Granulite Facies Metapelites Research Articles

Adaptation of the SELECTOR-C program package for solving petrogenetic problems of metamorphic rocks

The paper presents thermodynamic models for mineral solid solutions used in physicochemical simulations with the SELECTOR-C program package (PP) in application to metamorphic mineral-forming processes. It is demonstrated that the simulated FeO and MgO distribution in the mineral pairs garnetbiotite, garnet-orthopyroxene, orthopyroxene-biotite, orthopyroxene-olivine, garnet-cordierite, garnetclinopyroxene, and clinopyroxene-orthopyroxene in the model samples satisfactorily corresponds to available experimental and empirical data. Simulations of naturally occurring mineral associations are employed to demonstrate the capabilities of the new version of the SELECTOR-C PP as a tool for studying the evolution of mineral assemblages at varying P-T conditions and fluid regime, the perfectly mobile and inert behaviors of certain fluid components during the origin of mineral associations are demonstrated, the pseudosection method applied over a broad P-T range is used to trace systematic variations in the composition of mineral associations in granulite-facies metabasites and metapelites, and the upper limit of plagioclase stability is estimated for these rocks at pressures of 11–12 kbar. Principal differences are elucidated in the effect of rocks rich and poor in Fe3+ on the percolation of metamorphic fluid through them: Fe3+-rich rocks retain their own redox potential at a certain level by buffering reactions, whereas Fe3+-poor rocks rapidly exhaust their buffer capacity and acquire the redox potential of the inflowing external fluid. This allowed us to evaluate the logfO2 at no higher than −17 (at T = 700°C and P = 6.8 kbar). Our simulation of the equilibrium of natural rock samples provides good reasons to believe that natural mineral assemblages can be formed at low fluid/rock ratios of no higher than 0.01–0.06.

A ternary feldspar-mixing model based on calorimetric data: development and application

A mixing model for high structural state ternary feldspars in the NaAlSi3O8–KAlSi3O8–CaAl2Si2O8 system is presented based exclusively on calorimetric and volumetric measurements. Comparisons with existing mixing models, which are based on phase-equilibrium experiments, reveal distinct differences. The incorporation of K into Ca-rich plagioclase and of Ca into K-rich alkali feldspar is more strongly limited by our calorimetry-based model, whereas the stability field of Na-rich feldspars is broadened. Natural feldspar assemblages from well-studied magmatic and high-grade metamorphic rocks (i.e. a teschenite sill in Scotland, the Klokken syenogabbro in Greenland, and a granulite-facies metapelite in Sri Lanka) were used to test the mixing models. It was found that the new model largely eliminates discrepancies between observed and predicted feldspar compositions that were present in earlier attempts. The reasons for the problems associated with phase-equilibrium based mixing models are discussed.

U–Pb and Sm–Nd dating of high-pressure granulites from Tcholliré and Banyo regions: Evidence for a Pan-African granulite facies metamorphism in north-central Cameroon

U–Pb (TIMS–ID and SIMS) and Sm–Nd analyses of zircons and garnet-whole rock pairs were applied on high-pressure granulite facies metapelites and metagranodiorite from Tcholliré and Banyo regions, respectively in the Adamawa–Yadé and Western Domains of the Central-African Fold Belt (CAFB) of Cameroon. Cathodoluminescence (CL) images of zircons reveal that they are made up of ubiquitous magmatitic xenocrystic cores, surrounded and/or overprinted by light unzoned recrystallized domains. U–Pb data on cores yield ages ranging from Paleoproterozoic to Neoproterozoic, which we consider as dating inheritances. Data on overgrowths and recrystallized domains give ages ranging between 594 and 604 Ma, interpreted as the time of HP granulite-facies metamorphism in the Tcholliré and Banyo regions. This is also supported by ages derived from Sm–Nd garnet-whole rock pairs. Sediments of the Tcholliré region were deposited after ca. 620 Ma from Paleoproterozoic, Mesoproteroszoic and Neoproterozoic protoliths, while those from the Banyo region were deposited after 617.6 ± 7.1 Ma essentially from Neoproterozoic protoliths.

