Cordierite-bearing schists and gneisses from Timor, eastern Indonesia: P-T conditions of metamorphism and tectonic implications

In the Boi Massif of Western Timor the Mutis Complex, which is equivalent to the Lolotoi Complex of East Timor, is composed of two lithostratigraphical components: various basement schists and gneisses; and the dismembered remnants of an ophiolite. Cordierite-bearing pelitic schists and gneisses carry an early mineral assemblage of biotite + garnet + plagioclase + Al-silicate, but contain no prograde muscovite; sillimanite occurs in a textural mode which suggests that it replaced and pseudomorphed kyanite at an early stage and some specimens of pelitic schist contain tiny kyanite relics in plagioclase. Textural relations between, and mineral chemistries of, ferro-magnesian phases in these pelitic chists and gneisses suggest that two discontinuous reactions and additional continuous compositional changes have been overstepped, possibly with concomitant anatexis, as a result of decrease in P_load during high temperature metamorphism. The simplified reactions are: garnet and/or biotite + sillimanite + quartz + cordierite + hercynite + ilmenite + excess components. P-T conditions during the development of the early mineral assemblage in the pelitic gneisses are estimated to have been P + 10 kbar and T > 750°C, based upon the plagioclase-garnet-Al-silicate-quartz geobarometer and the garnet-biotite geothermometer. P-T conditions during the subsequent development of cordierite-bearing mineral assemblages in the pelitic gneisses are estimated to have been P + 5 kbar and T + 700°C with X_H2O < 0.5, based upon the Fe content of cordierite occurring in the assemblage quartz + plagioclase + sillimanite + biotite + garnet + cordierite coexisting with melt. Final equilibration between some of the phases suggests that conditions dropped to P > 2.3 kbar and T > 600°C. A similar exhumation P-T path is suggested for the pelitic schists with early metamorphic conditions of P > 6.2 kbar and T > 745°C and subsequent development of cordierite under conditions in the range P = 3-4 kbar and T = 600-700°C. The tectonic implications of these P-T estimates are discussed and it is concluded that the P-T path followed by these rocks was caused by decompression during rifting and synmetamorphic ophiolite emplacement resulting from processes during the initiation and development of a convergent plate junction located in Southeast Asia during late Jurassic to Cretaceous time.     

Convergent phase equilibria at the massif anorthosite—granulite interface near Bolangir,Orissa, India and thermal evolution of a part of the Indian shield

This investigation is based on detailed geological mapping of the western part of the Bolangir anorthosite massif of Orissa, India and its granulite borders, detailed petrography, whole-rock chemistry, mineralogy and an equilibrium thermodynamic analysis of the mineral phase relations. Structural analysis of the foliations of the granulites and the primary flow layers and the joint system of the anorthositic rocks strongly indicates that the pluton was forcefully intruded into the granulitic cover with considerable stretching and extension and approached the form and structure of a schlieren dome. The anorthositic suite of rocks includes anorthositic norites (median plagioclase composition An₇₅), noritic anorthosites (median plagioclase composition An₇₀) and anorthosites (median plagioclase composition An₅₂), while the bordering granulites include leptynites (K-feldspar + plagioclase + quartz + orthopyroxene + biotite + garnet + ilmenite), khondalites (K-feldspar + quartz + sillimanite + garnet + graphite + ilmenite ± biotite), basic granulites (plagioclase + diopsidic clinopyroxene + orthopyroxene + garnet + hornblende + ilmenite ± K-feldspar ± quartz ± magnetite) and calc-granulites (diopside + scapolite + calcite + garnet + microline + quartz + sphene ± magnetite ± apatite). The anorthositic rocks have a relatively high K₂O/SiO₂ ratio with the MgO/FeO ratio mainly between 1 and 2. The MgO/FeO vs. plagioclase/mafics relations of the anorthositic suite indicate the fractionation trend: anorthositic norite → noritic anorthosite → anorthosite.The calculated P−T curves for nine different mineral equilibria in the anorthosites and the granulites converge to a broad cluster within the region, 3–7 kbar and 600–740°C. The orthopyroxene—garnet thermometer (Harley) and orthopyroxene—plagioclase—garnet—quartz barometer (Newton and Perkins) restrict the convergence to a slightly narrower P−T region: 4.7–7 kbar and 620–740°C. The two-pyroxene equilibria were possibly quenched at a somewhat higher temperature region and the temperature spread of the order of 200°C at pressures between 4.7 and 7 kbar may represent the P−T path over which the pluton cooled in the final stage of its evolution. The parent magma of the anorthosites, believed to be coeval with the 1312 Ma old Chilka Lake massif anorthosites of Orissa, may have formed under a Proterozoic continental crust of well over 20 km thickness.     

