P-T path determinations in the Tormes Gneissic Dome, NW Iberian Massif, Spain

The Tormes Gneissic Dome (TGD, NW sector of the Iberian Massif, Spain) is a high-grade metamorphic complex affected by a major episode of extensional deformation (D2). The syn-D2 P–T  path of the Lower Unit of the TGD was deduced from the analysis of reaction textures related to superimposed fabrics developed during exhumation, analysis of mineral zoning and thermobarometric calculations. It comprises an initial phase of decompression, determined using the tweequ thermobarometric technique, from 6.4–8.1 kbar at 735–750 °C (upper structural levels) and 7.2 kbar at 770 °C (lower structural levels) to 3.3–3.9 kbar and 645–680 °C. This evolution is consistent with the observed sequence of melting reactions and the generation of garnet- and cordierite-bearing anatectic granitoids. The later part of the syn-D2 P–T  path consisted of almost isobaric cooling associated with the thermal re-equilibration of the unit in the new structural position. This segment of the P–T  path is recorded by assemblages with And +Bt+Ms and Ms+ Chl +Ab related to the later mylonitic S2 fabrics, which indicate retrogression to low-amphibolite and greenschist facies conditions.     

Transitional blueschist–epidote amphibolite facies metamorphism in the Frankenberg massif, Germany, and geotectonic implications

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.     

Equilibration and reaction in Archaean quartz-sapphirine granulite xenoliths from the Lace kimberlite pipe, South Africa

Ultrahigh-temperature quartz-sapphirine granulite xenoliths in the post-Karoo Lace kimberlite, South Africa, comprise mainly quartz, sapphirine, garnet and sillimanite, with rarer orthopyroxene, antiperthite, corundum and zinc-bearing spinel; constant accessories are rutile, graphite and sulphides. Comparison with assemblages in the experimentally determined FMAS and KFMASH grids indicates initial equilibration at >1040 °C and 9–11  kbar. Corona assemblages involving garnet, sillimanite and minor cordierite developed on a near-isobaric cooling P–T  path as both temperature and, to a lesser extent, pressures decreased. Garnet-orthopyroxene Fe-Mg exchange thermometers record temperatures of only 830–916 °C. These estimates do not indicate the peak metamorphic conditions but instead reflect the importance of post-peak Fe-Mg exchange during cooling. Correction of mineral Fe-Mg compositions for this exhange using a convergence approach of Fitzsimons & Harley (1994 ) leads to retrieved P–T  estimates from garnet-orthopyroxene thermobarometry ( c . 1000 °C and 10.5±0.7  kbar) that are consistent with the petrogenetic grid constraints. U-Pb dating of a single zircon grain gives an age of 2590±83  Ma, interpreted as the age of the metamorphic event. Protolith major and trace element chemistries of the xenoliths differ from sapphirine-quartzites typical of the Napier Complex (Antarctica) but are comparable to less siliceous, high Cr and Ni, sapphirine granulites reported from several ultrahigh temperature granulite terranes.     

Hot contacts of garnet peridotites in middle/upper crustal levels: new constraints on the nature of the late Variscan high-T/low-P event in the Moldanubian (Central Vosges/NE France)

P–T  paths based on parageneses in the immediate vicinity of former high-temperature contact zones between mantle peridotites and granulitic country rocks of the Central Vosges (NE France) were derived by applying several conventional thermometers and thermobarometric calculations with an internally consistent dataset. The results indicate that former garnet peridotites and garnet–spinel peridotites were welded together with crustal rocks at depths corresponding to 1–1.2 GPa. The temperature of the crustal rocks was about 650–700 °C at this stage, whereas values of 1100 °C (garnet peridotites) and 800–900 °C (garnet–spinel peridotites) were calculated for the ultramafic rocks. After emplacement of the mantle rocks, exhumation of the lower crust took place to a depth corresponding to 0.2–0.3 GPa. The temperatures of the incorporated peridotite slices were still high (900–1000 °C) at this stage. This is indicated by the presence of high- T  /low- P parageneses ( c . 800 °C, 0.2–0.3 GPa) in a small (1–10 m) contact aureole around a former garnet peridotite. Crustal rocks distant to the peridotites equilibrated in the same pressure range at lower temperature (650–700 °C). High cooling rates (10²–10³ °C Ma⁻¹) were calculated for a garnet–biotite rock inclusion in the peridotites and for the crustal rocks at the contact by applying garnet–biotite diffusion modelling. Minimum rates of 0.75–7.5 cm a⁻¹ are required for vertical ascent of rock units (30 km vertical distance) derived from the crust–mantle boundary, resulting in a late Variscan (340 Ma) high- T  /low- P event.     