Constraints on crustal structure and composition within a continental suture zone in the Irish Caledonides from shear wave wide-angle reflection data and lower crustal xenoliths

SUMMARY Shear wave seismic velocities (Vs) when used together with compressional wave velocities (Vp) are a powerful diagnostic of the chemical composition and mineralogy of the continental lithosphere. In this paper, we present whole crustal models of Vs and Vp velocities from two wide-angle seismic profiles from southwest Ireland, which straddle the Eastern Avalonian Terrane within the Iapetus Suture Zone. These models are based on traveltime interpretations of primary and reflected P- and S-wave seismic phases generated by explosive underwater sources. The quality of the shear wave data is exceptional when compared to similar data sets recorded elsewhere and allows the distribution of Poisson's ratio (σ) within the entire crust to be accurately determined. Models of Poisson's ratio, Vp and Vs are then used to constrain crustal composition at various depths down to the Moho (ca. 32 km depth) using published laboratory velocity measurements from rocks of different metamorphic grades and chemistry. The results indicate that the Irish crust in this region of the Caledonian orogenic belt is unusually felsic in bulk composition with a σ value of 0.247, which is below the global average value of 0.265. Variations in σ within the upper ca. 5 km of crust are greatest (0.20–0.28) and can be correlated with siliciclastic and carbonate sediments in Devonian and Carboniferous extensional sedimentary basins that were formed during the early part of the Variscan orogenic cycle. The typically more uniform variation in the mid crust down to 15 km depth is consistent with a granite/granodiorite or metagreywacke composition of greenschist to amphibolite facies mineralogy. Within the lower crust the value of σ (0.257–0.265) is significantly less than the global mean value of 0.281 predicted from an average crustal petrology model. This suggests a bulk silica content (%SiO2) of ∼64 per cent for the lower crust, based on an empirical relationship between σ and %SiO2, determined from compilations of laboratory data. These results are consistent with petrophysical, geochemical and petrological data from an unusually well-preserved lower crustal xenolith suite from the Irish Midlands, hosted in early Carboniferous volcanic rocks. The xenoliths (mainly partially melted granulite facies metapelites) evidently represent the bulk composition of the lower crust, which was largely derived by accretion of sedimentary material, derived from oceanic, island arc and continental margin sources, during oblique Caledonian collision.

Fluid and deformation induced partial melting and melt volumes in low-temperature granulite-facies metasediments, Damara Belt, Namibia

Fluid-present partial melting has generally been regarded a poor candidate for effecting crustal differentiation. In this study we report on granulite-grade metasediments from the Pan-African Damara Belt in Namibia that have undergone fluid-present biotite melting at a relatively low-temperature, yet appear to have lost a significant volume of melt. In situ anatectic features have been identified on the basis of the existence of new generations of cordierite and/or garnet produced as the solid products of incongruent anatexis within or adjacent to leucosomes. Leucosomes occur in lens-shaped pods that are orientated at high angles to the regional stretching lineation that formed during orogen-parallel extrusion of the rocks during the main collisional event in the Damara Belt. Within these sites biotite underwent incongruent melting via the reaction Bt + Qtz + Pl + H 2O = Melt + Grt + Crd. Cordierite nucleated on pre-existing crystals within the bounding gneiss; garnet nucleated within the fracture sites (leucosomes) and typically occurs as individual, large (50 to 120 mm) poikiloblastic crystals. Thermobarometry applied to the anatectic assemblage yields low-temperature, granulite-facies peak conditions of 750 °C, 0.5 GPa. This temperature is approximately 100 °C lower than the accepted conditions for the onset of fluid-absent biotite melting. This, coupled to the focussing of anatexis on dilational sites, suggests that anatexis occurred through water-present biotite incongruent melting. In order to better understand this process, both fluid-absent and water-present partial melting experiments were conducted within the temperature interval 700 to 900 °C at 0.7 GPa. In the fluid-absent experiments, biotite incongruent melting started between 800 and 850 °C to produce melt coexisting with peritectic garnet and cordierite. In contrast, in water-saturated experiments, biotite melted via the reaction Bt + Qtz + Pl + H 2O = Melt + Grt + Crd, between 700 and 750 °C, to produce melt, cordierite and garnet in the proportions 73:24:3. The garnet compositions produced in these low-temperature experiments match very well with the relatively high spessartine content (10%), low Mg# garnets formed with the leucosomes, confirming the water-saturated, low-temperature nature of anatexis in the study area. Mass balance calculations indicate that melting in typical dilational sites has produced 6 dm 3 of melt. This melt has been sourced from some 0.08 m 3 of rock, a volume that coincides well with the typical frequency of extensional fracture spacing. This was probably important in enabling efficient melt extraction which has caused a slight shift towards more refractory compositions in the granulite-facies metapelites over amphibolite-facies equivalents. However, the attendant expansion in chemical equilibration volume has resulted in a marked compositional homogenization of the high-grade gneisses over the rhythmically bedded amphibolite-facies metapelites.