Oxygen Fugacity and Water Activity during Thermal Peak and Retrogression of Granulites around the Sarvapuram Area,Karimnagar Granulite Terrane,Andhra Pradesh,India

The investigated area around Sarvapuram represents a part of the Karimnagar granulite terrane of the Eastern Dharwar Craton, India. Garnet–bearing gneiss is hosted as enclaves, pods within granite gneiss and charnockite. It is largely made up of garnet, orthopyroxene, cordierite, biotite, plagioclase, K–feldspar, sillimanite and quartz. The peak metamorphic stage is represented by the equilibrium mineral assemblage i.e. garnet, orthopyroxene, cordierite, biotite, plagioclase, sillimanite and quartz. Breakdown of the garnet as well as preservation of the orthopyroxene–cordierite symplectite, formation of cordierite with the consumption of the garnet + sillimanite + quartz represents the decompressional event. The thermobarometric calculations suggest a retrograde P–T path with a substantial decompression of c. 3.0 kbar. The water activity(XH2 O) conditions obtained with the win TWQ program for core and symplectite compositions from garnet–bearing gneiss are 0.07–0.14 and 0.11–0.16 respectively. The quantitative estimation of oxygen fugacity in garnet–bearing gneiss reveal log f O2 values ranging from-11.38 to-14.05. This high oxidation state could be one of the reasons that account for the absence of graphite in these rocks.     

Pro- and retrograde P–T evolution of granulites of the Beit Bridge Complex (Limpopo Belt, South Africa): constraints from quantitative phase diagrams and geotectonic implications

Interpretations based on quantitative phase diagrams in the system CaO–Na₂O–K₂O–TiO₂–MnO–FeO–MgO–Al₂O₃–SiO₂–H₂O indicate that mineral assemblages, zonations and microstructures observed in migmatitic rocks from the Beit Bridge Complex (Messina area, Limpopo Belt) formed along a clockwise P–T path. That path displays a prograde P–T increase from 600 °C/7.0 kbar to 780 °C/9–10 kbar (pressure peak) and 820 °C/8 kbar (thermal peak), followed by a P–T decrease to 600 °C/4 kbar. The data used to construct the P–T path were derived from three samples of migmatitic gneiss from a restricted area, each of which has a distinct bulk composition: (1) a K, Al‐rich garnet–biotite–cordierite–sillimanite–K‐feldspar–plagioclase–quartz–graphite gneiss (2) a K‐poor, Al‐rich garnet–biotite–staurolite–cordierite–kyanite–sillimanite–plagioclase–quartz–rutile gneiss, and (3) a K, Al‐poor, Fe‐rich garnet–orthopyroxene–biotite–chlorite–plagioclase–quartz–rutile–ilmenite gneiss. Preservation of continuous prograde garnet growth zonation demonstrates that the pro‐ and retrograde P–T evolution of the gneisses must have been rapid, occurring during a single orogenic cycle. These petrological findings in combination with existing geochronological and structural data show that granulite facies metamorphism of the Beit Bridge metasedimentary rocks resulted from an orogenic event during the Palaeoproterozoic (c. 2.0 Ga), caused by oblique collision between the Kaapvaal and Zimbabwe Cratons. Abbreviations follow Kretz (1983 ).     

The Imataca Complex near Cerro Bolivar,Venezuela — a calc-alkaline Archean protolith

The Imataca Complex in an area near Cerro Bolivar, Venezuela, consists of a conformable, predominantly acid, compositionally-intergradational, sequence of acid, intermediate and mafic granulites, granitic gneisses and amphibolites with minor iron-formation and other metasedimentary interlayers.Major- and trace-element compositions of granulites, granitic gneisses, and amphibolites, and compositional relations in pyroxenes and irontitanium oxides indicate an igneous protolith. Pyroxenes and oxides in the granulites appear to be relict igneous phases, and record Tf_O2 of original igneous crystallization, despite high-grade metamorphism. Mineral thermometers in granitic gneisses (biotite, FeTi oxides) and in metasedimentary biotite gneisses (cordieritegarnet, FeTi oxides) indicate metamorphic TP_T of 625–675°C, 4–6.5 kbar.Major- and trace-element variations in meta-igneous granulites, granitic gneisses, and amphibolites are calc-alkaline in character, and the sequence, particularly in its predominantly acid composition, most closely resembles continental (i.e., “Andean”) calc-alkaline series. Mafic granulites and amphibolites, however, are tholeiitic and relatively iron-rich.The compositional similarity between the Imataca series and other granulite series raises the possibility that acidic calc-alkaline continental volcanism may be more important in the Archean than recent emphasis on greenstones and island-arc oceanic types of volcanism would suggest.     

A multi-stage pressure—temperature record in the Chilka Lake granulites: the epitome of the metamorphic evolution of Eastern Ghats,India?

