First Report of Sapphirine+Quartz Assemblage from Southern India: Implications for Ultrahigh-temperature Metamorphism

We report here for the first time, the occurrence of sapphirine+quartz assemblage in textural equilibrium from quartzo-feldspathic and pelitic granulites from southern India. The sapphirine-bearing rocks occur as layered gneisses associated with pink granite within massive charnockite in Rajapalaiyam area in the southern part of Madurai Block. Sapphirine occurs in three associations: (i) fine-grained subhedral mineral associated with quartz enclosed in garnet, (ii) intergrowth with Al-rich orthopyroxene (up to 9.7 wt.% Al₂O₃), and (iii) in symplectitic intergrowth with orthopyroxene (Al₂O₃= 5.9–6.7 wt.%) and cordierite surrounding garnet. The sapphirine in association with quartz is slightly magnesian (X_Mg = 0.79–0.80) and low in Si content (1.55–1.56 pfu) as compared with those associated with orthopyroxene and cordierite (X_Mg= 0.77–0.79, Si = 1.59–1.63 pfu). The sapphirine+quartz assemblage suggests that the granulites underwent T>1050 °C peak metamorphism. Cores of porphyroblastic orthopyroxene in the sapphirine-bearing rocks shows high-Al₂O₃ content of up to 9.7 wt.%, suggesting T = 1040–1060°C and P = 8 kbar. FMAS reaction of sapphirine+quartz→garnet+sillimanite+cordierite indicates a cooling from sapphirine+quartz stability field after the peak ultrahigh-temperature metamorphism. Slightly lower temperature estimates from ternary feldspar and sapphirine-spinel geothermometers (T = 950–1000°C) also support a post-peak isobaric cooling. Corona textures of orthopyroxene+cordierite (±sapphirine), orthopyroxene+sapphirine, and cordierite+spinel around garnet suggest subsequent decompression. The sapphirine-quartz association and related textures reported in this study have important bearing on the ultrahigh-temperature metamorphism and exhumation history of the Madurai Block as well as on the tectonic evolution of the continental deep crust in southern India.     

http://www.sciencedirect.com/science/article/pii/S1674987111000843

We report equilibrium sapphirine t quartz assemblage in biotiteeorthopyroxeneegarnet granulites from a new locality in Panasapattu of Paderu region in the Eastern Ghats granulite belt, which provide new evidence for ultrahigh-temperature (UHT) metamorphism at 1030e1050 C and 10 kbar in this region. The development of migmatitic texture, stabilization of the garneteorthopyroxenee plagioclaseeK-feldspar association, prograde biotite inclusions within garnet and sapphirine as well as sapphirine and cordierite inclusions within garnet in these granulites indicate that the observed peak assemblages probably formed during prograde dehydration melting of a BteSilleQtz assemblage, and constrain the prograde stage of the PeT path. The core domains of orthopyroxene porphyroblasts have up to wt(Al2O3) 9.6%, which suggest that the temperatures reached up to 1150 C suggesting extreme crustal metamorphism. These conditions were also confirmed by the garneteorthopyroxene thermobarometery, which yields a PeT range of 1012e960 C and 9.4 kbar. The PeT phase topologies computed using isochemical sections calculated in the model system Na2OeCaOeK2OeFeOeMgOeAl2O3eSiO2 eH2O (NCKFMASH) for metapelites, garnet-free sapphirine granulites and garnet-bearing sapphirine granulites match the melt-bearing assemblages observed in these rocks. Isochemical sections constructed in the NCKFMASH system for an average sub-aluminous metapelite bulk composition, and contoured for modal proportions of melt and garnet, as well as for the compositional isopleths of garnet, predict phase and reaction relations that are consistent with those observed in the rocks. Garnet and orthopyroxene contain Ti-rich phlogopite inclusions, suggesting formation by prograde melting reactions at the expense of phlogopite during ultrahigh-temperature conditions. These PeT results underestimate ‘peak’ conditions, in part as a result of the modification of garnet compositions in the domains where some melt was retained. The post-peak evolution is constrained by a succession of melt-present reactions that occur at P < 10 kbar, inferred from micro-structural relations among various minerals. After high-temperature decompression from the metamorphic peak, the PeT path followed a near isobaric cooling stage to T < 900 C. The UHT rocks investigated in this study occur within a continental collision suture which witnessed prolonged subductioneaccretion history prior to the final collision. We correlate the extreme metamorphism and the stabilization of UHT mineral assemblages to heat and volatile input from an upwelled asthenosphere during subductionecollision tectonics in a Proterozoic convergent plate margin.     