Metamorphic P-T path of the eastern Trivandrum Granulite Block, southern India: implications for regional correlation of lower crustal fragments

We report here the metamorphic P-T path for granulite-facies metapelites from the eastern part of the Trivandrum Granulite Block (TGB), southern India, and compare with the paths from the western part of the block as well as from other granulite blocks and shear zones in this region for understanding of exhumation history of the southern granulite terrane. The peak temperature condition of the eastern part (900-1000 °C) is almost consistent with that from the western part, suggesting regional ultrahigh-temperature (UHT) metamorphism throughout the TGB. However, there is a slight difference in peak pressure conditions between the eastern part (12 kbar) and the western part (10 kbar) estimated from different Zn content within spinel in equilibrium with quartz. Such P ∼ 12 kbar high-pressure event is consistent with that obtained from granulite-facies rocks further north in the Palghat-Cauvery Shear Zone system (PCSZ) and Madurai Granulite Block (MGB). The results therefore suggest that the TGB, MGB, and PCSZ underwent similar prograde high-pressure metamorphism followed by peak UHT event along a clockwise P-T trajectory, related to the collisional orogeny during the assembly of the Gondwana Supercontinent.

Diffusion-controlled biotite breakdown reaction textures at the solid/liquid transition in the continental crust

Two-phase quartz intergrowths with garnet, cordierite and tourmaline occur commonly in prograde high-temperature migmatites, granulites, as well as in the last crystallization stages of biotite granites. Structural, microtextural and mineralogical data show that they result from the breakdown of biotite in the presence of a melt phase associated with incongruent dissolution of feldspars into the melt and silica release (giving quartz in silica saturated rocks). Biotite breakdown and growth of Al-rich ferromagnesian minerals, occurring at the solid–liquid transition in the crust (early melting or final crystallization), is kinetically controlled by Fe and Mg mass transport, the network-forming cations Si and Al being locally compensated for by feldspar dissolution/crystallization. This process leads to significant changes with respect to equilibrium dehydration-melting reactions wherein quartz is a reactant and K-feldspar a reaction product. Therefore, quartz inclusions commonly occurring in garnets from granulite-facies metapelites and metagraywackes are not simply grains passively included during garnet growth. They may also correspond to newly crystallized phases. Resorption of feldspar may lead to more alkaline melt and to crystalline residue richer in Al than expected under equilibrium conditions. Hence, excess alumina in granulite-facies rocks is not necessarily related to initial alumina-rich whole-rock compositions (as currently considered), but may be due, at least partly, to kinetics of melting.

On the application of in situ monazite chemical geochronology to constraining P – T – t histories in high-temperature ( > 850 ° C) polymetamorphic granulites from Prydz Bay, East Antarctica

We present electron microprobe-based in situ (Th + U)–Pb monazite chemical age data from granulite-facies metapelites in Prydz Bay, East Antarctica. The monazite age data define two distinct age populations, Late Neoproterozoic to Early Palaeozoic ( c . 570–520 Ma) and Neoproterozoic ( c . 950–820 Ma), that confirm the polytectonic nature of Prydz Bay. Our data suggest that similarity in lineation orientation along the Prydz Bay coast is not sufficient to necessarily indicate time-equivalance. The minimum duration of Early Palaeozoic tectonism, spanning at least 60 Ma, is constrained from an Mg–Al-rich metapelite: monazite hosted by coarse-grained orthopyroxene defines the oldest Early Palaeozoic population, whereas cordierite + orthopyroxene symplectites define the youngest Early Palaeozoic population. The spatial distribution of monazites and their ages is correlatable with the inferred mineral assemblage evolution. We are able to characterize the evolution of Early Palaeozoic tectonism in quantitative P – T – t –mineral assemblage space, demonstrating that ultrahigh-temperature tectono-metamorphism in Prydz Bay is of Early Palaeozoic age. The survival of Neoproterozoic inheritance in Fe–Al-rich metapelites has implications for high Pb retentivity in monazite when chemical and kinetic conditions are favourable. The approach and logic applied herein are entirely and directly transferable to the interrogation of any other metamorphic terrane.