Abstract Metapelitic and charnockitic granulites exposed around Chilka Lake in the northern sector of the Eastern Ghats, India, preserve a multi-stage P—T record. A high-T decompression from above 10 kbar to 8 kbar around 1100°C has been determined from Mg-rich metapelites (X_Mg>0.60) with quartz-cordierite-orthopyroxene-sillimanite and cordierite—orthopyroxene—sapphirine—spinel assemblages. Between this and a second decompression to 6.0 kbar, isobaric cooling from 830 to 670°C at 8 kbar is evident. These changes are registered by the rim compositions of orthopyroxene and garnet in charnockites and metapelites with an orthopyroxene—quartz—garnet—plagioclase—cordierite assemblage, and are further supported by the garnet + quartz ± orthopyroxene + cordierite and biotite-producing reactions in sapphirine-bearing metapelites. Another indication of isobaric cooling from 800 to 650°C at 6.0 kbar is evident from rim compositions of orthopyroxene and garnet in patchy charnockites. Two sets of P—T values are obtained from metapelites with a quartz—plagioclase—garnet—sillimanite—cordierite assemblage: garnet and plagioclase cores yield 6.2 kbar, 700°C and the rims 5 kbar, 650°C, suggesting a third decompression. The earliest deformation (F1) structures are preserved in the larger charnockite bodies and the metapelites which retain the high P—T record. The effects of post-crystalline F2 deformation are observed in garnet megacrysts formed during or prior to F1 in some metapelites. Fold styles indicate a compressional regime during F1 and an extensional regime during F2. These lines of evidence and two phases of cooling at different pressures point to a discontinuity after the first cooling, and imply reworking. Two segments of the present P—T path replicate parts of the P—T paths suggested for four other granulite terranes in the Eastern Ghats, and the sense of all the paths is the same. This, plus the signature of three phases of deformation identified in the Eastern Ghats, suggests that the Chilka Lake granulites could epitomize the metamorphic evolution of the Eastern Ghats.     

P–T–t evolution of spinel–cordierite–garnet gneisses from the Sauwald Zone (Southern Bohemian Massif, Upper Austria): is there evidence for two independent late-Variscan low-P/high-T events in the Moldanubian Unit?

The Sauwald Zone, located at the southern rim of the Bohemian Massif in Upper Austria, belongs to the Moldanubian Unit. It exposes uniform biotite + plagioclase ± cordierite paragneisses that formed during the post-collisional high-T/low-P stage of the Variscan orogeny. Rare metapelitic inlayers contain the mineral assemblage garnet + cordierite + green spinel + sillimanite + K-feldspar + plagioclase + biotite + quartz. Mineral chemical and textural data indicate four stages of mineral growth: (1) peak assemblage as inclusions in garnet (stage 1): garnet core + cordierite + green spinel + sillimanite + plagioclase (An_35–65); (2) post-peak assemblages in the matrix (stages 2, 3): cordierite + spinel (brown-green and brown) ± sillimanite ± garnet rim + plagioclase (An_10–45); and (3) late-stage growth of fibrolite, muscovite and albite (An_0–15) during stage 4. Calculation of the P–T conditions of the peak assemblage (stage 1) yields 750–840°C, 0.29–0.53 GPa and for the stage 2 matrix assemblage garnet + cordierite + green spinel + sillimanite + plagioclase 620–730°C, 0.27–0.36 GPa. The observed phase relations indicate a clockwise P–T path, which terminates below 0.38 GPa. The P–T evolution of the Sauwald Zone and the Monotonous Unit are very similar, however, monazite ages of the former are younger (321 ± 9 Ma vs. 334 ± 1 Ma). This indicates that high-T/low-P metamorphism in the Sauwald Zone was either of longer duration or there were two independent phases of late-Variscan low-P/high-T metamorphism in the Moldanubian Unit.     

The P–T record of synchronous magmatism, metamorphism and deformation at Petrel Cove, southern Adelaide Fold Belt

Porphyroblastic schists in the thermal aureole of the Victor Harbor Granite at Petrel Cove, in the southern Adelaide Fold Belt, South Australia, preserve a record of sequential cordierite, andalusite, staurolite, fibrolite, chlorite and muscovite growth (along with biotite+plagioclase+quartz+ilmenite) during progressive deformation. A P–T pseudo‐section appropriate to biotite‐saturated assemblages in KFMASH shows that the sequence of mineral reactions records increasing pressure of at least 1 kbar (from c. 3 to c. 4 kbar) during cooling from around 580 °C. Heating at pressures below c. 3 kbar is inferred for growth of early formed cordierite porphyroblasts, and is attributed in part to the thermal effects of granite emplacement, while the pressure increase is attributed to tectonic burial accruing from ongoing deformation. The ‘anticlockwise’P–T path is consistent with convergent deformation being focussed as a consequence of heating, as to be expected for a lithospheric rheology that is strongly temperature dependent.     

Petrology of garnet — cordierite — sillimanite gneisses from the El Tormes thermal dome,Iberian Hercynian foldbelt (W Spain)

