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.     

Cr-rich magnesiochloritoid eclogites from the Monviso ophiolites (Western Alps, Italy)

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

Petrochemical constraints for dual origin of garnet peridotites from the Dabie-Sulu UHP terrane, eastern-central China

Garnet peridotites occur as lenses, blocks or layers within granulite–amphibolite facies gneiss in the Dabie-Sulu ultra-high-pressure (UHP) terrane and contain coesite-bearing eclogite. Two distinct types of garnet peridotite were identified based on mode of occurrence and petrochemical characteristics. Type A mantle-derived peridotites originated from either: (1) the mantle wedge above a subduction zone, (2) the footwall mantle of the subducted slab, or (3) were ancient mantle fragments emplaced at crustal depths prior to UHP metamorphism, whereas type B crustal peridotite and pyroxenite are a portion of mafic–ultramafic complexes that were intruded into the continental crust as magmas prior to subduction. Most type A peridotites were derived from a depleted mantle and exhibit petrochemical characteristics of mantle rocks; however, Sr and Nd isotope compositions of some peridotites have been modified by crustal contamination during subduction and/or exhumation. Type B peridotite and pyroxenite show cumulate structure, and some have experienced crustal metasomatism and contamination documented by high ⁸⁷Sr/⁸⁶Sr ratios (0.707–0.708), low ε_Nd( t ) values (−6 to −9) and low δ¹⁸O values of minerals (+2.92 to +4.52). Garnet peridotites of both types experienced multi-stage recrystallization; some of them record prograde histories. High- P–T  estimates (760–970 °C and 4.0–6.5±0.2 GPa) of peak metamorphism indicate that both mantle-derived and crustal ultramafic rocks were subducted to profound depths >100 km (the deepest may be ≥180–200 km) and experienced UHP metamorphism in a subduction zone with an extremely low geothermal gradient of <5 °C km⁻¹.     

Metamorphic phase relations in orthopyroxene-bearing granitoids: implication for high-pressure metamorphism and prograde melting in the continental crust

In this work, the factors controlling the formation and preservation of high-pressure mineral assemblages in the metamorphosed orthopyroxene-bearing metagranitoids of the Sandmata Complex, Aravalli-Delhi Mobile Belt (ADMB), northwestern India have been modelled. The rocks range in composition from farsundite through quartz mangerite to opdalite, and with varying K₂O, Ca/(Ca + Na)_rock and FeO_tot + MgO contents. A two stage metamorphic evolution has been recorded in these rocks.
An early hydration event stabilized biotite with or without epidote at the expense of magmatic orthopyroxene and plagioclase. Subsequent high-pressure granulite facies metamorphism (∼15 kbar, ∼800 °C) of these hydrated rocks produced two rock types with contrasting mineralogy and textures. In the non-migmatitic metagranitoids, spectacular garnet ± K-feldspar ± quartz corona was formed around reacting biotite, plagioclase, quartz and/or pyroxene. In contrast, biotite ± epidote melting produced migmatites, containing porphyroblastic garnet incongruent solids and leucosomes.
Applying NCKFMASHTO T–M (H₂O) and P–T pseudosection modelling techniques, it is demonstrated that the differential response of these magmatic rocks to high-pressure metamorphism is primarily controlled by the scale of initial hydration. Rocks, which were pervasively hydrated, produced garnetiferous migmatites, while for limited hydration, the same metamorphism formed sub-solidus garnet-bearing coronae. Based on the sequence of mineral assemblage evolution and the mineral compositional zoning features in the two metagranitoids, a clockwise metamorphic P–T path is constrained for the high-pressure metamorphic event. The finding has major implications in formulating geodynamic model of crustal amalgamation in the ADMB.     

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.     

Dehydration melting, fluid buffering and decompressional P–T path in a granulite complex from the Eastern Ghats, India

A suite of metapelites, charnockites, calc-silicate rocks, quartzo-feldspathic gneisses and mafic granulites is exposed at Garbham, a part of the Eastern Ghats granulite belt of India. Reaction textures and mineral compositional data have been used to determine the P–T–X evolutionary history of the granulites. In metapelites and charnockites, dehydration melting reactions involving biotite produced quartzofeldspathic segregations during peak metamorphism. However, migration of melt from the site of generation was limited. Subsequent to peak metamorphism at c . 860° C and 8 kbar, the complex evolved through nearly isothermal decompression to 530–650° C and 4–5 kbar. During this phase, coronal garnet grew in the calc-silicates, while garnet in the presence of quartz broke down in charnockite and mafic granulite. Fluid activities during metamorphism were internally buffered in different lithologies in the presence of a melt phase. The P–T path of the granulites at Garbham contrasts sharply with the other parts of the Eastern Ghats granulite belt where the rocks show dominantly near-isobaric cooling subsequent to peak metamorphism.     