Fluorapatite-monazite relationships in granulite-facies metapelites, Schwarzwald, southwest Germany

Abstract Fluorapatite grains with monazite inclusions and/or rim grains are described in two of four samples from a set of granulite-facies metapelites collected from the Variscan Schwarzwald, southern Germany. Fluorapatite in all four samples appears to have experienced some dissolution in the partial granitic melt formed during granulite-facies metamorphism. Monazite inclusions and rim grains are highly deficient in Th and are presumed to have formed from fluorapatite in association with partial melting during granulite-facies metamorphism. Monazite inclusions range from very small (<1 μm) and very numerous to small (1–2 μm), sometimes elongated, and less numerous; both types are evenly distributed throughout the fluorapatite grain interior. Monazite rim grains tend to be 1–10 μm. The formation of monazite inclusions is proposed to be due to dissolution-reprecipitation of the fluorapatite by the aqueous fluids inherent in the granitic melt. We propose that an increase in inclusion size coupled with a decrease in inclusion number is due to Ostwald ripening (interfacial energy reduction), which is greatly facilitated by the presence of an interconnected, fluid-filled porosity in the metasomatized fluorapatite. We further propose that monazite rim grains formed principally during partial dissolution of the fluorapatite in the granitic melt and to a lesser extent by partial dissolutionreprecipitation of the fluorapatite grain rim area allowing for the partial removal of (Y+REE). We conclude that fluorapatite, with monazite inclusions and rim grains, experienced partial dissolution in a H2O-rich peraluminous granitic melt compared to fluorapatite with monazite rim grains and no inclusions which reacted with a similar, relatively less H2O-rich melt. In contrast, monazite-free fluorapatite experienced partial dissolution in a comparatively H2O-poor, subaluminous, possibly peralkaline melt.

Mesoproterozoic granulites of the Shillong–Meghalaya Plateau: Evidence of westward continuation of the Prydz Bay Pan-African suture into Northeastern India

Thermo-chronologic considerations in the Australo-Antarctic domains suggest that the Prydz Bay Pan-African suture of East Antarctica continues westward into India. However, the location of the suture within Eastern India has so far been uncertain because of a lack of adequate thermo-chronological information. In this study, electron microprobe (EPMA) monazite dates and mineral paragenesis of granulite facies metapelites are reported from two areas of the Shillong–Meghalaya gneissic complex (SMGC), a crustal block located in the extreme northeast of the Indian shield close to the Australo-Antarctic block in Neoproterozoic-Cambrian paleomagnetic reconstructions of the Rodinia supercontinent. In the Garo-Goalpara Hills region, a well constrained Mesoproterozoic age of 1596 ± 15 Ma ( n = 103) is correlated with a counterclockwise pressure–temperature path with near peak conditions of 7–8 kbar and 850 °C. Rare matrix monazite rims record younger ages (1032–1273 Ma). At Sonapahar region, 50 km ESE of Garo-Goalpara Hills, homogenous monazite grains in granulite facies metapelites yield EPMA dates tightly clustered at 500 ± 14 Ma ( n = 36) irrespective of their textural setting in a well-annealed mineral matrix. In a few zoned monazite grains, the cores yield older ages of 1078 ± 31 Ma ( n = 10) and 1472 ± 38 Ma ( n = 13). The 500 Ma date corresponds with the ca. 880–480 Ma Rb–Sr dates of porphyritic granites that predominantly intruded the east-central part of the SMGC. We propose that the progressively eastward dominance of Cambro-Ordovician ages in the SMGC indicates a Pan-African final amalgamation of the Indian plate with the Australo-Antarctic plate and a northward extension of the Prydz Bay suture through the SMGC, with the western boundary of the suture possibly located between the Garo-Goalpara Hills and Sonapahar areas.