The core of the El Tormes thermal dome, situated in the central part of one of the main metamorphic belts of the Iberian Peninsula, is formed by garnet-cordierite-biotite-sillimanite pelitic gneisses. These rocks, that very often are cut by minor intrusions of Al-rich S-type granites, are metatexitic gneisses in which there exists garnet showing different stages of resorption and transformation into an aggregate of cordierite±plagioclase±biotite. The garnet, mantled and corroded mainly by cordierite, has never been found to occur in contact with the prismatic sillimanite of the matrix, thus indicating that the continuous reaction Gr+Sill+Q = Cd has taken place. The presence of corroded biotite inside the garnet-rimming cordierite of the aggregates as well as inside the cordierite of the matrix, which usually includes remmants of sillimanite, indicates that the continuous reaction Bi+Sill+Q = Cd+FK+H₂O has occurred too. Therefore, a realistic net reaction for these aggretates should be represented by the univariant, at a given , equilibrium: Biotite+Sillimanite+Garnet+Quartz = Cordierite+K-feldspar+H₂O (1)The important garnet resorption near the anatectic granitic veins implies that this process is favoured by a decrease in , this factor being otherwise buffered by the reaction (1) assemblage.The most probable P-T path, assuming these conditions, consistent with the AFM projection of the former (inferred) and present assemblages in the aggregates and in the matrix, implies a decrease in P coeval with a decrease in T (Fig. 4, path 2).The most reliable P-T determination for the final stage of garnet breakdown through reaction (1), based on the coexistence of the seven phase assemblage garnet — cordierite — biotite — sillimanite — plagioclase — potash feldspar — quartz plus melt, gives 695° C, 4.3 kbar, = 0.5, The maximum pressure for this process, obtained from the garnet — plagioclase equilibrium, is 6.5±1 kbar at the same temperature.The estimates of the T for the garnet core-garnet included biotite pairs are consistently lower, ca. 550° C, than those obtained for the garnet rim-biotite in aggregates, ca. 645° C, or garnet rim-adjacent cordierite pairs, ca. 695° C.It may, therefore, be supposed that, during their evolution these rocks underwent first an increase in T and then, during the last stages, as garnet and biotite brokedown, a decrease in P and T. This represents an uplift of the core of El Tormes dome under high grade amphibolite to low pressure granulite facies conditions, accompanied by a process of partial melting with local decrase in . It is suggested, from mineral growth-deformation relationships, that this process took place during the late hercynian deformation phases, P-3 or doming stage.     

Metamorphic history of eclogites and country rock gneisses in the Aktyuz area,Northern Tien‐Shan,Kyrgyzstan: a record from initiation of subduction through to oceanic closure by continent–continent collision

Eclogites and related high‐P metamorphic rocks occur in the Zaili Range of the Northern Kyrgyz Tien‐Shan (Tianshan) Mountains, which are located in the south‐western segment of the Central Asian Orogenic Belt. Eclogites are preserved in the cores of garnet amphibolites and amphibolites that occur in the Aktyuz area as boudins and layers (up to 2000 m in length) within country rock gneisses. The textures and mineral chemistry of the Aktyuz eclogites, garnet amphibolites and country rock gneisses record three distinct metamorphic events (M1–M3). In the eclogites, the first MP–HT metamorphic event (M1) of amphibolite/epidote‐amphibolite facies conditions (560–650 °C, 4–10 kbar) is established from relict mineral assemblages of polyphase inclusions in the cores and mantles of garnet, i.e. Mg‐taramite + Fe‐staurolite + paragonite ± oligoclase (An_<16) ± hematite. The eclogites also record the second HP‐LT metamorphism (M2) with a prograde stage passing through epidote‐blueschist facies conditions (330–570 °C, 8–16 kbar) to peak metamorphism in the eclogite facies (550–660 °C, 21–23 kbar) and subsequent retrograde metamorphism to epidote‐amphibolite facies conditions (545–565 °C and 10–11 kbar) that defines a clockwise P–T path. thermocalc (average P–T mode) calculations and other geothermobarometers have been applied for the estimation of P–T conditions. M3 is inferred from the garnet amphibolites and country rock gneisses. Garnet amphibolites that underwent this pervasive HP–HT metamorphism after the eclogite facies equilibrium have a peak metamorphic assemblage of garnet and pargasite. The prograde and peak metamorphic conditions of the garnet amphibolites are estimated to be 600–640 °C; 11–12 kbar and 675–735 °C and 14–15 kbar, respectively. Inclusion phases in porphyroblastic plagioclase in the country rock gneisses suggest a prograde stage of the epidote‐amphibolite facies (477 °C and 10 kbar). The peak mineral assemblage of the country rock gneisses of garnet, plagioclase (An_11–16), phengite, biotite, quartz and rutile indicate 635–745 °C and 13–15 kbar. The P–T conditions estimated for the prograde, peak and retrograde stages in garnet amphibolite and country rock are similar, implying that the third metamorphic event in the garnet amphibolites was correlated with the metamorphism in the country rock gneisses. The eclogites also show evidence of the third metamorphic event with development of the prograde mineral assemblage pargasite, oligoclase and biotite after the retrograde epidote‐amphibolite facies metamorphism. The three metamorphic events occurred in distinct tectonic settings: (i) metamorphism along the hot hangingwall at the inception of subduction, (ii) subsequent subduction zone metamorphism of the oceanic plate and exhumation, and (iii) continent–continent collision and exhumation of the entire metamorphic sequences. These tectonic processes document the initial stage of closure of a palaeo‐ocean subduction to its completion by continent–continent collision.     

