Osumilite–melt interactions in ultrahigh temperature granulites: phase equilibria modelling and implications for the P–T–t evolution of the Eastern Ghats Province,India

The exposed residual crust in the Eastern Ghats Province records ultrahigh temperature (UHT) metamorphic conditions involving extensive crustal anatexis and melt loss. However, there is disagreement about the tectonic evolution of this late Mesoproterozoic–early Neoproterozoic orogen due to conflicting petrological, structural and geochronological interpretations. One of the petrological disputes in residual high Mg–Al granulites concerns the origin of fine‐grained mineral intergrowths comprising cordierite + K‐feldspar ± quartz ± biotite ± sillimanite ± plagioclase. These intergrowths wrap around porphyroblast phases and are interpreted to have formed by the breakdown of primary osumilite in the presence of melt trapped in the equilibration volume by the melt percolation threshold. The pressure (P)–temperature (T) evolution of four samples from three localities across the central Eastern Ghats Province is constrained using phase equilibria modelling in the chemical system Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–Fe₂O₃ (NCKFMASHTO). Results of the modelling are integrated with published geochronological results for these samples to show that the central Eastern Ghats Province followed a common P–T–t history. This history is characterized by peak UHT metamorphic conditions of 945–955 °C and 7.8–8.2 kbar followed by a slight increase in pressure and close‐to‐isobaric cooling to the conditions of the elevated solidus at 940–900 °C and 8.5–8.3 kbar. In common with other localities from the Eastern Ghats Province, the early development of cordierite before osumilite and the peak to immediate post‐peak retrograde reaction between osumilite and melt to produce the intergrowth features requires that the prograde evolution was one of contemporaneous increasing pressure with increasing temperature. This counter‐clockwise (CCW) evolution is evaluated for one sample using inverse phase equilibria modelling along a schematic P–T path of 150 °C kbar^?1 starting from the low P–T end of the prograde P–T path as constrained by the phase equilibria modelling. The inverse modelling is executed by step‐wise down temperature reintegration of sufficient melt into the residual bulk chemical composition at the P–T point of the 1 mol.% melt isopleth at each step, representing the melt remaining on grain boundaries after each prograde drainage event, to reach the melt connectivity transition (MCT) of 7 mol.%. The procedure is repeated until a plausible protolith composition is recovered. The result demonstrates that clastic sedimentary rocks that followed a CCW P–T evolution could have produced the observed mineral assemblages and microstructures preserved in the central Eastern Ghats Province. This study also highlights the role of melt during UHT metamorphism, particularly its importance to both chemical and physical processes along the prograde and retrograde segments of the P–T path. These processes include: (i) an increase in diffusive length scales during the late prograde to peak evolution, creating equilibration volumes larger than a standard thin section; (ii) the development of retrograde mineral assemblages, which is facilitated if some melt is retained post‐peak; (iii) the presence of melt as a weakening mechanism and the advection of heat by melt, allowing the crust to thicken; and (iv) the effect of melt loss, which makes the deep crust both denser and stronger, and reduces heat production at depth, limiting crustal thickening and facilitating the transition to close‐to‐isobaric cooling.     

Spinel–cordierite symplectites replacing andalusite: evidence for melt-assisted diapirism in the Bushveld Complex, South Africa

Spinel–cordierite symplectites partially replacing andalusite occur in metapelitic rocks within the cores of several country rock diapirs that have ascended into the upper levels of layered mafic/ultramafic rocks in the Bushveld Complex. We investigate the petrogenesis of these symplectites in one of these diapirs, the Phepane dome. Petrographic evidence indicates that at conditions immediately below the solidus the rocks were characterized by a cordierite‐, biotite‐ and K‐feldspar‐rich matrix and 5–10 mm long andalusite porphyroblasts surrounded by biotite‐rich fringes. Phase relations in the MnNCKFMASHT model system constrain the near‐solidus prograde path to around 3 kbar and imply that andalusite persisted metastably into the sillimanite + melt field, where the fringing relationship between biotite and andalusite provided spatially restricted equilibrium domains with silica‐deficient effective bulk compositions that focused suprasolidus reaction. MnNCKFMASHT pseudosections that model these compositional domains suggest that volatile phase‐absent melting reactions consuming andalusite and biotite initially produced a moat of cordierite surrounding andalusite; reaction progressed until all quartz was consumed. Spinel is predicted to grow with cordierite at around 720 °C. Formation of the aluminous solid products was strongly controlled by the receding edge of andalusite grains, with symplectites forming at the andalusite‐cordierite moat interface. Decompression due to melt‐assisted diapiric rise of the floor rocks into the overlying mafic/ultramafic rocks occurred close to the thermal peak. Re‐crossing of the solidus at P = 1.5–2 kbar, T > 700 °C resulted in preservation of the symplectites. Two features of the silica‐deficient domains inhibited resorption of spinel. First, the cordierite moat armoured the symplectites from reaction with crystallizing melt in the outer part of the pseudomorphs. Second, an up‐T step in the solidus at low‐P, which may be in excess of 100 °C higher than the quartz‐saturated solidus, resulted in high‐T crystallization of melt on decompression. Even in metapelitic rocks where melt is retained, preservation of spinel is favoured by decompression.     