Extreme crustal metamorphism during Columbia supercontinent assembly: Evidence from North China Craton

A vast supracrustal belt of khondalites (granulite facies metapelites) occur along the northern margin of the North China Craton. We report here for the first time spinel + quartz equilibrium assemblage from these rocks in two textural settings: (1) high ZnO (up to 14.47 wt.%) spinel with quartz as inclusions within garnet porphyroblasts defining pressure above 12 kbar and temperature of 900 °C; and (2) low ZnO (down to 1.2 wt.%) spinel in association with quartz in the matrix assemblage formed during peak ultrahigh-temperature conditions (ca. 975 °C and 9 kbar). We present a unique case of decompression where the metamorphic conditions of the rocks traversed mostly through the spinel + quartz (extended) stability field. Monazite grains in textural association with both types of spinel + quartz textures were analysed for age determination, and the data define two age peaks at 1927 ± 11 Ma and 1819 ± 11 Ma. Since the peak thermal regime of the khondalites was close to or exceeded the theoretical closure temperature of Pb in monazite, we infer the 1819 ± 11 Ma age as the timing of ultrahigh-temperature event in this craton. Our data lend support to the idea of ca. 1.9–1.8 Ga E–W collisional orogen at the northern margin of the North China Craton. We correlate the extreme crustal metamorphism with tectonics associated with the assembly of the North China Craton within the Columbia supercontinent.

A semi-quantitative technique for determination of CO2 in cordierite by Raman spectroscopy in thin sections

Confocal micro-Raman spectroscopic measurements at high spatial resolution of about 5 μm 3 were performed to determine the CO 2 contents of natural cordierite. Nine cordierite single-crystals with known CO 2 concentrations, previously determined by colorimetric titration, were studied to obtain a calibration curve for this method. Measurements were also performed to quantify the CO 2 content of cordierite porphyroblasts in a thin section of a granulite-facies metapelite from Kosseldorf, Sauwald Area, Southern Bohemian Massif, Upper Austria. Fifteen points along a traverse through a large (560 μm) single crystal revealed low CO 2 contents of 0.14 ± 0.11 wt.% CO 2 . These results provide some constraints on the activity of CO 2 [a(CO 2 )] during granulite-facies metamorphism, only if fluid-saturated conditions prevailed or independent H 2 O measurements were made.

Two‐stage metamorphic evolution of the Bemarivo Belt of northern Madagascar: constraints from reaction textures and in situ monazite dating

AbstractNew results on the pressure–temperature–time evolution, deduced from conventional geothermobarometry and in situ U‐Th‐total Pb dating of monazite, are presented for the Bemarivo Belt in northern Madagascar. The belt is subdivided into a northern part consisting of low‐grade metamorphic epicontinental series and a southern part made up of granulite facies metapelites. The prograde metamorphic stage of the latter unit is preserved by kyanite inclusions in garnet, which is in agreement with results of the garnet (core)‐alumosilicate‐quartz‐plagioclase (inclusions in garnet; GASP) equilibrium. The peak metamorphic stage is characterized by ultrahigh temperatures of ∼900–950 °C and pressures of ∼9 kbar, deduced from GASP equilibria and feldspar thermometry. In proximity to charnockite bodies, garnet‐sillimanite‐bearing metapelites contain aluminous orthopyroxene (max. 8.0 wt% Al2O3) pointing to even higher temperatures of ∼970 °C. Peak metamorphism is followed by near‐isothermal decompression to pressures of 5–7 kbar and subsequent near‐isobaric cooling, which is demonstrated by the extensive late‐stage formation of cordierite around garnet. Internal textures and differences in chemistry of metapelitic monazite point to a polyphasic growth history. Monazite with magmatically zoned cores is rarely preserved, and gives an age of c. 737 ± 19 Ma, interpreted as the maximum age of sedimentation. Two metamorphic stages are dated: M1 monazite cores range from 563 ± 28 Ma to 532 ± 23 Ma, representing the collisional event, and M2 monazite rims (521 ± 25 Ma to 513 ± 14 Ma), interpreted as grown during peak metamorphic temperatures. These are among the youngest ages reported for high‐grade metamorphism in Madagascar, and are supposed to reflect the Pan‐African attachment of the Bemarivo Belt to the Gondwana supercontinent during its final amalgamation stage. In the course of this, the southern Bemarivo Belt was buried to a depth of >25 km. Approximately 25–30 Myr later, the rocks underwent heating, interpreted to be due to magmatic underplating, and uplift. Presumably, the northern part of the belt was also affected by this tectonism, but buried to a lower depth, and therefore metamorphosed to lower grades.