Cordierite-Cummingtonite Facies Rocks from the Gold Brick District, Colorado

The unusual association of cordierite and cummingtonite (? gedrite+ chlorite + biotite + ilmenite + plagioclase + quartz) definesa metamorphic facies within aluminous, low-Ca amphibolites fromthe Proterozoic rocks of the Gold Brick District, east of Gunnison,Colorado. More Fe-rich bulk chemistries in the same facies arecharacterized by assemblages consisting of cordierite+-gedrite+ garnet + chlorite + biotite + ilmenite + plagioclase + quartz,whereas more Mg-rich compositions are characterized by cordierite+ anthophyllite + chlorite + biotite + ilmenite ? plagioclase+ quartz. The assemblage gedrite 4- cummingtonite + chlorite+ biotite + ilmenite + plagioclase + quartz was also observed.Coexisting cordierite+ anthophyllite + cummingtonite was notobserved in any rocks, apparently because this assemblage isstable over only a very narrow range of bulk compositions. Metamorphosedpelitic rocks are more iron rich than the assemblage cordierite+ gedrite + garnet + chlorite + biotite + ilmenite + plagioclase+ quartz and consist of garnet ?cordierite ?staurolite ? chlorite? andalusite + biotite + ilmenite + plagioclase + quartz? microclineor muscovite. Mineral rim compositions from cordierite-bearing amphibolitesand metapelites determined by electron microprobe analysis showsystematic Fe/Mg partitioning and define assemblages that occupynon-overlapping regions of the compositional system SiO₂-TiO₂-Al₂O₃-MnO-FeO-MgO-CaO-Na₂O-K₂O-H₂Oas determined by algebraic and statistical methods developedby Braun & Stout (1975) and Fisher (1989). Graphical methods(projections) produced spurious overlaps not confirmed by themore rigorous algebraic tests. The spurious overlaps were generatedbecause standard projective analysis was not able simultaneouslyto account for the important effects of the components Na₂O,CaO, and MnO on the AFM topologies. The results of algebraicand statistical analysis are consistent with an equilibriumorigin at constant values of temperature and pressure. The cordierite-cummingtonite facies encompasses the relativelylow-pressure and moderate-temperature conditions associatedwith the stability field of andalusite. Garnet-biotite geothermo-metry,and garnet, aluminosilicate, silica, plagioclase (GASP) geobarometrysuggest that temperatures and pressures were nearly constantacross the study area at 550( ? 70) ?C and 3 kb, respectively,near the peak of metamorphism. Other geothermometers and geobarometers,and independent pressure and temperature estimates, are compatiblewith garnet-biotite thermometry and GASP geo-barometry. Gradientsin fO₂ or H₂O are not required to explain the compatibilityof these assemblages at constant T and P. Cordierite + cummingtonite-bearingrocks can apparently be derived from anthophyllite +garnet-bearingrocks by increasing temperature or decreasing pressure.     

High-pressure phengite decomposition in the Weissenstein eclogite,Münchberger Gneiss Massif,Germany

The petrography, mineral chemistry and petrogenesis of a sample from the Weissenstein eclogite, Bavaria, Germany, has been investigated. The total mineral assemblage comprises garnet, clinopyroxene_I+II, quartz, amphibole_I+II, rutile, phengite, epidote/allanite, plagioclase, biotite, apatite, pumpellyite, titanite (sphene), zircon, alkali feldspar and calcite. Textural observations combined with geothermobarometry (Fe/Mg distribution between clinopyroxene/garnet and phengite/garnet; jadeite-content of omphacite, Si-content of phengite, and An-content of plagioclase) provide indications of two different stages in the metamorphic evolution of the rock. The main phengitequartz-eclogite mineral equilibration occurred at minimum P=13–17kbar, minimum T=620±50° C; the retrograde symplectite stage (clinopyroxene_II, amphibole_II, biotite, plagioclase) occurred at P _total between 12 and 8.5 kbar. Reactions of the symplectite stage are:

1. phengite (core) + Na₂O_aq + CaO_aq=phengite (rim) + biotite + plagioclase + K₂O_aq + H₂O
2. phengite (core) + clinopyroxene_I + Na₂O_aq=phengite (rim + biotite + plagioclase + amphibole_II + SiO₂ + K₂O_aq + CaO_aq + H₂O
3. clinopyroxene_I + SiO₂ + K₂O_aq + H₂O=clinopyroxene_II + plagioclase+amphibole_II + Na₂O_aq + CaO_aq

The phengite decomposition produces H₂O, whereas the clinopyroxene decomposition consumes H₂O. The estimated P-T-conditions for the Weissenstein eclogite are in the same order of magnitude as those for other eclogite bodies from the Alps and Caledonides believed to be related to subduction processes.     

Tectonothermal evolution of the gneiss complex at Salur in the Eastern Ghats Grnaulite Belt of India