Modelling of mineral equilibria in ultrahigh-temperature metamorphic rocks from the Anápolis–Itauçu Complex, central Brazil

A new quantitative approach to constraining mineral equilibria in sapphirine‐bearing ultrahigh‐temperature (UHT) granulites through the use of pseudosections and compatibility diagrams is presented, using a recently published thermodynamic model for sapphirine. The approach is illustrated with an example from an UHT locality in the Anápolis–Itauçu Complex, central Brazil, where modelling of mineral equilibria indicates peak metamorphic conditions of about 9 kbar and 1000 °C. The early formed, coarse‐grained assemblage is garnet–orthopyroxene–sillimanite–quartz, which was subsequently modified following peak conditions. The retrograde pressure–temperature (P–T) path of this locality involves decompression across the FeO–MgO–Al₂O₃–SiO₂ (FMAS) univariant reaction orthopyroxene + sillimanite = garnet + sapphirine + quartz, resulting in the growth of sapphirine–quartz, followed by cooling and recrossing of this reaction. The resulting microstructures are modelled using compatibility diagrams, and pseudosections calculated for specific grain boundaries considered as chemical domains. The sequence of microstructures preserved in the rocks constrains a two‐stage isothermal decompression–isobaric cooling path. The stability of cordierite along the retrograde path is examined using a domainal approach and pseudosections for orthopyroxene–quartz and garnet–quartz grain boundaries. This analysis indicates that the presence or absence of cordierite may be explained by local variation in a_H2O. This study has important implications for thermobarometric studies of UHT granulites, mainly through showing that traditional FMAS petrogenetic grids based on experiments alone may overestimate P–T conditions. Such grids are effectively constant a_H2O sections in FMAS‐H₂O (FMASH), for which the corresponding a_H2O is commonly higher than that experienced by UHT granulites. A corollary of this dependence of mineral equilibria on a_H2O is that local variations in a_H2O may explain the formation of cordierite without significant changes in P–T conditions, particularly without marked decompression.     

Phase equilibria constraints on melting of stromatic migmatites from Ronda (S. Spain): insights on the formation of peritectic garnet

Stromatic metatexites occurring structurally below the contact with the Ronda peridotite (Ojén nappe, Betic Cordillera, S Spain) are characterized by the mineral assemblage Qtz+Pl+Kfs+Bt+Sil+Grt+Ap+Gr+Ilm. Garnet occurs in low modal amount (2–5 vol.%). Very rare muscovite is present as armoured inclusions, indicating prograde exhaustion. Microstructural evidence of melting in the migmatites includes pseudomorphs after melt films and nanogranite and glassy inclusions hosted in garnet cores. The latter microstructure demonstrates that garnet crystallized in the presence of melt. Re‐melted nanogranites and preserved glassy inclusions show leucogranitic compositions. Phase equilibria modelling of the stromatic migmatite in the MnO–Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂–O₂–C (MnNCaKFMASHOC) system with graphite‐saturated fluid shows P–T conditions of equilibration of 4.5–5 kbar, 660–700 °C. These results are consistent with the complete experimental re‐melting of nanogranites at 700 °C and indicate that nanogranites represent the anatectic melt generated immediately after entering supersolidus conditions. The P–T estimate for garnet and melt development does not, however, overlap with the low‐temperature tip of the pure melt field in the phase diagram calculated for the composition of preserved glassy inclusions in garnet in the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O (NCKFMASH) system. A comparison of measured melt compositions formed immediately beyond the solidus with results of phase equilibria modelling points to the systematic underestimation of FeO, MgO and CaO in the calculated melt. These discrepancies are present also when calculated melts are compared with low‐T natural and experimental melts from the literature. Under such conditions, the available melt model does not perform well. Given the presence of melt inclusions in garnet cores and the P–T estimates for their formation, we argue that small amounts (<5 vol.%) of peritectic garnet may grow at low temperatures (≤700 °C), as a result of continuous melting reactions consuming biotite.     

