A corundum–quartz assemblage from the Eastern Ghats Granulite Belt,India: evidence for high P–T metamorphism?

Corundum+quartz-bearing assemblages occur in small lenses in granulite facies metapelites in Rayagada, north-central part of the Eastern Ghats Granulite Belt, India. Corundum porphyroblasts and quartz coexist with porphyroblastic almandine-rich garnet, hercynite spinel, ilmenite and magnetite. Corundum and quartz are separated by sillimanite or a composite corona consisting of sillimanite and garnet, whereas corundum shows sharp grain boundaries with spinel, ilmenite and magnetite. Porphyroblastic corundum contains prismatic sillimanite inclusions in which irregularly shaped quartz is enclosed. Two distinct reactions are inferred from the textural features: corundum+quartz=sillimanite and spinel+quartz=garnet+sillimanite. From the petrographical features, we infer that corundum–quartz–garnet–spinel was the peak metamorphic assemblage. Although large uncertainties exist regarding the positions of the respective reactions in P–T  space, from several published experimental results and theoretical calculations a peak metamorphic condition of 12  kbar and 1100  °C is estimated as the lower stability limit of the corundum–quartz assemblage. Decompression from the peak P–T  condition to c .  9  kbar, 950  °C is inferred.     

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.     

Two-stage decompression in orthopyroxene–sillimanite granulites from Forefinger Point, Enderby Land, Antarctica: implications for the evolution of the Archaean Napier Complex

Magnesian metapelites of probable Archaean age from Forefinger Point, SW Enderby Land, East Antarctica, contain very-high-temperature granulite facies mineral assemblages, which include orthopyroxene (8–9.5 wt% Al₂O₃)–sillimanite ± garnet ± quartz ± K-feldspar, that formed at 10 ± 1.5 kbar and 950 ± 50°C. These assemblages are overprinted by symplectite and corona reaction textures involving sapphirine, orthopyroxene (6–7 wt% Al₂O₃), cordierite and sometimes spinel at the expense of porphyroblastic garnet or earlier orthopyroxene–sillimanite. These textures mainly pre-date the development of coarse biotite at the expense of initial mesoperthite, and the subsequent formation of orthopyroxene (4–6 wt% Al₂O₃)–cordierite–plagioclase rinds on late biotite.
The early reaction textures indicate a period of near-isothermal decompression at temperatures above 900°C. Decompression from 10 ± 1.5 kbar to 7–8 kbar was succeeded by biotite formation at significantly lower temperatures (800–850°C) and further decompression to 4.5 ± 1 kbar at 700–800°C.
The later parts of this P–T evolution can be ascribed to the overprinting and reworking of the Forefinger Point granulites by the Late-Proterozoic ( c . 1000 Ma) Rayner Complex metamorphism, but the age and timing of the early high-temperature decompression is not known. It is speculated that this initial decompression is of Archaean age and therefore records thinning of the crust of the Napier Complex following crustal thickening by tectonic or magmatic mechanisms and preceding the generally wellpreserved post-deformational near-isobaric cooling history of this terrain.     

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.     

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.     

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.     

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.     

P–T–t paths and tectonic history of an early Precambrian granulite facies terrane, Jining district, south-east Inner Mongolia, China

Abstract The widespread khondalite series of south-east Inner Mongolia consists largely of biotite–sillimanite–garnet gneiss and quartzo-feldspathic gneiss with some marble and mafic granulite layers. It has experienced two metamorphic events at c. 2500 and 1900–2000 Ma.
A pre-peak stage of the first metamorphism at T = 600–700°C and P > 6–7 kbar is recognized by the relict amphibolite facies assemblage Ky–Grt–Bt–Pl–Qtz and 'protected'inclusions of biotite, hornblende, sodic plagioclase and quartz in garnet or orthopyroxene. The peak stage, with T = c. 800 ± 50°C and P 8–10 kbar, is characterized by the widespread granulite facies assemblages Sil–Grt–Bt–Kfs–Pl–Qtz in gneiss and Opx–Cpx–Pl ± Hbl ± Grt in granulite. The P–T–t path suggests that the supracrustal sequence was buried in the lower crust by tectonic thickening during D1–D2.
The beginning of the second metamorphism is characterized by further temperature rise to 700°C or more at lower pressure. This stage is manifested by the appearance of cordierite after garnet, fibrolite (Sil2) after biotite in gneiss and transformation of Hbl1 into Opx2 and Cpx2 in granulite. Coronas of symplectitic Opx2 + Pl2 surrounding Grt1 and Cpx1 in mafic granulite are interpreted as products of near-isothermal decompression. The P–T–t path may be related tectonically to waning extension of the crust by the end of the early Proterozoic.     