Granulite facies metamorphism in the Mallee Bore area, northern Harts Range: implications for the thermal evolution of the eastern Arunta Inlier, central Australia

The Mallee Bore area in the northern Harts Range of central Australia underwent high-temperature, medium- to high-pressure granulite facies metamorphism. Individual geothermometers and geobarometers and average P–T  calculations using the program Thermocalc suggest that peak metamorphic conditions were 705–810 °C and 8–12 kbar. Partial melting of both metasedimentary and meta-igneous rocks, forming garnet-bearing restites, occurred under peak metamorphic conditions. Comparison with partial melting experiments suggests that vapour-absent melting in metabasic and metapelitic rocks with compositions close to those of rocks in the Mallee Bore area occurs at 800–875 °C and >9–10 kbar. The lower temperatures obtained from geothermometry imply that mineral compositions were reset during cooling. Following the metamorphic peak, the rocks underwent local mylonitization at 680–730 °C and 5.8–7.7 kbar. After mylonitization ceased, garnet retrogressed locally to biotite, which was probably caused by fluids exsolving from crystallizing melts. These three events are interpreted as different stages of a single, continuous, clockwise P–T  path. The metamorphism at Mallee Bore probably occurred during the 1745–1730 Ma Late Strangways Orogeny, and the area escaped significant crustal reworking during the Anmatjira and Alice Springs events that locally reached amphibolite facies conditions elsewhere in the Harts Ranges.     

The assemblage diaspore+quartz in metamorphic rocks: a petrological, experimental and thermodynamic study

The upper pressure limit of pyrophyllite is given by the equilibria (i) pyrophyllite=diaspore+quartz and (ii) pyrophyllite=diaspore+coesite. High- P experimental investigations carried out to locate equilibrium (i) yield brackets between 497 °C/24.8  kbar and 535 °C/25.1  kbar, and between 500 °C/23  kbar and 540 °C/23  kbar. Equilibrium (ii) was bracketed at 550 °C between 26.0 and 28.3  kbar. In the experimental P–T  range, equilibria (i) and (ii) are metastable with respect to kyanite. A stable P–T  grid is calculated using thermodynamic data derived under consideration of the present experimental results. According to these data, the lower pressure limit of the assemblage diaspore+quartz according to equilibrium (i) range from about 12  kbar/300 °C to 20  kbar/430 °C (in the presence of pure water). The upper stability of diaspore+quartz is limited by the reaction diaspore+quartz=kyanite+H₂O at about 450 °C (nearly independent of pressure) and, to higher pressure, by the quartz=coesite transition. Equilibrium (ii) is metastable over the whole P–T  range.
Natural occurrences600.S of the diaspore–quartz assemblage in metamorphic rocks in Sulawesi, New Caledonia, Amorgos and the Vanoise are characterized by minerals indicative of high- P such as ferro-magnesiocarpholite, glaucophane, sodic pyroxene and lawsonite. The metamorphic P–T  conditions of these rocks are estimated to be in the range 300–400 °C, >8  kbar. These data are compatible with the derived P–T  stability field of the diaspore+quartz assemblage. We conclude that, in metamorphic rocks, diaspore+quartz is, as ferrocarpholite, an indicator for unusual low- T  /very high- P settings.     