Phase equilibria constraints on the melt fertility of crustal rocks: the effect of subsolidus water loss

During regional prograde metamorphism, H₂O generated by ongoing dehydration reactions is likely to be continuously lost from a rock by compaction. Classical melting experiments cannot easily simulate this phenomenon, because ideally, all run products are conserved within the experimental charge, although significant equilibration and H₂O generation may occur during heating. Phase equilibria modelling is used to consider the effect of subsolidus water loss (SWL) on subsequent melting relationships of felsic lithologies (including metapelite, metagreywacke and metatonalite) in the suprasolidus. SWL drives the bulk composition towards the minimum saturation point on the boundary of the wet‐melting field and results in significantly reduced subsequent melt generation when compared to melting experiments involving conservation of subsolidus H₂O. This effect is most significant at P–T conditions just above the solidus. For initially hydrated rocks, the reduction in melt production causes rheologically critical thresholds (e.g. melt connectivity threshold, melt escape threshold and the solid‐to‐liquid transition) to be intersected at temperatures generally more than 100 °C, higher than predicted by idealized experimental melting curves.     

Dehydration of metapelites during high‐P metamorphism: The coupling between fluid sources and fluid sinks

Polymetamorphic metapelites and embedded eclogites share a complex, episodic interplay of dehydration and fluid infiltration at the eclogite type‐locality (Saualpe–Koralpe, Eastern Alps, Austria). The metapelites inherited a fluid content (i.e. mineral‐bound OH expressed in terms of mol.% H₂O) of ～6–7 mol.% H₂O from high‐T–low‐P metamorphism experienced during the Permian. At or near P_max of the subsequent Eoalpine event (～20 kbar and 680^°C), the breakdown of paragonite to Na‐rich clinopyroxene and kyanite in metapelites released a discrete pulse of hydrous fluid. Prior to the dehydration event, the rocks were largely fluid absent, allowing only limited re‐equilibration during the prograde Eoalpine evolution. Similarly, Permian‐aged gabbros have persisted metastably due to the absence of a catalyst prior to fluid‐induced re‐equilibration. The fluid triggered partial to complete eclogitization along a fluid infiltration front partially preserved in metagabbro. Near‐isothermal decompression to ～7.5–10 kbar and 670–690°C took place under fluid‐absent conditions. After decompression, a second breakdown of phengitic white mica and garnet produced muscovite, biotite, plagioclase and ～0.1–0.7 mol.% H₂O that enhanced extensive fluid‐aided re‐equilibration of the metapelites. Potential relicts of high‐P assemblages were largely obliterated and replaced by the recurrent amphibolite facies assemblage garnet+biotite+staurolite+kyanite+muscovite+plagioclase+ilmenite+quartz. The hydrous fluid originating from the metapelites infiltrated the embedded eclogites at these P–T conditions and induced the local breakdown of the peak assemblage omphacite and garnet to fine‐grained symplectites of diopside and plagioclase. Further fluid infiltration led to the formation of hornblende–quartz poikiloblasts at the expense of the symplectites. The metapelites re‐equilibrated until the growth of retrograde staurolite consumed any remaining free fluid, thereby terminating the process. Further re‐equilibration is inhibited by both the lack of a catalytic fluid and H₂O as a reactant essential for rehydration reactions. The interplay between fluid sources and fluid sinks describes a closed cycle for the rocks at the eclogite type‐locality. Final, near‐isobaric cooling is indicated by a slight increase of X_Fe in garnet rims. Post‐decompression dehydration and fluid‐aided re‐equilibration arrested by the introduction of staurolite might explain the apparently homogeneous retrogression conditions as well as the notorious absence of diagnostic high‐P assemblages in metapelites at the eclogite type‐locality.     