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.     

Ultra-high-temperature metamorphism in Central Brazil: the Barro Alto complex

The Barro Alto complex, central Brazil, is a layered mafic–ultramafic intrusion, which was subjected to granulite facies metamorphism during the Neoproterozoic. Ultra-high-temperature conditions are recorded by parageneses that occur in some lenses of quartz-bearing rock (metagranite, metapelite and impure quartzite). The peak paragenesis consists of spinel+quartz±cordierite±leucosome (recording the former presence of melt with quartz in excess), which is replaced by either orthopyroxene+sillimanite or garnet+sillimanite. Quartz+biotite±sillimanite±garnet symplectites are ubiquitous and indicate reactions between Fe–Mg phases and melt. Late kyanite porphyroblasts have overgrown these symplectites. The direct replacement of spinel+quartz±cordierite by orthopyroxene+sillimanite or garnet+sillimanite occurred around the [Sa] invariant point, which appears only in a petrogenetic grid with inverted topology. The topology inversion occurs at conditions of high oxygen fugacity or due to the presence of ZnO-bearing spinel. Minimum peak conditions of ultra-high-temperature metamorphism were calculated as c. 980 °C and c. 7.9 kbar. The succession of observed mineral textures can be explained by a near-isobaric cooling P–T  path, with a cooling stage occurring between c. 980 and 750 °C.     

Evolution of high-Mg–Al granulites from Sunkarametta, Eastern Ghats, India: evidence for a lower crustal heating–cooling trajectory

A suite of high-Mg–Al granulites from Sunkarametta, Eastern Ghats Belt, India, shows contrasting prograde assemblages of extremely aluminous orthopyroxene+cordierite+sapphirine and similarly aluminous orthopyroxene+Ti-rich spinel in closely associated domains. Textural and compositional characteristics indicate that both were derived from prograde dehydration–melting of biotite–plagioclase–quartz-bearing protoliths. The former assemblage was stabilized at relatively more magnesian bulk composition. Geothermobarometric data and petrogenetic grid considerations place 'peak' metamorphic conditions at c. 950 °C and 9 kbar. Subsequent to peak metamorphism, the rocks cooled to c . 700–750 °C, with slight lowering of pressure, and the retrograde reactions also involved melt–solid interaction. The inferred P – T  trajectory is one of heating–cooling at lower crustal (25–30 km) depths.     

From granulites to eclogites in the Sesia zone (Italian Western Alps): a record of the opening and closure of the Piedmont ocean

The Sesia zone (Italian Western Alps) offers one of the best preserved examples of pre-Alpine basement reactivated, under eclogite facies conditions, during the Alpine orogenesis. A detailed mineralogical study of eclogitized acid and basic granulites, and related amphibolites, is presented. In these rare weak to undeformed rocks microstructural investigations allow three main metamorphic stages to be distinguished.
(a) A medium- to low- P granulite stage giving rise to the development of orthopyroxene + garnet + plagioclase + brown amphibole + ilmenite ± biotite in basic granulites and garnet + K-feldspar + plagioclase + cordierite + sillimanite + biotite + ilmenite in acid granulites.
(b) A post-granulite re-equilibration, associated with the development of shear zones, producing discrete amphibolitization of the basic granulites and widespread development of biotite + sillimanite + cordierite + spinel in the acid rocks.
(c) An eo-Alpine eclogite stage giving rise to the crystallization of high- P and low- T assemblages.
In an effort to quantify this evolution, independent well-calibrated thermobarometers were applied to basic and acid rocks. For the granulite event, P-T estimates are 7–9 kbar and 700–800° C, and for subsequent retrograde evolution, P-T was 4–5 kbar and 600° C. For the eo-Alpine eclogite metamorphism, pressure and temperature conditions were 14–16 kbar and 550° C.
The inferred P-T path is consistent with an uplift of continental crust produced by crustal thinning prior to the subduction of the continental rocks. In the light of the available geochronological constraints we propose to relate the pre-Alpine granulite and post-granulite retrograde evolution to the Permo-Jurassic extensional regime. The complex granulite-eclogite transition is thus regarded as a record of the opening and of the closure of the Piedmont ocean.     