Low-P / high-T metamorphism in the Okiep Copper District, western Namaqualand, South Africa

The Okiep Copper District, part of the 1.2–1.0 Ga high-grade terrane in western Namaqualand, is composed of a mid-Proterozoic supracrustal sequence and several pre- to post-orogenic intrusive suites affected by two high-grade events (M2a/M2b, M3) of Kibaran and one low-grade event (M4) of Pan-African age. Peak assemblages in quartz-bearing pelites are characterized either by garnet+cordierite coexisting with sillimanite/biotite, or by biotite+sillimanite±garnet; a difference controlled by bulk composition and variation in water activities (0.1–0.7) during dehydration melting. Maximum P–T conditions were reached during M2a coevally with the major deformational event (D2a) and are estimated at 750–820  °C and 5–6  kbar. A counterclockwise P–T  path is indicated by regionally occurring pseudomorphs of sillimanite after andalusite and by prograde reaction textures preserved as relics in M2a porphyroblasts. Two stages of retrograde metamorphism are distinguished: M2a garnet+cordierite-bearing assemblages were retrogressed to biotite+sillimanite+quartz (M2b) along discontinuous foliation planes and shear zones (D2b). Retrograde M3 corona assemblages formed at similar P–T  conditions (580–660  °C and 5.8±0.5  kbar) to the M2b assemblages but M3 crystallization postdates penetrative D2 deformation, intrusion of 1.06 Ga granitoids and formation of associated W–Mo deposits. It is concluded that: (a) Kibaran high-grade metamorphism in the Okiep Copper District is thermally punctuated and (b) reaction textures documenting apparent isobaric cooling of this low- P high- T  terrane must be interpreted with caution.     

Pressure–temperature conditions and retrograde paths of eclogites, garnet–glaucophane rocks and schists from South Sulawesi, Indonesia

High-pressure metamorphic rocks exposed in the Bantimala area, c . 40  km north-east of Ujung Pandang, were formed as a Cretaceous subduction complex with fault-bounded slices of melange, chert, basalt, turbidite, shallow-marine sedimentary rocks and ultrabasic rocks. Eclogites, garnet–glaucophane rocks and schists of the Bantimala complex have estimated peak temperatures of T  =580–630 °C at 18  kbar and T  =590–640 °C at 24  kbar, using the garnet–clinopyroxene geothermometer. The garnet–omphacite–phengite equilibrium is used to estimate pressures. The distribution coefficient K _D1=[( X _pyr)³( X _grs)⁶/( X _di)⁶]/[(Al/Mg)_M2,wm (Al/Si)_T2,wm]³ among omphacite, garnet and phengite is a good index for metamorphic pressures. The K _D1values of the Bantimala eclogites were compared with those of eclogites with reliable P–T  estimates. This comparison suggests that peak pressures of the Bantimala eclogites were P =18–24  kbar at T  =580–640 °C. These results are consistent with the P–T  range calculated using garnet–rutile–epidote–quartz and lawsonite–omphacite–glaucophane–epidote equilibria.     

Metamorphic field gradients in the Central Alps

Metamorphic field gradients were determined across the entire amphibolite grade Central Alps ( c . 50×100 km). P – T  were calculated from 116 samples acquired from our own field work, from samples provided to us by others, and from rocks with mineral compositions described in the literature. Only fluid-conserved equilibria were used to determine P – T  . The use of an internally consistent thermodynamic database and mineral solid solution models makes the results robust and reduces relative errors. The results are presented in contour maps. Temperature increases from 500 to 550 °C along the limit of amphibolite grade metamorphism in the north and west, to c . 675 °C toward the south at the Insubric line near the town of Bellinzona. Maximum recorded pressures of c . 7 kbar are in a central region c . 20 km north of the Insubric line, and decrease both to the north (5.5 kbar) and south (4.5 kbar). The P–T  results indicate that there is a relatively large area that reached conditions in the sillimanite stability field but developed neither sillimanite nor fibrolite; this is interpreted as a result of kinetic constraints on nucleation and growth because of the small amounts of thermal overstep (<40 °C) of the kyanite-sillimanite phase boundary. Comparison of P–T  conditions with carbonate isograds in the region indicate that fluids present during metamorphism were not dominated by a homogeneous external source. Examination of the two-dimensional distribution of pressure and temperature in the context of thermal and tectonic models indicates that two thermal pulses affected the Central Alps during the Tertiary. In the second, heat affected only the southern parts of the area and overprinted the previously established P–T  gradients.     

