ALBERT

Notes: Abstract Spinel-quartz-cordierite and spinel-quartz are found as relic prograde assemblages in Fe-rich granulites from the Araku area, Eastern Ghats belt, India. Subsequent reactions produced orthopyroxene + sillimanite in the former association and garnet + sillimanite in the latter. The first reaction is univariant in the FMAS system, but is trivariant in the present case because of the presence of Zn and Fe3+ in spinel. The second reaction also has high variance because of Zn and Fe3+, but also because of the presence of Ca in garnet. Thermobarometry shows that the metamorphic conditions were approximately 950° C and 8.5 kbar and the fo2 was near the NNO buffer. In Fe-rich bulk compositions and low-P-high-T conditions of metamorphism, two of the univariant reactions around the invariant point [Sa], namely (Sa, Hy) and (Sa, Cd), change topology due to reverse partitioning of Fe-Mg between coexisting garnet and spinel. An alternative partial petrogenetic grid in the system FMAS is constructed for such conditions and is applied satisfactorily to several sapphirine-free spinel granulites. It is shown that bulk composition (XFe and Zn) exerts greater control on the stability of spinel + quartz than fo2. The effect of the presence of Zn and Fe3+ in spinel on the proposed grid is evaluated. Reaction textures in the Araku spinel granulites can be explained from the petrogenetic grid as due to near-isobaric cooling.

Notes: The impure marbles of the internal Sesia-Lanzo Zone underwent a multi-stage metamorphic evolution of Alpine age and retain early-Alpine eclogitic assemblages, partially recrystallized under blueschist to greenschist facies conditions. These high-P assemblages consist of carbonates, phengite, quartz, omphacite, grossular-rich (locally spessartinic) garnet, zoisite and Al-rich titanite. Retrogressive stages are characterized by the growth of glaucophane, paragonite, phlogopite, tremolite and albite. Halogen-rich biotite and amphibole are also present. P-T estimates of the early-Alpine metamophism have been calculated from these unique high-P assemblages, in order to test the applicability of some calibrations to impure carbonate systems. In particular, some Gt-Cpx calibrations and the phengite geobarometer give results (T= 575 ± 45° C at 15 kbar for the eclogitic climax and T≤ 500° C at PH2O ≤ 9 kbar for early-Alpine retrogressive stages) which are within the range obtained from the surrounding lithologies.Phase relationships in P-T-XCO2 space indicate that mineral assemblages in the impure marbles coexisted with H2O-rich fluids (XCO2 〈0.03) during their entire Alpine evolution.

Notes: Analysis of the precision of the illite ‘crystallinity’technique shows that machine errors are 〈5%, while intra- and inter-sample errors are variable but are up to 12% and 14%, respectively (1σ). Consideration of this error analysis shows that the isocryst approach, which involves close contouring (e.g. 0.03 Δ2°) of illite ‘crystallinity’data, has a very low degree of confidence (〈0.5) and thus is not regarded as statistically valid. If contouring is to be undertaken with a high degree of confidence (〉0.8) it is necessary that contours should be at intervals of 0.1 ΔΘ2°, which is equivalent to subdivision of the anchizone into upper and lower units. Where previous interpretations have relied upon an isocryst method of contouring at less than 0.1 ΔΘ2° the conclusions must be regarded as unsubstantiated.Centrifuge separation of clay fractions (based on a Stokes’law application) gives separations in which a significant, but variable, percentage of grains have long axes greater than the size calculated. For the typical 〈2-μm fraction utilized, some 20% of grains lie in the 2–4-μm range, although the proportion is not believed to have a significant effect upon ‘crystallinity’values. The formula is applicable for grain-sizes down to 0.5 μm. Illite ‘crystallinity’values on samples prepared by an ultrasonic disaggregation method show a small increase on those prepared by ball mill crushing. The differences are minimal at the epi/anchizone level but increase to some 10% at the anchizone/diagenetic level. The effect on grade determinations is again thought to be minimal and indicates that concern over unsuitability of the ultrasonic disaggregation method is unfounded.