Textural relations, thermobarometry and petrogenetic grid considerations in the syn-tectonic granitoid massif and the enveloping metasedimentary gneisses at Salur are consistent with a counter-clockwise P–T –t path for the rocks. The low-P/high-T prograde sector is documented by the pre- to syn-D₁ cordierite±orthopyroxene±garnet±spinel–bearing metatexite leucosomes in metapelites. Heating and loading of the rocks (syn- to post-D₁) resulted in the formation of garnet+orthopyroxene± cordierite-bearing diatexites, and decomposition of cordierite in metatexite leucosomes to orthopyroxene+sillimanite+biotite+quartz symplectites. Near-peak temperature, 850 °C at 8.0 kbar, was reached syn- to post-D₂. Post-peak cooling resulted in the stabilization of coronal grossular and anorthite+calcite symplectites at the expense of scapolite+wollastonite+calcite assemblages in calc-silicate gneisses, and the resetting of cation exchange temperatures at 700–750 °C. Near-isothermal decompression at c. 700–750 °C is manifested by the decomposition of garnet porphyroblasts in the granitoid gneisses to plagioclase+orthopyroxene/ilmenite/biotite two-phase coronas and restabilization of cordierite at garnet margins in metapelites. Subsequent low-P, near-isobaric cooling led to the overprinting of granulite facies assemblages by muscovite+calcite assemblages, and further resetting of cation exchange thermometers to lower temperatures c. 600 °C. The tectonothermal evolution of the Salur gneiss complex vis-a-vis the Eastern Ghats Belt is therefore consistent with high degrees of lower crustal melting, followed by prograde heating of the cover rocks due to magma invasion synchronous with crustal compression, and finally thermal relaxation over a protracted period punctuated by tectonic/erosional denudation of the thickened crust.     

Cordierite gneisses of southern Kerala,India: petrology,fluid inclusions and implications for crustal uplift history

The cordierite-bearing gneisses occurring as elongate patches in an 8- to 10-km-wide zone along the Achankovil fault-lineament at the northern margin of the southern Kerala crustal segment represent an important lithological unit in the Archaean granulite terrane of south India. The textural relationships in these rocks are consistent with the following main reactions: (1) garnet+quartz=cordierite+hypersthene; (2) garnet+sillimanite+quartz=cordierite; (3) hypersthene+sillimanite+quartz=cordierite; (4) sillimanite+spinel=cordierite+corundum; and (5) biotite+quartz+sillimanite=cordierite+K-feldspar. Many of the mineral associations and reaction textures, including the remarkable preservation of symplectites, are indicative of partial replacement of high-pressure assemblages by cordierite-bearing lower-pressure ones during an event of rapid decompression. Temperature estimates from coexisting mineral phases show 710° (garnet-biotite), 791° (garnet-cordierite) and 788° C (garnet-orthopyroxene). Pressure estimates from mineral assemblages range from 5.4 to 7 kb. Detailed fluid inclusion studies in quartz associated with cordierite show high-density CO₂ (0.80–0.95 g/cm³) as the dominant fluid phase, with traces of probable CH₄ (?) in the sillimanite-bearing rocks. The isochore for the higher-density fluid inclusions defines a pressure of 5.5 kb. The fracture-bound CO₂ and CO₂-H₂O (±CH₄?) inclusions indicate simultaneous entrapment at 400° C and 1.7 kb in the cordierite-hypersthene assemblage and 340° C and 1.2 kb in the cordierite-sillimanite assemblage. The P-T path delineated from combined solid and fluid data corresponds to the piezothermic array of the gneisses and is characterized by T-convex nature, indicative of rapid and virtually isothermal crustal uplift, probably aided by extensional tectonics.     

Reaction Textures in a Suite of Spinel Granulites from the Eastern Ghats Belt, India: Evidence for Polymetamorphism, a Partial Petrogenetic Grid in the System KFMASH and the Roles of ZnO and Fe2O3

Spinel granulites, with or without sapphirine, occur as lensesin garnetiferous quartzofeldspathic gneisses (leptynites) nearGokavaram in the Eastern Ghats Belt, India. Spinel granulitesare mineralogically heterogeneous and six mineral associationsoccur in closely spaced domains. These are (I) spinel–quartz–cordierite,(II) spinel–quartz–cordierite–garnet–orthopyroxene–sillimanite,(III) spinel–cordierite–orthopyroxene–sillimanite,(IV) spinel–quartz–sapphirine–sillimanite–garnet,(V) spinel–quartz-sapphirine–garnet and (IV) rhombohedral(Fe–Ti) oxide–cordierite–orthopyroxene–sillimanite.Common to all the associations are a porphyroblastic garnet(containing an internal schistosify defined by biotite, sillimaniteand quartz), perthite and plagioclase. Spinel contains variableamounts of exsolved magnetite and is distinctly Zn rich in thesapphirine-absent associations. X_Mg in the coexisting phasesdecreases in the order cordierite–biotite–sapphirine–orthopyroxene–spinel–garnet–(Fe–Ti)oxides. Textural criteria and compositional characteristicsof the phases document several retrograde mineral reactionswhich occurred subsequent to prograde dehydration melting reactionsinvolving biotite, sillimanite, quartz, plagioclase and spinel.The following retrograde mineral reactions are deduced: (1)spinel + quartz cordierite, (2) spinel + quartz garnet + sillimanite,(3) garnet + quartz cordierite + orthopyroxene, (4) garnet+ quartz + sillimanite cordierite, (5) spinel + cordierite orthopyroxene + sillimanite, (6) spinel + sillimanite + quartz sapphirine, (7) spinel + sapphirine + quartz garnet + sillimanite,and (8) spinel + quartz sapphirine + garnet. A partial petrogeneticgrid for the system FeO–MgO–Al₂O₃–SiO₂–K₂O–H₂Oat high fo₂, has been constructed and the effects of ZnO andFe₂O₃ on this grid have been explored Combining available experimentaland natural occurrence data, the high fo₂ invariant points inthe partial grid have been located in P–T space. Geothermobarometricdata and consideration of the deduced mineral reactions in thepetrogenetic grid show that the spinel granulites evolved throughan anticlockwise P–T trajectory reaching peak metamorphicconditions >9 kbar and 950C, followed by near-isobaric cooling(dT/dP = 150C/kbar). This was superimposed by an event of near-isothermaldecompression (dT/dP = 15C/kbar). The studied spinel granulites,therefore, preserve relic prograde mineral associations andreaction textures despite being metamorphosed at very high temperatures,and bear evidence of polymetamorphism. KEY WORDS: spinel granulite; Eastern Ghats; India; polymetamorphism; geothermometry; geobarometry Corresponding author     