Calculated mineral equilibria for eclogites in CaO–Na

Carson  Powell  & Clarke 《Journal of Metamorphic Geology》1999,17(1):9-24

Regional high-pressure metamorphism during intracratonic deformation: the Petermann Orogeny, central Australia

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.     

Empirical calibration of the silica–Ca-tschermak's–anorthite (SCAn) geobarometer

The pressure-sensitive equilibrium among anorthite, quartz and the Ca-tschermak component in clinopyroxene (CaAl₂SiO₆; CaTs), CaAl₂SiOCpx6+SiOQtz2=CaAl₂Si₂OPl8 (SCAn) ,can be used as a geobarometer in granulites with the proper assemblage, and has been calibrated using mineral composition data from partial melting experiments of natural assemblages and from phase equilibrium experiments on the end-member CMAS system. The experimental data cover the P – T  range 4–32  kbar and 900–1400  °C. Linear least-squares regression analysis of the experimental data resulted in the following empirical expressions for pressure in terms of composition and temperature: P = 5.066 [±0.760]+ 1300 [±800] T  −ln K 276 [±16] · T  [±2.5  kbar]or P = 6.330 [±0.116]−ln K 301 [±9]· T  [±1.0  kbar] ,where K = a PlAn a CpxCaTs  .The first equation incorporates an enthalpy term, but is less accurate than the second equation, in which the enthalpy of reaction is ignored. Application of these expressions to natural and experimental equilibrium mineral assemblages demonstrates that the empirical barometers are applicable over a wide range of pressures (≥4  kbar), temperatures (≥700  °C) and bulk compositions (Mg#≥32.5).     

Thermal and baric evolution of garnet granulites from Sri Lanka

Abstract Garnet granulites from Sri Lanka preserve textural and chemical evidence for prograde equilibration at temperatures of at least 700–750°C and pressures in the vicinity of 6–8 kbar. Associated strain patterns suggest prograde metamorphism occurred during and immediately following an episode of crustal thickening, with the prograde P–T conditions probably reflecting a combination of the conductive and advective transport of heat at the mid-levels of tectonically thickened crust. The occurrence of prograde wollastonite provides evidence for internally buffered fluid compositions, or fluid absent conditions, during peak metamorphism and precludes pervasive advection of a CO₂-rich fluid. The advective heat component is therefore likely to have been provided by the transport of silicate melt. Intricate symplectitic textures record partial re-equilibration of the garnet granulites to lower pressures (˜ 4–6 kbar) at high temperatures (600–750°C), and testify either to the erosional denudation of the overthick crust prior to significant cooling (i.e. quasi-isothermal decompression) or to a subsequent static heating possibly of early Palaeozoic age (Pan-African). The metamorphic history of the Sri Lankan granulites is compared with high grade terrains in the neighbouring fragments of Gondwana, with the emphasis on similarities with Proterozoic granulites of the East Antarctic craton.     

Preservation of old U–Th–Pb ages in shielded monazite: example from the Beni Bousera Hercynian kinzigites (Morocco)

Four samples from the metamorphic aureole around the Beni Bousera ultramafic massif were studied in detail for U–Th–Pb electron microprobe dating on monazite. The samples include three meta-sedimentary granulites (kinzigites), collected at variable distance from the peridotites, and one kyanite-bearing leucosome in the kinzigite. Two types of monazite were identified in thin section, using SEM. The main population consists of interstitial grains, 20–70  μ m in size, while the second population consists of small grains (<20  μ m), included in garnet. A total of 64 U–Th–Pb electron microprobe measurements on 53 monazite crystals were undertaken. Most crystals have a Pb content lower than the Pb detection limit, indicating that they crystallized, or were reset, during a young event, probably Cainozoic in age. Few crystals, all entirely included in garnet, have Hercynian age, the best estimate of which is 284±27 Ma. This is a direct demonstration of the shielding effect of garnet for the U–Th–Pb system in monazite. The grains in inclusion in garnet are not reset by the post-Hercynian events, despite the high temperature reached at this time (>850 °C). Thus, the monazite closure temperature depends on its textural position in the host rock. The data also show that a Hercynian event occurred in the Beni Bousera granulitic metapelites, which equates with a high- P , high- T  event. The emplacement of the peridotite in the Cainozoic may be linked a low- P , high- T  event, followed by a low- P , low- T  retrogression. These two events reset the U–Th–Pb system in almost all monazite grains, except for the few crystals shielded by garnet.     