Notes: The amphibolite facies Puolankajärvi Formation (PjF) occupies the western margin of the Early Proterozoic Kainuu Schist Belt (KSB) of northern Finland. The lower and middle parts of the PjF consist of turbiditic psammites and pelites and tempestitic semipelites. This report concentrates on the pelitic lithologies which include quartz–two-mica–plagioclase schists with variable amounts of garnet, staurolite, andalusite and biotite porphyroblasts as well as sillimanite and cordierite segregations.The KSB forms a major north–south-trending synclinorium between two Archaean blocks. It contains both autochthonous and allochthonous units and is cut by faults and shear zones. The PjF lies on the western side of the KSB and is probably allochthonous. The formation has undergone six major deformation phases (D1, D2, D3a, D3b, D4 and D5). During D3a-D5 the maximum principal stress (σ1) changed in a clockwise direction from south-west to north-east. Between D2 and D3 the intermediate principal stress (σ2) changed from horizontal to vertical and the interval between D2 and D3 marks a transition from thrust to strike-slip tectonics.Relict structures in the porphyroblasts indicate the following mineral growth–deformation evolution in the PjF. (1) Throughout the PjF there was a successive crystallization of garnet (syn-D1), poryphyroblastic biotite (inter-D3/4) and staurolite (inter-D3/4) during the pre-D4 stage. (2) A syn-D4-inter-D4/5 crystallization of kyanite, sillimanite (fibrolite), porphyroblastic tourmaline, magnetite, rutile, cordierite and muscovite–biotite–plagioclase pseudomorphs after staurolite was most localized at and near D4 shear zones. (3) A syn- to post-D5 generation of andalusite, ilmenohematite and sheet silicates after staurolite and after cordierite occurred near D5 faults.The evolution outlined here permits the relative dating of the PjF parageneses, which is used in the second part of the study (Tuisku & Laajoki, 1990), and, together with the knowledge of the pressure–temperature conditions during various growth events, makes it possible to compile pressure–temperature–deformation paths for the PjF.

Notes: Petrographical and mineral chemical data are given for the eclogites which occur in the garnet-kyanite micaschists of the Penninic Dora-Maira Massif between Brossasco, Isasca and Martiniana (Italian Western Alps) and for a sodic whiteschist associated with the pyrope-coesite whiteschists of Martiniana. The Brossasco-Isasca (BI) eclogites are fine grained, foliated and often mica-rich rocks with a strong preferred orientation of omphacite crystals and white micas. Porphyroblasts of hornblende are common in some varieties, whilst zoisite and kyanite occur occasionally in pale green varieties associated with leucocratic layers with quartz, jadeite and garnet. These features differentiate the BI eclogites from the eclogites that occur in other continental units of the Western Alps, which all belong to type C. Garnet, sodic pyroxene and glaucophane are the major minerals in the sodic whiteschist.Sodic pyroxene in the eclogites is an omphacite often close to Jd50Di50, with very little acmite and virtually no AlIV, and impure jadeite in the leucocratic layers and in the sodic whiteschist. Garnet is almandine with 20–30 mol. % for each of the pyrope and grossular components in the eclogites and a pyrope-rich variety in the sodic whiteschist. White mica is a variably substituted phengite, and paragonite apparently only occurs as a replacement product of kyanite. Amphibole is hornblende in the eclogites, but the most magnesian glaucophane yet described in the sodic whiteschist. Quartz pseudomorphs of coesite were found occasionally in a few pyroxenes and garnets.The P-T conditions during the VHP event are constrained in the eclogites by reactions which define a field ranging from 27–28 kbar to 35 kbar and from 680 to 750° C. These temperatures are consistent with the results of garnet-pyroxene and garnet-phengite geothermometry which suggest that the eclogites may have equilibrated at around 700° C. In the sodic whiteschist pressures ranging from 29 to 35 kbar can be deduced from the stability of the jadeite-pyrope garnet-glaucophane compatibility. As in the eclogites water activity must have been low. Such conditions are close to the P-T values estimated for the early Alpine recrystallization of the pyrope-coesite rock and, like petrographical and mineralogical features, set aside the BI eclogites from the other eclogites of the Western Alps, instead indicating a close similarity to some of the eclogite bodies occurring in the Adula nappe of the Central Alps. An important corollary is that glaucophane stability, at least in Na- and Mg-rich compositions and under very high pressures, may extend up to 700° C, in agreement with the HT stability limit suggested by experimental studies.