Exsolution textures in orthopyroxene in aluminous granulites as indicators of UHT metamorphism: New evidence from the Eastern Ghats Belt, India

Highly aluminous orthopyroxene, coexisting with sapphirine, cordierite, sillimanite, quartz and garnet in various combinations, constitute granoblastic mosaic peak metamorphic assemblages in aluminous granulites from three localities in the Eastern Ghats Belt, India. Orthopyroxene contains four types of intergrowths: (a) involving sapphirine with or without cordierite, (b) involving spinel, but without sapphirine, (c) involving cordierite, but without sapphirine and spinel, and (d) involving garnet, without sapphirine, spinel or cordierite. On the basis of textural and compositional data, origin of the intergrowths is ascribed to breakdown of Mg-Tschermak component, locally also involving Fe- and Ti-Tschermak. An attempt is made to compute the “pre-breakdown” compositions of orthopyroxene by image analysis, which shows maximum Al₂O₃ content of 13.4 wt.% in the pristine orthopyroxene. Geothermometry, phase equilibria consideration and application of existing experimental data on alumina solubility in orthopyroxene coexisting with sapphirine and quartz, collectively indicate extreme thermal conditions of metamorphism (> 1000 °C) for the studied assemblages. This re-affirms the notion that Al₂O₃ solubility in orthopyroxene is the most powerful indicator of UHT metamorphism (Harley, S.L., 2004. Extending our understanding of ultrahigh temperature crustal metamorphism. J. Mineral. Petrol. Sci. 99, 140–158). The intergrowths are considered to have formed due to cooling from the thermal peak spanning a temperature range of approximately 150 °C. Appearance of diverse types of intergrowths is probably related to subtle differences in bulk composition, particularly Fe:Mg ratios.     

Sapphirine + quartz assemblage in contrasting textural modes from the Eastern Ghats Belt, India: Implications for stability relations in UHT metamorphism and retrograde processes

Stability of the assemblage sapphirine + quartz in Mg–Al-rich granulites implies ultrahigh temperature (UHT) condition of metamorphism but their direct contact is rarely preserved in natural rocks. The present study shows contrasting textural relations between sapphirine and quartz in different parts of the same occurrence of a Mg–Al-rich granulite, Eastern Ghats Belt, India. Textural data suggest stabilization of the assemblage sapphirine + quartz with orthopyroxene and cordierite during the metamorphic peak. Thermometric estimates yield temperature exceeding 950 °C for the stability of this assemblage. Most of such sapphirine grains (Spr₁) are texturally separated from quartz and cordierite grains by double corona of sillimanite + orthopyroxene that results due to isobaric cooling during the post-peak stage. Sapphirine (Spr₂) also forms a symplectic intergrowth with quartz and orthopyroxene at the fringe of coarse orthopyroxene. This textural feature can be explained by the breakdown of (Fe, Mg)-Tschermak components of orthopyroxene during the same isobaric cooling episode from UHT peak condition. The preservation of grain contact of this intergrown sapphirine and quartz can be attributed to a problem in reaction kinetics. In the other mode, sapphirine (Spr₃) occurs with quartz with a thin skin of cordierite near a quartz vein. Such texture could result from isothermal decompression of the cooled crust. Alternatively and more possibly, cordierite could form from ingress of CO₂–H₂O rich fluid during terminal stage of cooling. Finally, sapphirine (Spr₄) and quartz show direct contact close to the quartz vein. Direct contact of such sapphirine and quartz represents textural disequilibrium as this particular quartz is introduced as a vein much later than the peak metamorphism but prior to the major foliation-forming deformation. Coarse sapphirine and vein quartz, therefore, accidentally came in contact with each other and persisted metastably. Therefore, though coexistence of sapphirine and quartz is considered to be a strong evidence for ultrahigh temperature condition, care should be taken to decipher their stable coexistence. Different types of textural relations involving this mineral pair could originate in a single rock, probably in different stages of its metamorphic history.     