P–T conditions of decompression of the Limpopo high-grade terrane: record from shear zones

The Southern Marginal Zone of the late Archean Limpopo Belt of southern Africa is an example of a high‐grade gneiss terrane in which both upper and lower crustal deformational processes can be studied. This marginal zone consists of large thrust sheets of complexly folded low‐strain gneisses, bound by an imbricate system of kilometre‐wide deep crustal shear zones characterized by the presence of high‐strain gneisses (‘primary straight gneisses’). These shear zones developed during the decompression stage of this high‐grade terrane. Low‐ and high‐strain gneisses both contain similar reaction textures that formed under different kinematic conditions during decompression. Evidence for the early M1/D1 metamorphic phase (> 2690 Ma) is rarely preserved in low‐strain gneisses as a uniform orientation of relict Al‐rich orthopyroxene in the matrix and quartz and plagioclase inclusions in the cores of early (M1) Mg‐rich garnet porphyroblasts. This rare fabric formed at > 820 °C and > 7.5 kbar. The retrograde M2/D2 metamorphic fabric (2630–2670 Ma) is well developed in high‐strain gneisses from deep crustal shear zones and is microscopically recognized by the presence of reaction textures that formed synkinematically during shear deformation: M2 sigmoid‐shaped reaction textures with oriented cordierite–orthopyroxene symplectites formed after the early M1 Mg‐rich garnet porphyroblasts, and syn‐decompression M2 pencil‐shaped garnet with oriented inclusions of sillimanite and quartz formed after cordierite under conditions of near‐isobaric cooling at 750–630 °C and 6–5 kbar. The symplectites and pencil‐shaped garnet are oriented parallel to the shear fabric and in the stretching direction. Low‐strain gneisses from thrust sheets show similar M2 decompression cooling and near‐isobaric cooling reaction textures that formed within the same P–T range, but under low‐strain conditions, as shown by their pseudo‐idioblastic shapes that reflect the contours of completely replaced M1 garnet and randomly oriented cordierite–orthopyroxene symplectites. The presence of similar reaction textures reflecting low‐strain conditions in gneisses from thrust sheets and high‐strain conditions in primary straight gneisses suggests that most of the strain during decompression was partitioned into the bounding shear zones. A younger M3/D3 mylonitic fabric (< 2637 Ma) in unhydrated mylonites is characterized by brittle deformation of garnet porphyroclasts and ductile deformation of the quartz–plagioclase–biotite matrix developed at < 600 °C, as the result of post‐decompression shearing under epidote–amphibolite facies conditions.     

Metapelitic granulites from Jetty Peninsula,east Antarctica: formation during a single event or by polymetamorphism?

Granulite facies metasedimentary gneiss exposed on Jetty Peninsula, east Antarctica, contains assemblages involving garnet-sillimanite-biotite-cordierite-spinel-ilmenite-rutile and garnet-orthopyroxene-cordierite-biotite, as well as quartz and K-feldspar. Peak assemblages involve garnet + sillimanite + ilmenite (±rutile) and garnet + orthopyroxene. P-T calculations suggest formation conditions of approximately 800d? C at 7-7.5 kbar. Cooling from peak conditions is suggested by biotite + garnet (±sillimanite) overprinting some peak assemblages. A subsequent increase in temperature is inferred from the formation of cordierite + garnet + biotite + ilmenite, garnet + sillimanite + cordierite + ilmenite and cordierite + orthopyroxene assemblages during D2. In slightly zincian bulk compositions, hercynitic spinel + cordierite + sillimanite constitutes the peak D2 assemblage. Average pressure calculations indicate peak pressures of 5.9 ±0.4 kbar at 700d? C for the cordierite-bearing D2 assemblages. Available radiometric data suggest that peak metamorphism occurred at c. 1000 Ma and D2 occurred after 940 ± 20 Ma. The following two possibilities exist for the metamorphic evolution. (1) The formation of the lower pressure cordierite-bearing assemblages is associated with a separate metamorphic event (M2), unrelated to the peak assemblage (M1), and the lower pressure assemblages have no relevance in terms of a single tectonothermal event. (2) The cordierite-bearing assemblages formed during a progression from peak conditions. In this case, the lower pressure assemblages reflect a broadly decompressional metamorphic evolution, during which temperatures fluctuated. Comparison with P-T paths from granulites of similar age in adjacent areas suggests that the second possibility should be preferred. The cooling interval between peak conditions and the development of cordierite-bearing coronas and symplectites suggests affinities with isobarically cooled granulites of similar age immediately to the west, and the low-P/high-T post-peak conditions are similar to the later stages of decompressional paths recognized in much of east Antarctica.     

