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: Inclusion trails in garnet and albite porphyroblasts in the Fleur de Lys Supergroup preserve successive generations of microstructures, some of which correlate with equivalent microstructures in the matrix. Microstructure–porphyroblast relationships provide timing constraints on a succession of seven crenulation cleavages (S1–S7) and five stages of porphyroblast growth. Significant destruction and alteration of early fabrics has occurred during the microstructural development of the rock mass. Garnet porphyroblasts grew episodically through four growth stages (G1–G4) and preserve a succession of five fabrics (S1–S5) as inclusion trails. Garnet growth during each of the four growth phases did not occur on all pre-existing porphyroblasts, resulting in contrasting growth histories between individual garnet porphyroblasts from the same outcrop. Albite porphyroblasts grew during a single stage of growth and have overgrown microstructures continuous with the matrix. The garnet and albite porphyroblast inclusion trails record a succession of crenulation cleavages without any rotation of the porphyroblasts relative to other porphyroblasts in the population.Complex microstructural histories are best resolved by preparing multiple oriented thin sections from a large number of samples of different rock types within the area of study. The succession of matrix foliations must be understood, as it provides the most useful time-frame against which to measure the relative timing of phases of porphyroblast growth. Comparable microstructures must be identified in different porphyroblasts and in the rock matrix.

Notes: A second example of yoderite has been discovered in whiteschists from the Southern Chewore Hills of northern Zimbabwe. The mineral is pale green in colour and occurs in an equilibrium assemblage with talc+chlorite+kyanite+dravite+hematite. There is no quartz present. Recalculated microprobe analyses give a structural formula of Mg2Al5.7Fe0.3Si4O18(OH)2, similar to that obtained for the type locality at Mautia Hill, Tanzania, i.e. Mg2Al5.6Fe0.4Si4O18(OH)2. Textural relationships and relative proportions of minerals suggest that the yoderite was formed by reaction between talc, chlorite, kyanite and hematite. Experimental evidence suggests high-water-pressure metamorphic conditions at temperatures exceeding a reaction curve that extends between 13 kbar at 590 °C and 21 kbar at 650 °C. The yoderite-bearing whiteschist is associated with a 1.4 Ga dismembered ophiolite. It is proposed that this yoderite occurrence is associated with a relict subduction/suture zone.

Notes: Relict eclogites and associated high-pressure rocks are present in the Eastern Segment of the SW Swedish gneiss region (the tectonic counterpart of the Parautochthonous Belt of the Canadian Grenville). These rocks give evidence of Sveconorwegian eclogite facies metamorphism and subsequent pervasive reworking and deformation at granulite and amphibolite facies conditions. The best-preserved eclogite relics suggest a clockwise P–T –t history, beginning in the amphibolite facies, progressing through the eclogite facies, decompressing and partially reequilibrating through the high- and medium-pressure granulite facies, before cooling through the amphibolite facies. Textures demonstrate the former coexistence of the plagioclase-free assemblages garnet+clinopyroxene+quartz+rutile+ilmenite, garnet+clinopyroxene+ kyanite+rutile, and garnet+kyanite+quartz+rutile. The former existence of omphacite is evidenced by up to 45 vol.% plagioclase expelled as small grains within large clinopyroxene. Matrix plagioclase is secondary and occurs expelled from clinopyroxene or in fine-grained, granulite facies reaction domains formed during resorption of garnet and kyanite. Garnet shows preserved prograde growth zoning with rimward increasing pyrope content, decreasing spessartine content and decreasing Fe/(Fe+Mg) ratio, but is partly resorbed and reequilibrated at the rims. P–T estimates from microdomains with clinopyroxene+plagioclase+quartz+garnet indicate pressures of 9.5–12 kbar and temperatures of 705–795 °C for a stage of the granulite facies decompression. The preservation of the prograde zoning suggests that the rocks did not reside at these high temperatures for more than a few million years, and chemical disequilibrium and ‘frozen’ reaction textures indicate heterogeneous reaction progress and overstepping of reactions during the decompression through the granulite facies. Together these features suggest a rapid tectonic exhumation. The eclogite relics occur within a high-grade deformation zone with WNW–ESE stretching and associated oblique normal-sense, top-to-the-east (sensu lato) displacement, suggesting that extension was a main cause for the decompression and exhumation. Probable tectonic scenarios for this deformation are Sveconorwegian late-orogenic gravitational collapse or overall WNW–ESE extension.

