The In Ouzzal granulitic unit (IOGU) consists predominantly of felsic orthogneisses most of which correspond to granitoids emplaced during the Archaean, plus metasediments, including olivine-spinel marbles, of late Archaean age. All units were metamorphosed at granulite facies during the Eburnean (2 Ga). The stable isotope signature of the marbles (δ13C=–0.8 to –4.2‰/PDB; δ18O = 7.9 to 18.9‰/SMOW) does not record a massive streaming of C-bearing fluids during metamorphism. Most of the isotopic variation in the marbles is explained in terms of pregranulitic features. Metasomatic transformation of granulites into layered potassic syenitic rocks and emplacement of carbonate veins and breccias occurred during retrogressive granulite facies conditions. The chemistry of these rocks is comparable with that of fenites and carbonatites with high contents of (L)REEs, Th, U, F, C, Ba and Sr but, with respect to these elements, a relative depletion in Nb, Ta, Hf, Zr and Ti. The isotopic compositions of Nd (ɛNd(T)=–6.3 to –9.9), of Sr (87Sr/86Sr(T)= 0.7093–0.7104), and the O isotopic composition of metasomatic clinopyroxene (δ18O = 6.9 to 8‰), all indicate that the fluid had a strong crustal imprint. On the basis of the C isotope ratios (δ13C =–3.5 to –9.7‰), the fluid responsible for the crystallization of carbonates and metasomatic alteration is thought to be derived from the mantle, presumably through degassing of mantle-derived magmas at depth. Intense interaction with the crust during the upward flow of the fluid may explain its chemical and isotopic signatures. The zones of metasomatic alteration in the In Ouzzal granulites may be the deep-seated equivalents of the zones of channelled circulation of carbonated fluids described at shallower levels in the crust.

Notes: The In Ouzzal granulitic massif is composed mainly of various meta-igneous rocks which, in spite of Rb, U, Th, Cs and some K and Sr mobility, can be dated and generally classified according to their chemical composition as follows.Basic and ultrabasic granulites interlayered with the metasediments correspond to (1) ultrabasic cumulates from dislocated tholeiitic bodies, (2) ancient komatiitic to high-Mg tholeiitic basalts similar to the suites found in Archaean greenstone belts and (3) calcalkaline protoliths of high-K andesitic composition. No geochronological constraints are available apart from the depositional age of some associated sediments which is younger than 2.70 Ga detrital zircons, and the Nd model age of the andesitic granulites of c. 3.4 Ga.In spite of the high-grade metamorphism, the acidic magmatic precursors of the charnockites can be divided in three groups. (1) The most juvenile acid orthogneisses are trondhjemitic or tonalitic in composition, being similar to the TTG suites which are classically considered to be formed by partial melting of mantle-derived protoliths. The 3.3–3.2 Ga TDM indicates a possible age of separation from the mantle reservoir while the plutons may have been emplaced between 3.3 and 2.7 Ga (U–Pb zircon & Nd ages). (2) A group of alkaline granitic gneisses, similar in composition to rift-related-granites, were emplaced at 2650±10 Ma (U–Pb & Rb–Sr ages) in a thick continental crust. (3) Calcalkaline granodioritic and monzogranitic suites derived from the partial melting of continental precursors (3.5–3.3 Ga), in lower to middle levels of the continental crust. They were emplaced close to 2.5 Ga during crustal thickening.The very high-temperature metamorphism occurred at 2002±7 Ma from the age of synfoliation intrusions and was probably related to major overthrusting. Retrogressive metamorphism is dated at 1.95 Ga from garnet-Nd ages. In spite of the very high-temperature conditions, partial melting during granulite facies metamorphism may be restricted to scarce cordierite-bearing monzogranitic gneisses. The 2.0 Ga VHT metamorphism could be related to overthrusting, extensional or underplating processes.

Notes: The products of metamorphic fluid flow are preserved in zones within the marbles and metamorphosed semipelites of the Upper Calcsilicate Unit in the granulite portion of the Late Palaeoproterozoic Reynolds Range Group, northern Arunta Block, central Australia. The zones of retrogression, characterized by minerals such as wollastonite, grossular and clinohumite, local resetting of oxygen isotopic compositions and local major element metasomatism, were channelways for water-rich fluids derived from granulite facies metapelites. U–Th–Pb isotopic ages measured by the SHRIMP ion microprobe on zircon and monazite from a granulite facies semipelite, an early semiconcordant aluminous quartz-rich fluid-flow segregation and a late discordant quartz-rich segregation record some of the extended thermal history of the area. Zircon cores from the semipelite show its likely protolith to be an igneous rock 1812 ± 11 Ma old, itself derived from a source containing zircon as old as 2.2 Ga. Low-Th/U overgrowths on the zircon grew during granulite facies metamorphism at 1594 ± 6 Ma. Monazite cooled to its blocking temperature at 1576 ± 8 Ma. Zircon cores from the semiconcordant segregation are dominantly 〉2.3 Ga old, indicating that the source of the fluids was not the particular metamorphosed semipelite studied. Two generations of low-Th/U overgrowths on the zircon give indistinguishable ages for the older and younger of 1589 ± 8 and 1582 ± 8 Ma, respectively. The monazite age is the same, 1576 ± 12 Ma. Zircon from the late discordant segregation gave 1568 ± 4 Ma. Fluid flow occurred for at least 18 ± 3 (σ) Ma and ended 26 ± 3 (σ) Ma after the peak of metamorphism, suggesting a very slow cooling rate of ∼3°C Ma–1. The last regional high-grade metamorphism in the Reynolds Range occurred at ∼1.6 Ga, not ∼1.78 Ga as previously thought. The high-grade event at ∼1.78 Ga is a separate event that affected only the basement to the Reynolds Range Group.