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.     

Petrology and geothermobarometry of granulite facies metapelites from the Hisøy-Torungen area, south Norway: new data on the Sveconorvegian P–T–t path of the Bamble sector

ABSTRACT The high-grade rocks (metapelite, quartzite, metagabbro) of the Hisøy-Torungen area represent the south-westernmost exposures of granulites in the Proterozoic Bamble sector, south Norway. The area is isoclinally folded and a metamorphic P–T–t path through four successive stages (M1-M4) is recognized. Petrological evidence for a prograde metamorphic event (M1) is obtained from relict staurolite + chlorite + albite, staurolite + hercynite + ilmenite, cordierite + sillimanite, fine-grained felsic material + quartz and hercynite + biotite ± sillimanite within metapelitic garnet. The phase relations are consistent with a pressure of 3.6 ± 0.5 kbar and temperatures up to 750–850°C. M1 is connected to the thermal effect of the gabbroic intrusions prior to the main (M2) Sveconorwegian granulite facies metamorphism. The main M2 granulite facies mineral assemblages (quartz+ plagioclase + K-feldspar + garnet + biotite ± sillimanite) are best preserved in the several-metre-wide Al-rich metapelites, which represent conditions of 5.9–9.1 kbar and 790–884°C. These P–T conditions are consistent with a temperature increase of 80–100°C relative to the adjacent amphibolite facies terranes. No accompanying pressure variations are recorded. Up to 1-mm-wide fine-grained felsic veinlets appear in several units and represent remnants of a former melt formed by the reaction: Bt + Sil + Qtz→Grt + lq. This dehydration reaction, together with the absence of large-scale migmatites in the area, suggests a very reduced water activity in the rocks and XH₂O = 0.25 in the C–O–H fluid system was calculated for a metapelitic unit. A low but variable water activity can best explain the presence or absence of fine-grained felsic material representing a former melt in the different granulitic metapelites. The strongly peraluminous composition of the felsic veinlets is due to the reaction: Grt +former melt ± Sil→Crd + Bt ± Qtz + H₂O, which has given poorly crystalline cordierite aggregates intergrown with well-crystalline biotite. The cordierite- and biotite-producing reaction constrains a steep first-stage retrograde (relative to M2) uplift path. Decimetre- to metre-wide, strongly banded metapelites (quartz + plagioclase + biotite + garnet ± sillimanite) inter-layered with quartzites are retrograded to (M3) amphibolite facies assemblages. A P–T estimate of 1.7–5.6 kbar, 516–581°C is obtained from geothermobarometry based on rim-rim analyses of garnet–biotite–plagioclase–sillimanite–quartz assemblages, and can be related to the isoclinal folding of the rocks. M4 greenschist facies conditions are most extensively developed in millimetre-wide chlorite-rich, calcite-bearing veins cutting the foliation.     

Petrology and P–T history of the Wutai amphibolites: implications for tectonic evolution of the Wutai Complex,China

The Wutai Complex represents the best preserved granite-greenstone terrane in the North China Craton. The complex comprises a sequence of metamorphosed ultramafic to felsic volcanic rocks, variably deformed granitoid rocks, along with lesser amounts of siliciclastic and carbonate rocks and banded iron formations. Petrological evidence from the Wutai amphibolites indicates four metamorphic evolutionary stages. The M₁ assemblage is composed of plagioclase+quartz+actinolite+chlorite+epidote+biotite+rutile, preserved as mineral inclusions in garnet porphyroblasts. The metamorphic conditions for this assemblage cannot be quantitatively estimated. The M₂ stage is represented by garnet porphyroblasts in a matrix of quartz, plagioclase, amphibole, biotite, rutile and ilmenite. P–T conditions for this assemblage have been estimated using the program Tweequ at 10–12 kbar and 600–650°C. The M₃ assemblage is shown by amphibole+plagioclase±ilmenite symplectic coronas around embayed garnets and yields P–T conditions of 6.0–7.0 kbar and 600–650°C. M₄ is represented by chlorite and epidote rimming garnet, chlorite rimming amphibole and epidote replacing plagioclase under greenschist-facies conditions of 400–500°C and relatively lower pressures. Taken together, the qualitative P–T estimates from M₁ and M₄ and the quantitative P–T estimates from M₂ and M₃ define a clockwise P–T path for the Wutai amphibolites.The estimated P–T path from the four stages suggests that the Wutai Complex underwent initial burial and crustal thickening (M₁+M₂), subsequent isothermal exhumation (M₃), and finally cooling and retrogression (M₄). This tectonothermal path, along with those of the Fuping and Hengshan complexes, which bound the southeast and northwest margins, respectively, of the Wutai Complex, is considered to record the early Paleoproterozoic collision between the eastern and western segments of the North China craton.