Notes: The Strona-Ceneri Zone (Southern Alps) contains folds with moderately to steeply inclined axial planes and fold axes, and amplitudes of up to several kilometres (so-called ‘Schlingen’). These amphibolite facies folds deform the main schistosity of Late Ordovician metagranitoids and are discordantly overlain by unmetamorphic Permian sedimentary rocks. Mutually cross-cutting relationships between these folds and garnet-bearing leucotonalitic dykes indicate that these dykes were emplaced during folding. Sm–Nd systematics and the strongly peraluminous composition of these dykes point to an anatectic origin. Pb step leaching of magmatic garnet from a leucotonalitic dyke yielded a 321.3±2.3 Ma intrusive age. Rb–Sr ages on muscovites from leucotonalitic dykes range from 307 to 298 Ma, interpreted as cooling ages during retrograde amphibolite facies metamorphism. Conventional U–Pb data of zircons from an older granodioritic dyke that pre-dates the Schlingen folds yielded discordant U–Pb ages ranging from 371 to 294 Ma. These ages reflect a more complicated multi-episodic growth history which is consistent with the observed polyphase structural overprint of this dyke. Schlingen folding was accompanied by prograde amphibolite facies metamorphism, during the thermal peak of which the leucotonalitic dyke material was generated by partial melting in a deeper source region from where these S-type magmas intruded the presently exposed level. Because partial melting may occur in a relatively late stage of a clockwise P–T–t path, or even during decompression on the retrograde path, we do not exclude the possibility that Schlingen folding had already started in Early Carboniferous time. Schlingen folds also occur in Penninic and Austroalpine basement units with a very similar pre-Alpine history, indicating that Variscan folding affected large segments of the future Alpine realm.

Notes: Timing constraints on shear zones can provide an insight into the kinematic and exhumation evolution of metamorphic belts. In the Musgrave Block, central Australia, granulite facies gneisses have been affected, to varying degrees, by mylonitic deformation, some of which attained eclogite facies. The Davenport Shear Zone is a dominant strike-slip system that formed at eclogite facies conditions (T ≈650 °C and P≈12.0 kbar). Sm–Nd mineral isochrons obtained from equilibrated high-pressure assemblages, as well as 40Ar–39Ar data, show that the eclogite and greenschist facies high-strain overprints were coeval, at c. 550 Ma. Mylonitic processes do not appear to have reset the U–Pb system in zircon, but may have partially disturbed it. The thermal gradient in the Musgrave Block crust at c. 550 Ma was c. 16 °C km−1 and at c. 535 Ma was c. 18 °C km−1, based on P–T  estimates of eclogite and greenschist facies shear zones, respectively. These estimates are similar to present-day geothermal gradients in many stable continental shield areas, suggesting that the region did not undergo a significant transient perturbation of the geotherm. Therefore, in the Musgrave Block, cooling subsequent to eclogite facies metamorphism appears to have been controlled by exhumation, rather than by the removal of a heat source. Estimated exhumation rates in the range 0.2 to ≥1.5 mm year−1 are comparable with other orogenic belts, rather than cratonic areas elsewhere.

Notes: The Susunai Complex of southeast Sakhalin represents a subduction-related accretionary complex of pelitic and basic rocks. Two stages of metamorphism are recognized: (1) a local, low-P/T event characterized by Si-poor calcic amphiboles; (2) a regional, high-P/T event characterized by pumpellyite, actinolite, epidote, sodic amphibole, sodic pyroxene, stilpnomelane and aragonite. The major mineral assemblages of the high-P/T Susunai metabasites contain pumpellyite + epidote + actinolite + chlorite, epidote + actinolite + chlorite, epidote + Na-amphibole + Na-pyroxene + chlorite-(-haematite. The Na-amphibole is commonly magnesioriebeckite. The Na-pyroxene is jadeite-poor aegirine to aegirine-augite. Application of empirically and experimentally based thermobarometers suggests peak conditions of T= 250–300C, P= 4.7–6 kbar. Textural relationships in Susunai metabasite samples and a petrogenetic grid calculated for the Fe3+-rich basaltic system suggest that pressure and temperature increased during prograde metamorphism.