Calculated mineral equilibria for eclogites in CaO–Na2O–FeO–MgO–Al2O3–SiO2–H2O: application to the Pouébo Terrane, Pam Peninsula, New Caledonia

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  – X H2O 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.     

Assessment of the garnet–clinopyroxene Fe–Mg exchange thermometer using new experimental data

A detailed high-pressure experimental study of two mafic xenoliths, in which coexisting garnet and clinopyroxene (± plagioclase, spinel and olivine) were crystallized over a P–T range of 10–30 kbar and 950–1200°C, has revealed significant differences in temperatures from those estimated for coexisting garnets and clinopyroxenes using the Ellis & Green Fe–Mg exchange thermometer. The results show perfect matching at 30 kbar, 1150–1200°C, but increasing deviation at lower pressure and lower temperature, with the Ellis & Green calibration reaching a Δ T (overestimate) of c. 145°C at 10–12 kbar and 950°C. The grossular content of the garnet increases from c. 21 mol.% at 10 kbar to 26–31 mol.% at 30 kbar. These results confirm other recent experimental studies that show that the pressure correction, and possibly to a lesser extent the correction for grossular content, applied by Ellis & Green, are not appropriate for lower pressure conditions, and give estimated temperatures that are significantly high when applied to granulitic terranes formed at c. 10 kbar. The new reconnaissance results allow a graphical interpolation of a garnet–clinopyroxene geothermometer based on the Fe–Mg exchange reaction which should be applicable to assemblages formed under lower crustal conditions.     

Anti-clockwise P–T paths in the lower crust: an example from a kyanite-bearing regional aureole, George Sound, New Zealand

The George Sound Paragneiss (GSP) represents a rare Permo-Triassic unit in Fiordland that occurs as a km-scale pillar to gabbroic and dioritic gneiss of c . 120 Ma Western Fiordland Orthogneiss (WFO). It is distinguished from Palaeozoic paragneiss common in western Fiordland (Deep Cove Gneiss) by SHRIMP and laser-ablation U–Pb ages as young as c . 190 Ma and ¹⁷⁶Hf/¹⁷⁷Lu >0.2828 for detrital zircon grains. The Mesozoic age of the GSP circumvents common ambiguity in the interpretation of Cretaceous v. Palaeozoic metamorphic assemblages in the Deep Cove Gneiss. A shallowly dipping S₁ foliation is preserved in the GSP distal to the WFO, cut by 100 m scale migmatite contact zones. All units preserve a steeply dipping S₂ foliation. S₁ staurolite and sillimanite inclusions in the cores of metapelitic garnet grains distal to the WFO preserve evidence for prograde conditions of T  <   650 °C and P <  8 kbar. Contact aureole and S₂ assemblages include Mg-rich, Ca-poor cores to garnet grains in metapelitic schist that reflect WFO emplacement at ≈760 °C and ≈6.5 kbar. S₂ kyanite-bearing matrix assemblages and Ca-enriched garnet rims reflect ≈650 °C and ≈11 kbar. Poorly oriented muscovite–biotite intergrowths and rare paragonite reflect post-S₂ high- P retrogression and cooling. Pseudosection modelling in NCKFMASH defines a high- P anti-clockwise P–T history for the GSP involving: (i) mid- P amphibolite facies conditions; preceding (ii) thermal metamorphism adjacent to the WFO; followed by (iii) burial to high- P and (iv) high- P cooling induced by tectonic juxtaposition of cooler country rock.     