Sapphirine parageneses from the Caraiba complex, Bahia, Brazil: the influence of Fe2+–Fe3+ distribution on the stability of sapphirine in natural assemblages

Abstract Sapphirine-bearing rocks occur in three conformable, metre-size lenses in intrusive quartzo-feldspathic orthogneisses in the Curaçà valley of the Archaean Caraiba complex of Brazil. In the lenses there are six different sapphirine-bearing rock types, which have the following phases (each containing phlogopite in addition): A: Sapphirine, orthopyroxene; B: Sapphirine, cordierite, orthopyroxene, spinel; C: Sapphirine, cordierite; D: Sapphirine, cordierite, orthopyroxene, quartz; E: Sapphirine, cordierite, orthopyroxene, sillimanite, quartz; F: Sapphirine, cordierite, K-feldspar, quartz. Neither sapphirine and quartz nor orthopyroxene and sillimanite have been found in contact, however. During mylonitization, introduction of silica into the three quartz-free rocks (which represent relict protolith material) gave rise to the three cordierite and quartz-bearing rocks. Stable parageneses in the more magnesian rocks were sapphirine–orthopyroxene and sapphirine–cordierite. In more iron-rich rocks, sapphirine–cordierite, sapphirine-cordierite–sillimanite, cordierite–sillimanite, sapphirine–cordierite–spinel–magnetite and quartz–cordierite–orthopyroxene were stable. The iron oxide content in sapphirine of the six rocks increases from an average of 2.0 to 10.5 wt % (total Fe as FeO) in the order: C,F–A,D–B,E. With increase in Fe there is an increase in recalculated Fe₂O₃ in sapphirine. The four rock types associated with the sapphirine-bearing lenses are: I: Orthopyroxene, cordierite, biotite, quartz, feldspar tonalitic to grandioritic gneiss; II: Biotite, quartz, feldspar gneiss; III: Orthopyroxene, clinopyroxene, hornblende, plagioclase meta-norite; IV: Biotite, orthopyroxene, quartz, feldspar, garnet, cordierite, sillimanite granulite gneiss. The stable parageneses in type IV are orthopyroxene–cordierite–quartz, garnet–sillimanite–quartz and garnet–cordierite–sillimanite. Geothermobarometry suggests that the associated host rocks equilibrated at 720–750°C and 5.5–6.5 kbar. Petrogenetic grids for the FMASH and FMAFSH (FeO–MgO–Al₂O₃–Fe₂O₃–SiO₂–H₂O) model systems indicate that sapphirine-bearing assemblages without garnet were stabilized by a high Fe³⁺ content and a high X_Mg= (Mg/ (Mg+Fe²⁺)) under these P–T conditions.     

Sapphirine bearing granulites from the Sipiwesk Lake area of the late Archean Pikwitonei granulite terrain,Manitoba, Canada

Sapphirine occurs in the orthopyroxene-cordierite and feldspar-sillimanite granulites in the Sipiwesk Lake area of the Pikwitonei granulite terrain, Manitoba (97°40W, 55°05N). The orthopyroxene-cordierite granulites have extremely high Al₂O₃ (24.5 wt%) and MgO (24.6 wt%) contents and contain sapphirine (up to 69.2 wt% Al₂O₃), aluminous orthopyroxene (up to 8.93 wt% Al₂O₃), cordierite, spinel, phlogopite, and corundum. Sapphirine forms coronas mantling spinel and corundum. Corona sapphirine is zoned and its composition varies through the substitution (Mg, Fe, Mn) Si=2 Al as a function of the phases with which it is in contact. Textural and chemical relationships of sapphirine with coexisting phases indicate that spinel + cordierite reacted to form orthopyroxene + sapphirine under conditions of increasing pressure. Moreover, decreasing core to rim variation of Al₂O₃ in orthopyroxene porphyroblasts suggests decreasing temperature during sapphirine formation. On the basis of experimentally determined P-T stability of the assemblage enstatite + sapphirine + cordierite, and the Al content of hypothetical Fe²⁺-free orthopyroxene associated with sapphirine and cordierite, metamorphic temperatures and pressures are estimated to be 860–890° C and 3.0–11.2 kbar.In the feldspar-sillimanite granulites, sapphirine occurs as a relict phase mantled by sillimanite and/or by successive coronas of sillimanite and garnet. These textural relations suggest the reaction sapphirine + garnet + quartz = orthopyroxene + sillimanite with decreasing temperature. Compositions of minerals in the assemblage garnet-orthopyroxene-sillimanite-plagioclase-quartz, indicate metamorphic P-T conditions of 780–880° C and 9±1 kb.The metamorphic conditions estimated in this study suggest that the sapphirine bearing granulites in the Sipiwesk Lake area represent Archean lower crustal rocks. Their formation might be related to the crustal thickening processes in this area as suggested by Hubregtse (1980) and Weber (1983).     