Pressure–temperature evolution of blueschist facies rocks from Sifnos, Greece, and implications for the exhumation of high-pressure rocks in the Central Aegean

The blueschist and greenschist units on the island of Sifnos, Cyclades were affected by Eocene high‐pressure (HP) metamorphism. Using conventional geothermobarometry, the HP peak metamorphic stage was determined at 550–600 °C and 20 kbar, close to the blueschist and the eclogite facies transition. The retrograde P–T paths are inferred with phase diagrams. Pseudosections based on a quantitative petrogenetic grid in the model system Na₂O–CaO–FeO–MgO–Al₂O₃–SiO₂–H₂O reveal coeval decompression and cooling for both the blueschist and the greenschist unit. The conditions of the metamorphic peak and those of the retrograde stages conform to a similar metamorphic gradient of 10–12 °C km^?1 for both units. The retrograde overprint can be assigned to low‐pressure blueschist to HP greenschist facies conditions. This result cannot be reconciled with the (prograde) Barrovian‐type event, which affected parts of the Cyclades during the Oligocene to Miocene. Instead, the retrograde overprint is interpreted in terms of exhumation, directly after the HP stage, without a separate metamorphic event. Constraints on the exhumation mechanism are given by decompression‐cooling paths, which can be explained by exhumation in a fore‐arc setting during on‐going subduction and associated crustal shortening. Back‐arc extension is only responsible for the final stage of exhumation of the HP units.     

Calculation of partial melting equilibria in the system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (NCKFMASH)

A thermodynamic model for haplogranitic melts in the system Na₂O–CaO–K₂O–Al₂O₃–SiO₂–H₂O (NCKASH) is extended by the addition of FeO and MgO, with the data for the additional end‐members of the liquid incorporated in the Holland & Powell (1998) internally consistent thermodynamic dataset. The resulting dataset, with the software thermocalc , is then used to calculate melting relationships for metapelitic rock compositions. The main forms for this are P–T and T–X pseudosections calculated for particular rock compositions and composition ranges. The relationships in these full‐system pseudosections are controlled by the low‐variance equilibria in subsystems of NCKFMASH. In particular, the solidus relationships are controlled by the solidus relationships in NKASH, and the ferromagnesian mineral relationships are controlled by those in KFMASH. However, calculations in NCKFMASH allow the relationships between the common metapelitic minerals and silicate melt to be determined. In particular, the production of silicate melt and melt loss from such rocks allow observations to be made about the processes involved in producing granulite facies rocks, particularly relating to open‐system behaviour of rocks under high‐grade conditions.     

Anatectic record and contrasting P–T paths of aluminous gneisses from the central Grenville Province

Anatectic aluminous gneisses, some derived from sedimentary rocks of broadly pelitic composition and others from hydrothermally altered felsic volcanic rocks, are exposed in the mid‐P and high‐P segments of the hinterland in the central Grenville Province. These gneisses consist dominantly of garnet, biotite, K‐feldspar, plagioclase and quartz, with sillimanite or kyanite, and display microstructural evidence of anatexis by fluid‐absent reactions consuming muscovite and/or biotite. Melt‐related microstructures, such as inter‐granular films and/or interstitial quartz or feldspar enclosing relict phases, are most abundant in the metasedimentary samples. Despite anatexis at granulite facies conditions, the hydrothermally altered rocks preserve earlier features attributed to the circulation of hydrothermal fluids, such as sillimanite seams, dismembered quartz veins and garnet‐rich aluminous nodules in a K‐feldspar‐dominated matrix. Microstructural and mineral chemical data, integrated with P–T pseudosections calculated with thermocalc for the metasedimentary rocks, permit qualitative constraints on the P–T paths. Data from a high‐P kyanite‐bearing sample are consistent with a steep prograde P–T path up to ~14.5 kbar and 860–900 °C, followed by decompression with minor cooling to the solidus at ~11 kbar and 870 °C. This pressure‐dominated P–T path is similar to those inferred in other parts of the high‐P segment in the central Grenville Province. In contrast, the P–T path predicted from a mid‐P sillimanite‐bearing paragneiss has a strong temperature gradient with P–T of ~9.5 kbar and 850 °C at the thermal peak, and a retrograde portion down to ~8 kbar and 820 °C. In a broad sense, these two contrasting P–T patterns are consistent with predictions of thermo‐mechanical modelling of large hot orogens in which P–T paths with strong pressure gradients exhume deeper rocks in the orogenic flanks, whereas P–T paths with strong temperature gradients in the orogenic core reflect protracted lateral transport of ductile crust beneath a plateau.     