Reconstruction of the exhumation path of the Alpe Arami'ql garnet-peridotite body from depths exceeding 160 km

Chemical disequilibrium exists between all phases of the Alpe Arami garnet-peridotite body (Ticino, Switzerland) which hampers the evaluation of P–T  conditions of origin, yet disequilibrium offers the inherent possibility to derive a P–T–t path for this mantle slice. We tried to tackle this problem by carrying out new mineral analyses and taking diffusion rates and bulk-rock compositional effects into consideration. Peak metamorphic conditions from mineral core compositions were estimated as 1120±50 °C/5±0.2 GPa. These values are significantly higher than previously published results and were determined from a combination of the O'Neill & Wood (1979) Fe/Mg garnet–olivine exchange thermometer and the Al-in-orthopyroxene barometer (Brey & Köhler, 1990), and are supported by the Ca/Cr ratios in garnet, which are in accord with these conditions. Details of the exhumation path were derived from (1) rim compositions of minerals that yield a first retrograde stage of 720±50 °C/2±0.25 GPa (2) a spinel lherzolite assemblage in narrow shear zones (tectonic phase F0', after Möckel, 1969) which documents a second retrograde stage at 500–600 °C/0.8–1.5 GPa. The Ca content in olivine (Köhler & Brey, 1989) can be used to evaluate further P–T  conditions along the retrograde path. We measured very low values (30–40  ppm Ca) in the cores of olivine and a remarkable increase towards the rim (120  ppm). The low core values may reflect an equilibrium stage during the main Alpine metamorphism. The increasing values towards the olivine rims probably represent a late-stage heating event. The initial cooling rates for the peridotite body are between 2700 and 5100 °C Ma⁻¹, depending on which diffusion data are used.     

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.     

Cretaceous high-temperature rapid loading and unloading in the Abukuma metamorphic terrane, Japan

The Cretaceous Abukuma metamorphic terrane consists of the oceanic Gosaisyo Series overthrust onto the terrigenous Takanuki Series. Although the dominant mineralogy defines one of the classic areas of andalusite–sillimanite type progressive metamorphism, there are several lines of evidence suggesting an earlier higher-pressure (up to c . 12  kbar) history of the Takanuki Series and the nearby Gosaisyo Series. These are: (1) the occurrence of rare although widespread relic kyanite in sillimanite+K-feldspar zone-grade pelitic rocks; (2) the high grossular content of garnet interiors (up to c . 30  mol %) overgrown by Ca-poor rims ( c . 2  mol % grossular) in pelitic rocks containing Al₂SiO₅ minerals (sillimanite±relic kyanite±retrograde andalusite), plagioclase and quartz; (3) the occurrence of rutile as inclusions in garnet in pelitic rocks; and (4) the occurrence of relic corundum+almandine association in silica-poor and Al–Fe-rich rocks. Garnet in the Takanuki Series pelitic rocks commonly shows textural sector zoning and preserves growth zoning despite the high metamorphic grade, suggesting rapid changes in P–T  conditions and a relatively short duration of high-temperature conditions. Combined with radiometric dating, these observations suggest that the Abukuma sillimanite+K-feldspar zone-grade rocks underwent a clockwise P–T  path with very fast (>4  mm  y⁻¹) average burial and exhumation rates.     

Prograde eclogite in the Gföhl Nappe, Czech Republic: new evidence on Variscan high-pressure metamorphism

A layer of relict, high-temperature, prograde eclogite has been discovered within felsic granulite of the Gföhl Nappe, which is the uppermost tectonic unit in the Moldanubian Zone of the Bohemian Massif, the easternmost of the European Variscan massifs. Pressure-temperature conditions for eclogite (≥890  °C, 18.0  kbar) and felsic granulite ( c . 1000  °C, 16  kbar) place early metamorphism of the polymetamorphic Gföhl crustal rocks within the eclogite facies, and preservation of prograde compositional zoning in small garnet grains in high-temperature eclogite requires very rapid heating, as well as cooling. Mantle-derived garnet and spinel–garnet peridotites are associated with the high temperature-high pressure crustal rocks in the Gföhl Nappe, and this distinctive lithological suite appears to be unique among European Phanerozoic orogenic belts, implying that tectonic processes during the late stages in evolution of the Variscan belt were different from those in the Caledonian and Alpine belts. The unusually high temperatures and pressures in Gföhl crustal rocks, mineralogical evidence for rapid heating and cooling, juxtaposition of lithospheric and asthenospheric mantle with crustal rocks, and widespread production of late-stage granites indicate that culmination of the Variscan Orogeny may have been driven by lithospheric delamination and asthenospheric upwelling.     