Kaolin deposits at Melthonnakkal and Pallipuram within Trivandrum block, southern India

The kaolin deposits at Melthonnakkal and Pallipuram mines form part of the Warkalli Formation belonging to the Tertiary sequence in southern Kerala and occur at the boundary between the Tertiary sequence and Precambrian granulite facies metapelites (khondalites). The sedimentary clays are composed mainly of kaolinite, quartz and gibbsite. XRD and SEM studies have revealed that kaolinite is well-crystallized variety and the platy crystals are scarcely broken in the sedimentary clays. These sedimentary kaolins are considered to have been formed by intense tropical weathering of the khondalites, and subsequently transported and deposited with high organic input into lakes near the weathering crust over the basement rock. Besides, the surficial parts of the sedimentary deposits are extensively lateritized with the formation of goethite and hematite by Quaternary tropical weathering processes.

Depositional constraints and age of metamorphism in southern India: U–Pb chemical (EMPA) and isotopic (SIMS) ages from the Trivandrum Block

We report U–Pb electron microprobe (zircon and monazite) and Secondary Ion Mass Spectrometry (SIMS) U–Pb (zircon) ages from a granulite-facies metapelite and a garnet–biotite gniess from Chittikara, a classic locality within the Trivandrum Block of southern India. The majority of the electron-microprobe data on zircons from the metapelite define apparent ages between 1500 and 2500 Ma with a prominent peak at 2109±22 Ma, although some of the cores are as old as 3070 Ma. Zircon grains with multiple age zoning are also detected with 2500–3700 Ma cores, 1380–1520 mantles and 530–600 Ma outer rims. Some homogeneous and rounded zircon cores yielded late Neoproterozoic ages that suggest that deposition within the Trivandrum Block belt was younger than 610 Ma. The outermost rims of these grains are characterized by early Cambrian ages suggesting metamorphic overgrowth at this time. The apparent ages of monazite grains from this locality reveal multiple provenance and polyphase metamorphic history, similar to those of the zircons. In a typical case, Palaeoproterozoic cores (1759–1967 Ma) are enveloped by late Neoproterozoic rims (562–563 Ma), which in turn are mantled by an outermost thin Cambrian rim (∼515 Ma). PbO v. ThO*2 plots for monazites define broad isochrons, with cores indicating a rather imprecise age of 1913±260 Ma (MSWD=0.80) and late Neoproterozoic/Cambrian cores as well as thin rims yielding a well-defined isochron with an age of 557±19 Ma (MSWD=0.82). SIMS U–Pb isotopic data on zircons from the garnet–biotite gneiss yield a combined core/rim imprecise discordia line between 2106±37 Ma and 524±150 Ma. The data indicate Palaeoproterozoic zircon formation with later partial or non-uniform Pb loss during the late Neoproterozoic/Cambrian tectonothermal event. The combined electron probe and SIMS data from the metapelite and garnet–biotite gneiss at Chittikara indicate that the older zircons preserved in the finer-grained metapelite protolith have heterogeneous detrital sources, whereas the more arenaceous protolith of the garnet–biotite gniess was sourced from a single-aged terrane. Our data suggest that the metasedimentary belts in southern India may have formed part of an extensive late Neoproterozoic sedimentary basin during the final amalgamation of the Gondwana supercontinent.

Spinel+Quartz association from the Kerala khondalites, southern India: evidence for ultrahigh-temperature metamorphism

Granulite facies metapelites (khondalites) from Chittikara are characterized by the assemblage Grt + Sil + Bt + Crd + Spl + Kfs + Pl + Qtz ± graphite. The rocks show a series of mineral reactions such as: (1) Bt + Sil + Qtz = Grt + Crd ± Kfs, (2) Grt + Sil + Qtz = Crd, (3) Grt + Crd + Sil = Spl + Qtz , (4) Spl + Qtz = Crd, and (5) Grt + Sil = Spl + Crd. Spinel and quartz occur in direct grain contact and their stability considerations in appropriate reaction grids indicate > 950°C at 8 kbar. This, together with the ca. 1000°C temperatures derived from mesoperthites suggests ultrahigh temperature metamorphism in the khondalite belt. Electron microprobe data on the various minerals have been used to compute pressure-temperature estimates from mineral phase equilibria thermobaromtery. Temperature estimates from spinel-cordierite thermometry are around 900°C, while garnet-cordierite thermometer yields an upper estimate of 818°C. Estimates based on garnet-sillimanite-plagioclase-quartz barometer indicate pressures up to 8 kbar. Evaluation of the microstructures and reactions in the khondalites using available reaction grids for pelitic rocks provide evidence for peak metamorphism at high or ultrahigh temperatures followed by steep decompression from around 8 kbar down to 4 kbar. The P-T path attests to a rapid exhumation history probably associated with the extensional collapse following collision of continental blocks during the assembly of Gondwana in the late Pan-African.