A Variscan continental collision of the West Sudetes and the Brunovistulian terrane: a contribution from structural and metamorphic record of the Stronie Formation,the Orlica-Śnieżnik Dome,SW Poland

Structural, petrological and geochronological data from marbles and mica schists combined with those from highly disputable gneisses and eclogites of the Orlica-Śnieżnik Dome (OSD), gave new insights into Variscan evolution of eastern borderland of the West Sudetes. It is shown that the Variscan tectonometamorphic evolution of the OSD began with E–W oriented subhorizontal shortening (D1 stage) related to the collision of the West Sudetes terranes and the Brunovistulian terrane. The shortening led to generally upright folding, which resulted in formation of the steep N–S trending metamorphic planar fabric S1, thickening of the lithosphere and burial of the Stronie Formation under greenschist facies conditions. As a consequence of subsequent, near-coaxial gravity-controlled vertical shortening (D2), the S1 foliation was deformed in tight recumbent folds F2. The flattening strain was associated with the progression to amphibolite-facies conditions (from ca. 510 upto ca. 620°C) and uplift from depths corresponding to 9–10 kbar to depths corresponding to 7–8 kbar. On microscale, the flattening strain is documented by rotation of the mineral fabric overgrown by syn-D2 prograde garnet porphyroblasts. The Sm–Nd Grt–WR isochron age, correlated with the D2 event yields 346.5 ± 4.4 Ma. Further deformation and progressive metamorphism led to development of the subhorizontally disposed S2 axial plane schistosity, which terminated at the metamorphic temperature peak. During retrogression but still under ductile conditions of deformation, the S2 planes were reactivated during successive top-to-the-N shear movement of the OSD (D3 stage). Due to the mutual interaction of the OSD with the adjacent terranes the shear deformations were localized within marginal parts of these units. Finally, as a result of the NE–SW and the NW–SE oriented shortenings (D4 stage and D5 stage, respectively), both the structural surfaces and metamorphic isograds were regionally folded with W(NW)-ward plunges. A synthesis of new and existing data shows an overall similarity in sequence of Variscan deformations (D1–D2–D3) in the West Sudetes borderland to that observed in the eastern flank of the Moldanubian domain.     

Well-preserved garnet growth zoning in granulite from the Dabie Mountains, central China

Ultra-high pressure eclogites and granulites both occur in the Dabie Mountains, central China. A garnet porphyroblast from felsic granulite in the Dabie Mountains has been analysed for compositional zoning by electron microprobe. Two segments of the porphyroblast have opposite compositional variations. Segment I (from centre outward 9  mm to analytical point 18) has decreasing X_Sps and increasing X_Pyr, while Segment II (from analytical point 18, 1  mm outward to the rim) has increasing X_Sps and X_Alm and decreasing X_Pyr and X_Grs. The compositional zoning in segment I is considered as growth zoning and that in Segment II as diffusive retrograde zoning. Garnet growth zoning records a P–T  path prior to the peak granulite metamorphism. The minimum P – T  conditions are estimated to be 1.35  GPa and 850  °C for peak metamorphism, based on the highest Mg/(Fe+Mg) composition in the garnet (analytical point 18) and matrix hypersthene, biotite and plagioclase. A symplectitic corona surrounds the porphyroblast and appears to have formed at 0.6  GPa and 700  °C. The well-preserved growth zoning in garnet suggests a short residence time for the granulite at peak metamorphism and thus rapid tectonic uplift history. The P–T  path is consistent with that of ultra-high-pressure eclogite in the area. Tectonic movements during a collisional event could have brought both the granulite and the eclogite to their present positions.     