大别山北部超高压变质大理岩及其地质意义

岩石学研究表明 ,大别山北部镁铁 超镁铁质岩带中白云质大理岩至少经历过三期变质阶段 :(1)榴辉岩相峰期变质阶段 ,矿物组合主要为方解石 +白云石 +金红石 +镁橄榄石 +钛 斜硅镁石 +富镁的钛铁矿±文石±石榴子石 ;(2 )麻粒岩相退变质阶段 ,矿物组合主要为方解石 +白云石 +金云母 +镁橄榄石 +透辉石 +钛铁矿 +尖晶石±斜方辉石等 ;(3)角闪岩相退变质阶段 ,主要矿物组合为方解石 +白云石 +磷灰石 +磁铁矿+榍石等。它的峰期变质矿物组合 ,类似于苏 鲁超高压大理岩 ,形成压力至少大于 2 .5GPa。这进一步证明 ,大别山北部大多数高级变质岩 (包括大理岩等 )都曾经过超高压变质作用 ,应属于印支期扬子俯冲陆壳的一部分。     

Geochemical constraints of the eclogite and granulite facies metamorphism as recognized in the Raobazhai complex from North Dabie Shan,China

A combined study of major and trace elements, fluid inclusions and oxygen isotopes has been carried out on garnet pyroxenite from the Raobazhai complex in the North Dabie Terrane (NDT). Well‐preserved compositional zoning with Na decreasing and Ca and Mg increasing from the core to rim of pyroxene in the garnet pyroxenite indicates eclogite facies metamorphism at the peak metamorphic stage and subsequent granulite facies metamorphism during uplift. A P–T path with substantial heating (from c. 750 to 900 °C) after the maximum pressure reveals a different uplift history compared with most other eclogites in the South Dabie Terrane (SDT). Fluid inclusion data can be correlated with the metamorphic grade: the fluid regime during the peak metamorphism (eclogite facies) was dominated by N₂‐bearing NaCl‐rich solutions, whereas it changed into CO₂‐dominated fluids during the granulite facies retrograde metamorphism. At a late retrograde metamorphic stage, probably after amphibolite facies metamorphism, some external low‐salinity fluids were involved. In situ UV‐laser oxygen isotope analysis was undertaken on a 7 mm garnet, and impure pyroxene, amphibole and plagioclase. The nearly homogeneous oxygen isotopic composition (δ¹⁸O_VSMOW = c. 6.7‰) in the garnet porphyroblast indicates closed fluid system conditions during garnet growth. However, isotopic fractionations between retrograde phases (amphibole and plagioclase) and garnet show an oxygen isotopic disequilibrium, indicating retrograde fluid–rock interactions. Unusual MORB‐like rare earth element (REE) patterns for whole rock of the garnet pyroxenite contrast with most ultra‐high‐pressure (UHP) eclogites in the Dabie‐Sulu area. However, the age‐corrected initial ε_Nd(t) is ? 2.9, which indicates that the protolith of the garnet pyroxenite was derived from an enriched mantle rather than from a MORB source. Combined with the present data of oxygen isotopic compositions and the characteristic N₂ content in the fluid inclusions, we suggest that the protolith of the garnet pyroxenite from Raobazhai formed in an enriched mantle fragment, which has been exposed to the surface prior to the Triassic metamorphism.     

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 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.     

Phase Equilibria of Hornblende-Bearing Eclogite in the Western Dabie Mountain,Central China

The high-pressure (HP) eclogite in the western Dabie Mountain encloses numerous hornblendes,mostly barroisite.Opinions on the peak metamorphic P-T condition,PT path and mineral paragenesis of it are still in dispute.Generally,HP eclogite involves garnet,omphacite, hornblendes and quartz,with or without glaucophane,zoisite and phengite.The garnet has compositional zoning with X_(Mg) increase,X_(Ca) and X_(Mn) decrease from core to rim,which indicates a progressive metamorphism.The phase equilibria of the ...     

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.     

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.     

大别山铙钹寨被麻粒岩相变质作用改造的变质超镁铁岩体：石榴辉石岩证据

对大别山铙钹寨超镁铁岩体中石榴辉石岩的研究表明，铙钹寨岩体经历了从尖晶石－富铝辉石相（750度，1．1GPa)，尖晶石－石榴石相（850度，1．5GPa)到麻粒岩相（800度，0．85GPa)的变质演化，该岩体与南大别超高压变质杂岩的演化过程存在很大差异，它基本上处于一较高温地热体制之下，属于南大别俯冲陆壳的上盘杂岩，铙钹寨岩体及其它超镁铁岩体的普遍麻粒岩化与区域麻粒岩相的相关性可能说明大别变质基底已被彻底改造，北大别（安徽省内）目前所展示的高级变质作用仅是印支期后麻粒岩相变质事件的反映。     

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.