Analcime equilibria

The stability fields of analcime and analcime+quartz have been investigated using conventional hydrothermal techniques, over the approximate range of conditions 160–600 °C and 500–5000 bars fluid pressure. The dehydration of analcime (Na₂Al₂Si_3·3O_11·6 · nH₂O) to albite, nepheline and H₂O occurs at temperatures of 492±5 °C at 500 bars, 538±5 °C at 1000 bars, 578±5 °C at 2000 bars and 598±5 °C at 3000 bars. In the presence of quartz, analcine dehydrates to highly disordered albite and H₂O at about 200 °C and 2000 bars, 196°±5 °C and 3000 bars, about 190 °C and 4000 bars, and 183±5 °C at 5000 bars P_fluid. The synthetic phase equilibria appear to be compatible with field observations that primary analcimes occur as phenocrysts or in groundmass in some volcanic and hypabyssal rocks and secondary analcimes in sedimentary, hydrothermally altered and low-grade metamorphic rocks.     

Orthopyroxene–sillimanite–sapphirine granulites from the Bamble granulite terrane, southern Norway

Silica-deficient sapphirine-bearing rocks occur as an enclave within granulite facies Proterozoic gneisses and migmatites near Grimstad in the Bamble sector of south-east Norway (Hasleholmen locality). The rocks contain peraluminous sapphirine, orthopyroxene, gedrite, anthophyllite, sillimanite, sapphirine, corundum, cordierite, spinel, quartz and biotite in a variety of assemblages. Feldspar is absent.
Fe²⁺/(Fe²⁺+ Mg) in the analysed minerals varies in the order: spinel > gedrite ≥ anthophyllite ≥ biotite > sapphirine>orthopyroxene > cordierite.
Characteristic pseudomorph textures indicate coexistence of orthopyroxene and sillimanite during early stages of the reaction history. Assemblages containing orthopyroxene-sillimanite-sapphirine-cordierite-corundum developed during a high-pressure phase of metamorphism and are consistent with equilibration pressures of about 9 kbar at temperatures of 750–800°C. Decompression towards medium-pressure granulite facies generated various sapphirine-bearing assemblages. The diagnostic assemblage of this stage is sapphirine-cordierite. Sapphirine occurs in characteristic symplectite textures. The major mineralogical changes can be described by the discontinuous FMAS reaction: orthopyroxene + sillimanite → sapphirine + cordierite + corundum.
The disequilibrium textures found in the Hasleholmen rocks are characteristic for reactions which have been in progress but then ceased before they run to completion. Textures such as reaction rims, symplectites, partial replacement, corrosion and dissolution of earlier minerals are characteristic of granulite facies rocks. They indicate that, despite relatively high temperatures (700–800° C), equilibrium domains were small and chemical communication and transport was hampered as a result of dry or H₂O-poor conditions.     

Experimental Study of Fe-Mg Exchange Reaction Between Orthopyroxene and Sapphirine and its Calibration as a Geothermometer

The exchange of Fe²⁺ and Mg²⁺ between orthopyroxene and sapphirine has been investigated at pressures 7-16 kbar and temperatures 850-1300°C using a piston cylinder apparatus for the synthetic and natural systems. This reaction is temperature-dependent and would be a good geothermometer. The equilibration temperature T is approximately expressed by the distribution coefficient as follows: T(°C)=1515/(ln K_D+0.943)-273.

This empirical equation was applied to some Antarctic granulites and associated rocks. The new orthopyroxene-sapphirine geothermometer gives consistent results with those estimated from the Fe-Mg orthopyroxene-garnet geothermometer.     