New constraints on metamorphism in the Rauer Group, Prydz Bay, east Antarctica

Abstract Granulite facies metapelites of the Mather and Filla Paragneisses within the Rauer Group, east Antarctica, possess markedly different compositions. The metamorphic evolution of the two metapelite types has been interpreted as temporally distinct, with the Rauer Group preserving at least two distinct granulite facies tectonothermal episodes. Calculated P–T pseudosections and orthopyroxene Al content indicate the revised maximum‐preserved P–T conditions within the Mather Paragneiss to lie in the vicinity of 950–975 °C and 10–10.6 kbar, less extreme than previous estimates. The range of possible P–T paths for the Mather Paragneiss consistent with mineral textural relationships and pseudosections contoured for mineral proportion are significantly shallower (dP/dT) than previous estimates. A near‐isothermal decompression P–T path, and extreme peak metamorphic conditions, are not necessary to explain the development of preserved mineral reaction textures. The Filla Paragneiss contains pelitic assemblages less amenable to rigorous quantitative analysis. Nevertheless, possibilities for the shared or otherwise metamorphic evolution of the Mather and Filla Paragneisses may be postulated on the basis of calculated pseudosections in the context of existing geochronology for the Rauer Group and preserved microstructures. A shared evolution, most likely during Pan‐African granulite facies metamorphism, is plausible and consistent with mineral assemblage development, geochronology and microstructures. A revised interpretation of the Rauer Group's preserved metamorphic evolution may warrant the revision of existing tectonic models, applicable also to the remainder of Prydz Bay. More generally, the employed approach may incite a revision of peak P–T and P–T paths in other granulite facies terranes.

Local equilibrium in polymetamorphic gneiss and the titanium substitution in biotite

Secondary metamorphic biotite grew with sillimanite and garnet at the expense of cordierite during a Pan-African metamorphic event overprinting Grenville age granulite-facies metapelites at the western margin of the Namaqualand Metamorphic Complex, South Africa. Fe-Mg exchange thermometry and average pressure calculations constrain the conditions of the overprinting metamorphism to about 660 °C at 6 kbar. Biotite compositions reflect chemical equilibrium in local domains defined by partly to completely pseudomorphed cordierite grains. They show a continuous range of Ti content from 0.0 to 4.5 wt% TiO 2 depending on the proximity of the domain to a source of Ti such as ilmenite. Thus, they allow an assessment of the Ti substitution that is independent of metamorphic grade variation. Significant correlations between Ti and microprobe analysis total indicate that Ti is accommodated largely through a deprotonation substitution Ti 4 + + 2O 2 - + H 2 → M 2 + + 2OH - (where M 2 + is a divalent octahedral cation) rather than by substitutions involving octahedral vacancies. Ti shows a positive correlation with K, and strong negative correlations with octahedral Al and the sum of divalent cations. Ti content also varies with molar Mg/(Mg + Fe) in biotite (X M g ), because the process of cordierite replacement involves the mass transfer of Fe into cordierite domains. However, the above trends are also shown by a set of analyses with uniform X M g , In detail, the correlations depend on the method of formula recalculation. If the deprotonation substitution is dominant, normalization should not be made to a constant number of oxygen atoms. A statistical treatment of biotite analyses normalized to 14 tetrahedral and octahedral cations per formula unit suggests that the introduction of Ti involves deprotonation of two OH groups per Ti, and that the replacement of some octahedral Al by Ti is balanced by increases in K and/or Si in other sites. An appropriate Ti-biotite end-member may be K 2 [Mg 4 Ti 2 ]Al 2 Si 6 O 2 4 , rather than the vacancy-bearing end-members proposed in other studies. In view of the fact that even Ti-free biotite appears to contain a small proportion of octahedral vacancies, a suitable recalculation scheme for microprobe analyses may involve normalizing to 22 + Ti O atoms (neglecting H 2 O).