Pressure–temperature evolution of metapelitic granulites in a polymetamorphic terrane: the Rauer Group, East Antarctica

Distinctive lithological associations and geological relationships, and initial geochronological results indicate the presence of an areally extensive region of reworked Archaean basement containing polymetamorphic granulites in the Rauer Group, East Antarctica.
Structurally early metapelites from within this reworked region preserve complex and varied metamorphic histories which largely pre-date and bear no relation to a Late Proterozoic metamorphism generally recognized in this part of East Antarctica. In particular, magnesian metapelite rafts from Long Point record extreme peak P–T conditions of 10–12 kbar and 100–1050°C, and an initial decompression to 8 kbar at temperatures of greater than 900°C. Initial garnet–orthopyroxene–sillimanite assemblages contain the most magnesian (and pyrope-rich) garnets ( X _Mg= 0.71) yet found in granulite facies rocks. A high-temperature decompressional P–T history is consistent with reaction textures in which the phase assemblages produced through garnet breakdown vary systematically with the initial garnet X _Mg composition, reflecting the intersection of different divariant reactions in rocks of varied composition as pressures decreased. This history is thought to relate to Archaean events, whereas a lower-temperature ( c. 750–800°C) decompression to 5 kbar reflects Late Proterozoic reworking of these relict assemblages.
The major Late Proterozoic ( c. 1000 Ma) granulite facies metamorphism is recorded in a suite of younger Fe-rich metapelites and associated paragneisses in which syn- to post-deformational decompression, through 2–4 kbar from maximum recorded P–T conditions of 7–9 kbar and 800–850°C, is constrained by geothermobarometry and reaction textures. This P–T evolution is thought to reflect rapid tectonic collapse of crust previously thickened through collision.     

A recalibration of the garnet-olivine geothermometer and a new geobarometer for garnet peridotites and garnet-olivine-plagioclase-bearing granulites

The garnet-olivine Fe-Mg exchange geothermometer and the garnet-olivine-plagioclase geobarometer have been simultaneously calibrated using reversed experimental data based on the model reactions and between 900 and 1500 °C at 9.1–95.0 kbar and between 4.7 and 7.0 kbar at 750–1050 °C, respectively. The resulting garnet-olivine thermometer reproduces experimental temperatures mostly within ±75 °C and the garnet-olivine-plagioclase barometer reproduces experimental pressures well within ±0.19 kbar. These new thermobarometers use the same garnet and olivine activity models and are thermodynamically consistent. Application of these thermobarometers to garnet peridotites from mantle xenoliths, orogenic garnet peridotites over the world and the Adirondack olivine-bearing metagabbros yielded reasonable P–T results. The present garnet-olivine thermometer can be used to measure medium-high-grade to ultrahigh-grade, low-pressure to ultrahigh–high-pressure garnet peridotites and metagabbros, whereas the garnet-olivine-plagioclase barometer has limited application to garnet-olivine-plagioclase-bearing granulites.     

Influence of deformation partitioning and metamorphic re-equilibration on P–T path reconstruction in the pre-Alpine basement of central Southern Alps (Northern Italy)

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+Bt_I+Grt+Ms_I+Pl+Qtz and St+Bt_II+Grt+Ms_II+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.     

HT-fabrics in a garnet-bearing quartzite from Western Portugal: geodynamic implications for the Iberian Variscan Belt

ABSTRACT The study of a garnet-bearing quartzite from a major suture zone in Iberia reports an unusual high-T fabric. Quartz c -axis patterns were plotted using shaped garnet as reference axis for the finite stretch ( X -axis). The pole figures are characterized by a dominant single maximum around X together with other point maxima along the XY plane (mylonitic foliation). These patterns suggest that dominant < c > prism slip and subordinated < a > prism slip operated during quartz plastic deformation in agreement with P–T conditions for syntectonic garnet growth (4–5 kbar and 700 ± 50 °C) and, pre-dating the well-known (late) Variscan D1 event (<6 kb and 600 ± 30 °C). The geotectonic framework suggests that the fabrics were formed along the western shear margin of the Ossa-Morena Zone during the early stages of the Variscan orogeny.