Reaction history of sapphirine granulites and a decompressional P-T path in a granulite complex from the Eastern Ghats

The sapphirine granulites from G. Madugula, Eastern Ghats preserve a variety of mineral textures and reactions. Corona and reaction textures are used in conjunction with mineral compositions to construct a sequence of metamorphic reactions describing the mineralogical evolution of sapphirine granulites. An early stage is characterized by the development of sapphirine + quartz, spinel + quartz in textural equilibrium, and possible relicts after osumilite during peak metamorphic conditions. Sapphirine/spinel crystals were later detached from quartz in the form of mineral coronas. During a subsequent sapphirine-cordierite stage, several cordierite forming reactions reflect decreasingP-T conditions. Finally during the late stage, a few samples show evidence of retrogressive hydration. Sapphirine is rather iron-rich (12.8 wt%) and the Mg number in the analysed minerals varies in the order: cordierite > phlogopite > sapphirine > orthopyroxene > spinel > garnet.P-T conditions of metamorphism have been constrained through the application of geothermobarometry and thermodynamically calibrated MAS equilibria.P-T vectors from granulite facies rocks in the G. Madugula area indicate that the rocks experienced substantial decompression (up to 3 kbar) and moderate cooling (150–200°C) subsequent to peak conditions of metamorphism (8.4 kbar, > 900°C). The decompressionalP-T history of sapphirine granulites interpreted from textural features and thermobarometric estimates suggest that they may have eventually resulted from exhumation of thickened crust.     

Anticlockwise evolution of ultrahigh-temperature granulites within continental collision zone in southern India

We report three new localities of corundum and sapphirine-bearing hyper aluminous Mg-rich and silica-poor ultrahigh-temperature granulites formed during Late Neoproterozoic-Cambrian times within the Palghat–Cauvery Shear Zone system in southern India. From petrologic characteristics, mineral chemistry and petrogenetic grid considerations, the peak metamorphic conditions of these rocks are inferred to lie around 950–1000 °C (as suggested by Al in orthopyroxene thermometer) at pressures above 10 kbar (as indicated by the equilibrium orthopyroxene–sillimanite–gedrite ± quartz assemblage). These rocks preserve several remarkable reaction textures, the most prominent among which is the triple corona of spinel–sapphirine–cordierite on corundum, with the whole textural assembly embedded within the matrix of gedrite, suggesting the reaction: Ged + Crn = Spl + Spr + Crd. The formation of sapphirine–sillimanite assemblage/symplectite associated with relict corundum and porphyroblasitc cordierite is explained by the reaction: Crd + Crn = Spr + Sil. The association of sapphirine cordierite symplectite with gedrite–sillimanite assemblage as well as with aluminosilicate boundaries indicates the gedrite consuming reaction: Ged + Sil = Spr + Crd. Extensive growth of sapphirine–cordierite observed on the rim of gedrite porphyroblasts with spinel occurring as relict inclusions within the sapphirine indicates the reaction: Ged + Spl = Spr + Crd. The pressure–temperature (P–T) path defined from the observed mineral assemblages and reaction texture is characterized by anticlockwise trajectory, with a prograde segment of initial heating and subsequent deep burial, followed by retrograde near-isothermal decompression. Such an anticlockwise trajectory is being reported for the first time from southern India and has important tectonic implications since these rocks were developed at the leading edge of the crustal block that was involved in collisional orogeny and subsequent extension during the final phase of assembly of the Gondwana supercontinent. We propose that the rocks were subjected to deep subduction and rapid exhumation, and the extreme thermal conditions were attained either through input from underplated mantle-derived magmas, or convective thinning or detachment of the lithospheric thermal boundary layer during or after crustal thickening.     

Recrystallization of quartzofeldspathic rocks in a paleozoic thrust belt, Grand Lake, Newfoundland: Implications for garnet-biotite thermometry

Migmatitic granitic gneiss and associated garnetiferous granite at Grand Lake are tectonically interleaved with high-pressure ( 9 kbar) metapelites that contain Barrovian (Ky-St-Grt-Bt-Ms-Rt) mineral assemblages. The migmatites contain metabasites that are compositionally similar to the latest Proterozoic Long Range dykes, suggesting that the gneisses correlate to Grenvillian inliers in western Newfoundland.