Palaeoproterozoic high‐T, low‐P metamorphism and dehydration melting in metapelites from the Mopunga Range, Arunta Inlier, central Australia

AbstractA sequence of psammitic and pelitic metasedimentary rocks from the Mopunga Range region of the Arunta Inlier, central Australia, preserves evidence for unusually low pressure (c. 3 kbar), regional‐scale, upper amphibolite and granulite facies metamorphism and partial melting. Upper amphibolite facies metapelites of the Cackleberry Metamorphics are characterised by cordierite‐andalusite‐K‐feldspar assemblages and cordierite‐bearing leucosomes with biotite‐andalusite selvages, reflecting P–T conditions of c. 3 kbar and c. 650–680 °C. Late development of a sillimanite fabric is interpreted to reflect either an anticlockwise P–T evolution, or a later independent higher‐P thermal event. Coexistence of andalusite with sillimanite in these rocks appears to reflect the sluggish kinematics of the Al2SiO5 polymorphic inversion. In the Deep Bore Metamorphics, 20 km to the east, dehydration melting reactions in granulite facies metapelites have produced migmatites with quartz‐absent sillimanite‐spinel‐cordierite melanosomes, whilst in semipelitic migmatites, discontinuous leucosomes enclose cordierite‐spinel intergrowths. Metapsammitic rocks are not migmatised, and contain garnet–orthopyroxene–cordierite–biotite–quartz assemblages. Reaction textures in the Deep Bore Metamorphics are consistent with a near‐isobaric heating‐cooling path, with peak metamorphism occurring at 2.6–4.0 kbar and c. 750–800 °C. SHRIMP U–Pb dating of metamorphic zircon rims in a cordierite‐orthopyroxene migmatite from the Deep Bore Metamorphics yielded an age of 1730 ± 7 Ma, whilst detrital zircon cores define a homogeneous population at 1805 ± 7 Ma. The 1730 Ma age is interpreted to reflect the timing of high‐T, low‐P metamorphism, synchronous with the regional Late Strangways Event, whereas the 1805 Ma age provides a maximum age of deposition for the sedimentary precursor. The Mopunga Range region forms part of a more extensive low‐pressure metamorphic terrane in which lateral temperature gradients are likely to have been induced by localised advection of heat by granitic and mafic intrusions. The near‐isobaric Palaeoproterozoic P–T–t evolution of the Mopunga Range region is consistent with a relatively transient thermal event, due to advective processes that occurred synchronous with the regional Late Strangways tectonothermal event.

A petrogenetic grid for aluminous granulite facies metapelites in the KFMASH system

AbstractDue to the retrograde cation exchange problems experienced by conventional geothermobarometers above their closure temperatures, petrogenetic grids are a potentially powerful alternative to unravelling the P–T evolution of ultrahigh‐T granulite terranes. A new qualitative KFMASH (K2O–FeO–MgO–Al2O3–SiO2–H2O) petrogenetic grid for Mg–Al rich metapelites containing K‐feldspar, sillimanite and quartzofeldspathic melt that successfully accounts for the majority of assemblages composed of variations of sapphirine, spinel, garnet, orthopyroxene, cordierite, biotite and quartz is developed. Univariant reactions are predicted utilizing a newly derived ‘melt projection’ and these reactions are entirely consistent with algebraically calculated reaction coefficients obtained using a set of standard phase compositions. Based upon observations of commonly associated mineral assemblages in natural lithologies the [Spr, Spl], [Qtz, Spl], [Bt, Spl], [Opx, Spr], [Opx, Qtz] and [Bt, Opx] invariant points are assumed to be stable, whilst the [Grt, Spr], [Grt, Qtz], [Spr, Qtz] and [Crd, Qtz] are assumed to be metastable. Biotite‐bearing assemblages are confined to the lowest temperatures, and sapphirine + quartz to the highest temperatures. Orthopyroxene + sillimanite ± quartz assemblages are confined to the highest pressures, whilst spinel‐bearing assemblages are stabilized by lower pressures. The alternative choice of invariant point stability leads to significant differences between this grid and previously proposed topologies. Spinel cannot be stable along with the orthopyroxene and sillimanite assemblage as previously proposed. Further, more subtle differences in topology result from the treatment of H2O in the chemographic projection used to deduce univariant reactions, and projecting from a water‐bearing quartzofeldspathic melt does not yield the same reaction coefficients as projection from H2O. The new grid allows reinterpretation of previously proposed evolutionary P–T paths for Mg–Al rich granulites from the Napier Complex and Rauer Group, East Antarctica, and In Ouzzal, Algeria.