Orthopyroxene–sillimanite–quartz assemblages: distribution, petrology, quantitative P–T–X constraints and P–T paths

Granulite facies magnesian metapelites commonly preserve a wide array of mineral assemblages and reaction textures that are useful for deciphering the metamorphic evolution of a terrane. Quantitative pressure, temperature and bulk composition constraints on the development and preservation of characteristic peak granulite facies mineral assemblages such as orthopyroxene + sillimanite + quartz are assessed with reference to calculated phase diagrams. In NCKFMASH and its chemical subsystems, peak assemblages form mainly in high‐variance fields, and most mineral assemblage changes reflect multivariant equilibria. The rarity of orthopyroxene–sillimanite–quartz‐bearing assemblages in granulite facies rocks reflects the need for bulk rock X_Mg of greater than approximately 0.60–0.65, with pressures and temperatures exceeding c. 8 kbar and 850 °C, respectively. Cordierite coronas mantling peak minerals such as orthopyroxene, sillimanite and quartz have historically been used to infer isothermal decompression P–T paths in ultrahigh‐temperature granulite facies terranes. However, a potentially wide range of P–T paths from a given peak metamorphic condition facilitate retrograde cordierite growth after orthopyroxene + sillimanite + quartz, indicating that an individual mineral reaction texture is unable to uniquely define a P–T vector. Therefore, the interpretation of P–T paths in high‐grade rocks as isothermal decompression or isobaric cooling may be overly simplistic. Integration of quantitative data from different mineral reaction textures in rocks with varying bulk composition will provide the strongest constraints on a P–T path, and in turn on tectonic models derived from these paths.     

Testing the fidelity of thermometers at ultrahigh temperatures

A highly residual granulite facies rock (sample RG07‐21) from Lunnyj Island in the Rauer Group, East Antarctica, presents an opportunity to compare different approaches to constraining peak temperature in high‐grade metamorphic rocks. Sample RG07‐21 is a coarse‐grained pelitic migmatite composed of abundant garnet and orthopyroxene along with quartz, biotite, cordierite, and plagioclase with accessory rutile, ilmenite, zircon, and monazite. The inferred sequence of mineral growth is consistent with a clockwise pressure–temperature (P–T) evolution when compared with a forward model (P–T pseudosection) for the whole‐rock chemical composition. Peak metamorphic conditions are estimated at 9 ± 0.5 kbar and 910 ± 50°C based on conventional Al‐in‐orthopyroxene thermobarometry, Zr‐in‐rutile thermometry, and calculated compositional isopleths. U–Pb ages from zircon rims and neocrystallized monazite grains yield ages of c. 514 Ma, suggesting that crystallization of both minerals occurred towards the end of the youngest pervasive metamorphic episode in the region known as the Prydz Tectonic Event. The rare earth element compositions of zircon and garnet are consistent with equilibrium growth of these minerals in the presence of melt. When comparing the thermometry methods used in this study, it is apparent that the Al‐in‐orthopyroxene thermobarometer provides the most reliable estimate of peak conditions. There is a strong textural correlation between the temperatures obtained using the Zr‐in‐rutile thermometer––maximum temperatures are recorded by a single rutile grain included within orthopyroxene, whereas other grains included in garnet, orthopyroxene, quartz, and biotite yield a range of temperatures down to 820°C. Ti‐in‐zircon thermometry returns significantly lower temperature estimates of 678–841°C. Estimates at the upper end of this range are consistent with growth of zircon from crystallizing melt at temperatures close to the elevated (H₂O undersaturated) solidus. Those estimates, significantly lower than the calculated temperature of this residual solidus, may reflect isolation of rutile from the effective equilibration volume leading to an activity of TiO₂ that is lower than the assumed value of unity.     