In the metapelites, the Barrovian porphyroblastic assemblage overprints microfolds and is itself overprinted by greenschist-facies assemblages in thrust-related high-strain zones. Evidence that the quartzofeldspathic rocks also experienced Barrovian metamorphism is provided by low Ti/>Al amphiboles in metabasites in the gneiss, and, despite the migmatitic character of these rocks, the presence of growth-zoned garnets with bell-shaped compositional profiles (e.g., rimward increase in X_Prp and decrease in X_Sps) similar to those exhibited by garnet in the metapelite. The range of garnet-biotite paleotemperatures for the quartzofeldspathic rocks is only marginally higher (T_max=530–660°C) than that determined for the metapelites (T_max=500–615°C). There are no systematic differences in Grt-Bt temperatures between (proto) mylonitic rocks and nearby precursors.

These results indicate that the quartzofeldspathic rocks and metapelites share a common, post-migmatization (of the gneisses) metamorphic history; they may well have originally been linked by a basement/cover relationship. Mineralogical evidence for the earlier, higher grade metamorphic history of the migmatites has virtually been eradicated even outside the high-strain zones. In this area, there is no clear relation between qualitatively-estimated superimposed strain and the degree of resetting of the Grt-Bt geothermometer.     

Partial melting and decompression of the Thor-Odin dome, Shuswap metamorphic core complex, Canadian Cordillera

The Thor-Odin dome region of the Shuswap metamorphic core complex, British Columbia, contains migmatitic rocks exhumed from the deep mid-crust of the Cordilleran orogen. Extensive partial melting occurred during decompression of the structurally deepest rocks, and this decompression path is particularly well recorded by mafic boudins of silica-undersaturated, aluminous rocks. These mafic boudins contain the high-temperature assemblages gedrite+cordierite+spinel+corundum+kyanite/sillimanite±sapphirine±högbomite and gedrite+cordierite+spinel+corundum+kyanite/sillimanite+garnet±staurolite (relict)±anorthite. The boudins are interlayered with migmatitic metapelitic gneiss and orthogneiss in this region.

The mineral assemblages and reaction textures in these rocks record decompression from the kyanite zone (P>8–10 kbar) to the sillimanite–cordierite zone (P<5 kbar) at T750 °C, with maximum recorded temperatures of 800 °C. Evidence for high-temperature decompression includes the partial replacement of garnet by cordierite, the partial to complete replacement of kyanite by corundum+cordierite+spinel (hercynite)±sapphirine±högbomite symplectite, and the replacement of some kyanite grains by sillimanite. Kyanite partially replaced by sillimanite, and sillimanite with coronas of cordierite±spinel are also observed in the associated metapelitic rocks.

Partial melt from the surrounding migmatitic gneisses has invaded the mafic boudins. Cordierite reaction rims occur where minerals in the boudins interacted with leucocratic melt. When combined with existing structural and geochronologic data from migmatites and leucogranites in the region, these petrologic constraints suggest that high-temperature decompression was coeval with partial melting in the Thor-Odin dome. These data are used to evaluate the relationship between partial melting of the mid-crust and localized exhumation of deep, hot rocks by extensional and diapiric processes.     

Sapphirine-bearing granulites and related high-temperature metamorphic rocks from the Higo metamorphic terrane, west-central Kyushu, Japan

The Higo metamorphic unit in west-central Kyushu island, southwest Japan is an imbricated crustal section in which a sequence of units with increasing metamorphic grade from high (northern part) to low (southern part) structural levels is exposed. The basal part of the metamorphic sequence representing an original depth of 23–24  km consists mainly of garnet–cordierite–biotite gneiss, garnet–orthopyroxene gneiss, orthopyroxene-bearing amphibolite and orthopyroxene-bearing S-type tonalite. These metamorphic rocks underwent high amphibolite-facies up to granulite facies metamorphism with peak P – T  conditions of 720  MPa, 870  °C. In addition sapphirine-bearing granulites and related high-temperature metamorphic rocks also occur as tectonic blocks in a metamorphosed peridotite intrusion. The sapphirine-bearing granulites and their related high-temperature metamorphic rocks can be subdivided into five types of mineral assemblages reflecting their bulk chemical compositions as follows: (1) sapphirine–corundum–spinel–cordierite (2) corundum–spinel–cordierite (3) garnet–corundum–spinel–cordierite (4) garnet–spinel–gedrite–corundum, and (5) orthopyroxene–spinel–gedrite. These metamorphic rocks are characterized by unusually high Al₂O₃ and low SiO₂ contents, which could represent a restitic nature remaining after partial melting of pelitic granulite under the ultra high-temperature contact metamorphism at the peak metamorphic event of the Higo metamorphic unit. The metamorphic conditions are estimated to be about 800  MPa and above 950  °C which took place at about 250  Ma as a result of the thermal effect of the regional gabbroic rock intrusions.     