Contrasting sapphirine parageneses from Wilson Lake, Labrador and their tectonic implications

Abstract Regionally distributed pelitic granulites in the Wilson Lake region contain the assemblage sapphirine + hypersthene + sillimanite + quartz. Geochronology and geobarometry suggest it developed in early Proterozoic rocks at temperatures approaching 900°C and pressures above 10 kbar. Vein-like metasomatized rocks around a suite of mafic to ultramafic intrusions, emplaced near the peak of metamorphism about 1700 Ma ago, contain sapphirine, but these assemblages developed at temperatures near 750°C and pressures of 4.5 kbar. Both types of assemblage occur as relics in amphibolite-grade (biotite–sillimanite) migmatites.
P–T determinations indicate rapid isothermal uplift of 20 km accompanied by mafic intrusion and hydration. The metamorphic history and tectonic setting suggest exposure of deep continental crust by thrusting during continental collision, followed by essentially isothermal decompression.     

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.     

Granulites and garnet–cordierite gneisses from Dronning Maud Land, Antarctica

Abstract The central sector of Mühlig-Hofmannfjellet (3°E/71°S) in western Dronning Maud Land (East Antarctic shield) is dominated by large intrusive bodies of predominantly orthopyroxene-bearing quartz syenites (charnockites). Metasedimentary rocks are rare; however, two distinct areas with banded gneiss–marble–quartzite sequences of sedimentary origin were found during the Norwegian Antarctic Research Expedition NARE 1989/90. Cordierite-bearing metapelitic gneisses from two different localities contain the characteristic mineral assemblage: cordierite + garnet + biotite + K-feldspar + plagioclase + quartz ± sillimanite ± spinel. Thermobarometry indicates equilibration conditions of about 650°C and 4 kbar. Associated orthopyroxene–garnet granulites, on the other hand, revealed pressures of about 8 kbar and temperatures of 750°C. The earlier granulite facies metamorphism is not well preserved in the cordierite gneisses as a result of excess K-feldspar combined with interaction with an H₂O-rich fluid phase, probably released by the cooling intrusives. These two features allowed the original high-grade K-feldspar + garnet assemblages to recrystallize as cordierite–biotite–sillimanite gneisses, completely re-equilibrating them. Phase relationships indicate that the younger metamorphic event occurred in the presence of a fluid phase that varied in composition between the lithologies.     

Magmatic arc metamorphism: petrology and temperature history of metabasic rocks in the Coastal Cordillera of northern Chile

We investigated the metamorphic cooling history of underplated magmatic rocks at midcrustal depth. Granulites and amphibolites occur within the Jurassic magmatic belt of the Coast Range south of Antofagasta in northern Chile between 23°25' and 24°20' S. The protoliths of the metamorphic rocks are basic intrusions of Early Mesozoic age. They are part of the magmatically formed crust, and the essentially dry magmas were emplaced in an extensional regime. The granulites (clinopyroxene–orthopyroxene–plagioclase) show all stages of fabric development from magmatic to granoblastic fabrics. Pyroxene compositions were reset at temperatures around 800°  C independent of the stage of textural equilibration. The granulites were partially amphibolitized at upper amphibolite facies temperatures of 600–700°  C. Following cooling, a possible reheating to greenschist facies temperatures around 500°  C is indicated by prograde zoning in magnetite–ilmenite pairs. Mineral assemblages are not suitable for barometry, but a conservative estimation of the garnet-in reaction at given whole-rock compositions suggests maximum pressures in the granulite facies of around 5 kbar, and similar pressures are indicated by phengite barometry for the greenschist facies. The P–T  path of granulite–amphibolite metamorphism is one of slow cooling from magmatic temperatures with heterogeneous deformation. The thinning of the pre-Andean (Precambrian–Triassic) crust was apparently compensated by the magmatic underplating and this special tectonomagmatic setting caused the prolonged residence of the accreted rocks at midcrustal levels.