Notes: The retrograde P-T trajectory of the eclogitic Fe-Ti-gabbros from the Ligurian Alps is constrained by the appearance of mineral parageneses post-dating the Na-clinopyroxene + garnet eclogitic assemblage and indicating the following sequence of metamorphic events: (1) amphibolitic stage— edenite/katophorite + plagioclase (An33–43) + oxides in symplectitic aggregates; (2) glaucophanic stage— a porphyroblastic glaucophanic amphibole has overgrown the symplectite, winchite also occurs as thin rims around glaucophane and both amphiboles are, sometimes, armoured by atoll garnets; (3) albite-amphibolite stage—barroisite/katophorite + albite + epidote + oxides ± chlorite overprint the glaucophanic stage minerals; (4) greenschist stage—represented by actinolite + albite + epidote + oxide paragenesis.The metamorphic evolution is complex and the decompression path, on a P–T diagram, is significantly different from those defined in the literature for the Voltri eclogites. The main features inferred from the P–T path are the following: (1) the pressure climax does not match the thermal climax, the maximum temperature conditions are in fact achieved during the early stage of uplift; (2) a decrease in temperature, suggested by the appearance of glaucophane after the amphibolitic symplectite; (3) successive uplift, probably accompanied by an increase in temperature. The complexity of the P-T path drawn for the Voltri eclogites can be explained with a mechanism of successive underthrusts propagating from the innermost to the outermost sector of the Ligurian Alps.

Notes: Within the Çokkul synform, Caledonian metamorphic rocks of the Middle Köli Nappe Complex (MKNC) are in low-angle fault contact with the basement mylonites derived from the Precambrian Tysfjord granite-gneiss. In the synform, the MKNC is composed of four fault-bounded nappes each of which has a distinct tectonic stratigraphy composed of amphibolite-facies metamorphosed pelitic and psammitic schists with minor lensoidal bodies of mafic and ultramafic rocks.Pelitic rocks from the three structurally lowest nappes contain the low-variance AFM mineral assemblages gar + bio + staur and staur + ky + bio with mu + qtz + ilm, whereas staur and ky are absent from the highest nappe, the Kallakvare nappe. AFM mineral assemblages in the three lowest nappes indicate peak metamorphic temperatures of 610–660°C and peak pressures in excess of 600 MPa. Mineral assemblages from the Kallakvare nappe are not as diagnostic of metamorphic grade. However, rocks from that nappe contain coexisting plagioclases from both sides of the peristerite gap, suggesting lower-grade peak P–T conditions than those of the structurally lower nappes. In addition, biotite from the lower nappes is more Ti-rich than biotite from the Kallakvare nappe. However, gar–bio–mu–plag and gar–bio–ky–plag–qtz thermobarometry suggests that all four nappes equilibrated at approximately 525 ± 25°C and 700 ± 100 MPa.Gibbs method thermodynamic modelling of garnet zoning profiles suggests that the lower three nappes followed clockwise P–T paths that involved heating and compression to a metamorphic peak of approximately 575–625°C, 800 MPa followed by cooling and decompression to 525°C, 700 MPa. P–T paths calculated for the Kallakvare nappe show decompression and minor heating to a peak T of 500–525°C. In the lower nappes, staur and ky grew during the heating phase not seen by the highest nappe. The outer parts of the paths from all four nappes are approximately parallel, possibly recording the emplacement of the Kallakvare nappe onto the already stacked lower three nappes at some time following the metamorphic peak. These P–T paths suggest that the sole fault of the Kallakvare nappe is a normal fault. Garnet zonation thus appears to record a previously unrecognized phase of uplift and tectonic thinning of the MKNC. This event appears to be restricted to the MKNC and to have occurred prior to the emplacement of the MKNC onto the Tysfjord granite-gneiss basement of Baltoscandia under greenschist-facies conditions. It may have been responsible for the uplift and cooling of the MKNC from 25–30 km amphibolite-facies conditions prior to its emplacement onto Baltoscandia under 15–20 km greenschist-facies conditions.The deformation zone associated with this normal fault is relatively narrow, generally less than 1 m thick. If this is typical of other detachment faults in the metamorphic infrastructure of the Scandinavian Caledonides, they may be relatively common, but not often recognized due to the detailed study needed to document them.