Petrology of Granulites from Anakapalle--Evidence for Proterozoic Decompression in the Eastern Ghats, India

The granulite complex at Anakapalle, which was metamorphosedat 1000 Ma, comprises orthopyroxene granulites, leptynite, khondalite,mafic granulites, calc-silicate rock, spinel granulites, andtwo types of sapphirine granulites—one quartz-bearingand migmatitic and the other devoid of quartz and massive. Reactiontextures in conjunction with mineral-chemical data suggest severalcontinuous and discontinuous equilibria in these rocks. In orthopyroxenegranulites, dehydration-melting of biotite in the presence ofquartz occurred according to the reaction biotite+quartz= garnet (Py₃₇)+K-feldspar+orthopyroxene + liquid. Later, this garnet broke down by the reaction garnet (Py₃₇)+quartz= orthopyroxene + plagioclase. Subsequently, coronal garnet (Py₃₀) and quartz were producedby the same reaction but proceeding in the opposite direction.In spinel granulites, garnet (Py₄₂) and sillimanite were producedby the breakdown of spinel in the presence of quartz. In thetwo types of sapphirine granulites, garnet with variable pyropecontent broke down according to the reaction garnet = sapphirine + sillimanite + orthopyroxene. The highest pyrope content (59 mol %) was noted in garnets fromquartz-free sapphirine granulites compared with the quartz-bearingone (53 mol % pyrope). The calculated positions of the mineralreactions and diserete P-T points obtained by thermobarometrydefine a retrograde P-T trajectory during which a steep decompressionof 1.5 kbar from P-T_max of 8 kbar and 900C was followed bynear-isobaric cooling of 300C. During this decompression, garnetwith variable pyrope contents in different rocks broke downon intersection with various divariant equilibria. Near-isobariccooling resulted in the formation of coronal garnet around second-generationorthopyroxene and plagioclase replacing earlier porphyroblasticgarnet in orthopyroxene granulites. It has been argued thatthe deduced P-T trajectory originated in an extensional regimeinvolving either a crust of near-normal thickness of a slightlyoverthickened crust owing to magmatic underaccretion.     

东喜马拉雅地区高压麻粒岩岩石学研究及构造意义

将该区内的高喜马拉雅结晶岩划分为南部的角闪岩相岩石和北部的中低压麻粒岩相岩石,后者沿那木拉逆冲断层向南推覆于前者之上。高压麻粒岩相岩石仅以残余产出于后者,主要包括石榴石蓝晶石片麻岩和石榴石透辉石岩。前者的峰期矿物组合为石榴石+蓝晶石+三元长石+石英+金红石;后者的峰期组合为石榴石(铁铝榴石10_±钙铝榴石_>80)+透辉石+石英+方柱石+榍石(Al₂O₃为4%-4.5%).变质温压估计结果表明高压麻粒岩相岩石形成于大约1.7-1.8GPa,890℃,然后经历了近等温降压变质作用至0.5±0.1GPa,850±50℃。它们的原岩可能是大理岩及泥质岩。这表明在区内曾存在一高压麻粒岩带,那木拉冲断层可能是高喜马拉雅结晶岩内的一条重要的构造界线。     

Superposition of replacements in the mafic granulites of the Jijal complex of the Kohistan arc, northern Pakistan: dehydration and rehydration within deep arc crust