Notes: Numerical models of the progressive evolution of pelitic schists in the NCMnKFMASH system with the assemblage garnet + biotite + chlorite ± staurolite + plagioclase + muscovite + quartz + H2O are presented with the goal of predicting compositional changes in garnet and plagioclase along different P-T paths. The numerical models support several conclusions that should prove useful for interpreting the P-T paths of natural parageneses: (i) Garnet may grow along P-T vectors ranging from heating with decompression to cooling with compression. P-T paths deduced from garnet zoning that are inconsistent with these growth vectors are self-contradictory. (ii) There is a systematic relation between garnet and plagioclase composition and growth such that for most P-T paths, garnet growth requires plagioclase consumption. Furthermore, mass balance in a closed system requires that as plagioclase is consumed the remaining plagioclase becomes increasingly albitic. Inclusions of plagioclase in the core of garnet should be more anorthitic than those near the rim and zoned matrix plagioclase should have rims that are more albitic than the cores. Complex plagioclase textures may arise from the local variability of growth and precipitation kinetics. (iii) A decrease of Fe/(Fe + Mg) in a garnet zoning profile is a reliable indicator of increasing temperature for the assemblage modelled. However, there is no single reliable ΔP monitor and inferences about ΔP can only be made by considering plagioclase and garnet together. (iv) Consumption of garnet during the production of staurolite removes material from the outer shell of a garnet and may make recovery of peak metamorphic compositions and P-T conditions impossible. Low ‘peak’temperatures typically recorded by staurolite-bearing assemblages may reflect this phenomenon. (v) Diffusional homogenization of garnet affects the computed P-T path and results in a clockwise rotation of the computed P-T vector relative to the true P-T path.

Notes: Abstract Geological relationships and geochronological data suggest that in Miocene time the metamorphic core of the central Himalayan orogen was a wedge-shaped body bounded below by the N-dipping Main Central thrust system and above the N-dipping South Tibetan detachment system. We infer that synchronous movement on these fault systems expelled the metamorphic core southward toward the Indian foreland, thereby moderating the extreme topographic gradient at the southern margin of the Tibetan Plateau. Reaction textures, thermobarometric data and thermodynamic modelling of pelitic schists and gneisses from the Nyalam transect in southern Tibet (28°N, 86°E) imply that gravitational collapse of the orogen produced a complex thermal structure in the metamorphic core. Amphibolite facies metamorphism and anatexis at temperatures of 950 K and depths of at least 30 km accompanied the early stages of displacement on the Main Central thrust system. Our findings suggest that the late metamorphic history of these rocks was characterized by high-T decompression associated with roughly 15 km of unroofing by movement on the South Tibetan detachment system. In the middle of the metamorphic core, roughly 7–8 km below the basal detachment of the South Tibetan system, the decompression was essentially isothermal. Near the base of the metamorphic core, roughly 4–6 km above the Main Central thrust, the decompression was accompanied by about 150 K of cooling. We attribute the disparity between the P–T paths of these two structural levels to cooling of the lower part of the metamorphic core as a consequence of continued (and probably accelerated) underthrusting of cooler rocks in the footwall of the Main Central thrust at the same time as movement on the South Tibetan detachment system.