A deep-level crustal section of the Cretaceous Kohistan arc is exposed in the northern part of the Jijal complex. The occurrence of mafic to ultramafic granulite-facies rocks exhibits the nature and metamorphic evolution of the lower crust. Mafic granulites are divided into two rock types: two-pyroxene granulite (orthopyroxene+clinopyroxene+plagioclase±quartz [1]); and garnet–clinopyroxene granulite (garnet+clinopyroxene+plagioclase+quartz [2]). Two-pyroxene granulite occurs in the northeastern part of the Jijal complex as a relict host rock of garnet–clinopyroxene granulite, where the orthopyroxene-rich host is transected by elongated patches and bands of garnet–clinopyroxene granulite. Garnet–clinopyroxene granulite, together with two-pyroxene granulite, has been partly replaced by amphibolite (hornblende±garnet+plagioclase+quartz [3]). The garnet-bearing assemblage [2] is expressed by a compression–dehydration reaction: hornblende+orthopyroxene+plagioclase=garnet+clinopyroxene+quartz+H₂O↑. Subsequent amphibolitization to form the assemblage [3] is expressed by two hydration reactions: garnet+clinopyroxene+plagioclase+H₂O=hornblende+quartz and plagioclase+hornblende+H₂O=zoisite+chlorite+quartz. The mafic granulites include pod- and lens-shaped bodies of ultramafic granulites which consist of garnet hornblendite (garnet+hornblende+clinopyroxene [4]) associated with garnet clinopyroxenite, garnetite, and hornblendite. Field relation and comparisons in modal–chemical compositions between the mafic and ultramafic granulites indicate that the ultramafic granulites were originally intrusive rocks which dissected the protoliths of the mafic granulites and then have been metamorphosed simultaneously with the formation of garnet–clinopyroxene granulite. The results combined with isotopic ages reported elsewhere give the following tectonic constraints: (1) crustal thickening through the development of the Kohistan arc and the subsequent Kohistan–Asia collision caused the high-pressure granulite-facies metamorphism in the Jijal complex; (2) local amphibolitization of the mafic granulites occurred after the collision.     

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.     

Silica-undersaturated sapphirine, spinel and kornerupine granulite facies rocks, NE Strangways Range, Central Australia

Small pods of silica-undersaturated Al-rich and Mg-rich granulite facies rocks containing sapphirine, pleonastic spinel, kornerupine, cordierite, orthopyroxene, corundum, sillimanite and gedrite are scattered throughout the NE Strangways Range, Central Australia. These are divided into four distinct rock types, namely orthopyroxene-rich aluminous granofels and metapelitic gneisses containing sapphirine, spinel or kornerupine. Two granulite facies metamorphic events are recognized, of which only the first (M1) is considered in this paper. Peak metamorphic mineral parageneses indicate that the M1 thermal maximum occurred at approximately 900–950 °C and 8–9 kbar. All samples are characterized by profuse and diverse coronitic and symplectic reaction textures. These are interpreted as evidence for the sequential crossing of the following reactions in the system FMAS: cordierite + spinel + corundum = sapphirine + sillimanite, cordierite + spinel = orthopyroxene + sapphirine + sillimanite, sapphirine + spinel + sillimanite = orthopyroxene + corundum, sapphirine + sillimanite = cordierite + orthopyroxene + corundum. Phase stability relationships in FMAS and MASH indicate an anticlockwise P–T path terminated by isobaric cooling. Such a path is exemplified by early low-P mineral parageneses containing spinel, corundum and gedrite and the occurrence of both prograde and retrograde corundum. Reaction textures preserve evidence for an increase in aH₂O and aB₂O₃ with progressive isobaric cooling. This hydrous retrogression resulted from crystallization of intimately associated M1 partial melt segregations. There is no evidence for voluminous magmatic accretion giving rise to the high M1 thermal gradient. The M1 P–T path may be the result of either lithospheric thinning after both crustal thickening and burial of the supracrustal terrane, or concomitant crustal thickening and mantle lithosphere thinning.     

Metastable growth of corundum adjacent to quartz in a spinel-bearing quartzite from the Archaean Napier Complex, Antarctica

In a granulite-facies spinel-bearing quartzite, corundum, orthopyroxene and sapphirine (and rarely cordierite and sillimanite) form partial rims separating spinel from quartz. Textures indicate the reactions:
spinel + quartz = orthopyroxene + corundum, and
spinel + quartz = orthopyroxene + sapphirine.
Thus, corundum and sapphirine are produced by reactions involving quartz. The low Al-content of the orthopyroxene (0.5–2.8 wt %) and low values for Mg–Fe distribution coefficient for spinel–sapphirine and spinel–orthopyroxene reflect low-temperature conditions during formation of the reaction products. Absence of zoning in spinel and a constant Mg–Fe distribution coefficient for spinel–sapphirine and spinel–orthopyroxene, over a compositional range, indicate Mg–Fe equilibration. It is suggested that stable reactions such as spinel + quartz = cordierite or spinel + quartz = garnet + sillimanite were over-stepped and that metastable reactions give rise to the anomalous juxtaposition of corundum + quartz.