Near-isothermal decompression within a clockwise P–T evolution recorded in migmatitic mafic granulites from Clavering Ø, NE Greenland: implications for the evolution of the Caledonides

The high grade rocks (metapelites and metabasites) of Clavering Ø represent the easternmost exposures of granulites in the Palaeozoic Caledonian Orogen of East Greenland. Mafic granulites which occur as sheet‐like bodies and lenses within metapelitic migmatites and orthogneiss complexes have experienced migmatisation and mineral equilibria which define a clockwise P–T path incorporating a near‐isothermal decompression segment. Textures demonstrate the existence of early garnet‐clinopyroxene‐melt assemblages which equilibrated at >8–11 kbar and 850–915 °C. Subsequently, decompression melting led to formation of orthopyroxene‐plagioclase‐melt assemblages at conditions below >8–11 kbar. Continued syn‐deformational decompression is indicated by a combination of both static and syn‐deformational recrystallization textures which generated finer grained orthopyroxene‐plagioclase assemblages. P–T constraints indicate these assemblages equilibrated at c. 5.0–6.5 kbar at 850–915 °C. These data are consistent with the rocks undergoing a stage of rapid tectonic‐induced exhumation involving some 3.0–4.5 kbar (c.10–12 km) uplift as part of a clockwise P–T path in a collisional setting.     

Anatectic melt volumes in the thermal aureole of the Etive Complex,Scotland: the roles of fluid‐present and fluid‐absent melting

Pelitic hornfelses within the inner thermal aureole of the Etive igneous complex underwent limited partial melting, generating agmatic micro‐stromatic migmatites. In this study, observed volume proportions of vein leucosomes in the migmatites are compared with modelled melt volumes in an attempt to constrain the controls on melting processes. Petrogenetic modelling in the MnNCKFMASHT system was performed on the compositions of 15 analysed Etive pelite samples using THERMOCALC. Melt modes were calculated at 2.2 kbar (the estimated pressure in the southern Etive aureole) from solidus temperatures to 800 °C for both fluid‐absent and fluid‐present conditions. Volume changes accompanying fluid‐absent melting at 2.2 kbar were also calculated. P–T pseudosections reproduce the zonal sequence of the southern Etive aureole fairly well. The modelled solidus temperatures of silica‐rich pelitic compositions are close to 680 °C at 2.2 kbar and, in the absence of free fluid, melt modes in such compositions rise to between 12 and 29% at 800 °C, half of which is typically produced over the narrow reaction interval in which orthopyroxene first appears. Silica‐poor compositions have solidus temperatures of up to ～770 °C and yield <11.4% melt at 800 °C under fluid‐absent conditions. For conditions of excess H₂O, modelled melt modes increase dramatically within ～13 °C of the solidus, in some cases to >60%; by 800 °C they range from 61 to 88% and from 29 to 74% in silica‐rich and silica‐poor compositions, respectively. Calculated volume changes for fluid‐absent melting are positive for all modelled compositions and reach 4.5% in some silica‐rich compositions by 800 °C. Orthopyroxene formation is accompanied by a volume increase of up to 1.48% over a temperature increase of as little as 2.7 °C, supporting the arguments for melt‐induced ‘hydrofracturing’ as a viable melt‐escape mechanism in low‐P metamorphism. Mineral assemblages in the innermost aureole support previous conclusions that partial melting took place predominantly under fluid‐absent conditions. However, vein leucosome proportions, estimated by image analysis, do not show the expected correlation with grade, and are locally greatly in excess of melt modes predicted by fluid‐absent models, particularly close to the melt‐in isograd. Melting of interlayered psammites, addition of H₂O from interlayered melt‐free rocks, and metastable persistence of muscovite are ruled out as major causes of the excess melt anomaly. The most likely cause, we believe, is that local variations existed in the amount of fluid available at the onset of melting, promoted by focussing of fluid released by dehydration in the middle and outer aureole; however, some redistribution of melt by compaction‐driven flow through the vein channel network cannot be ruled out. The formation of melt‐filled fractures in the inner Etive aureole was assisted by stresses that caused extension at high angles to the igneous contact. The fractures were probably caused either by transient pressure reduction in the diorite magma chamber associated with a second phase of intrusion, or by sub‐solidus thermal contraction in the diorite pluton during the early stages of inner‐aureole cooling.     

Phase equilibria modelling of kyanite‐bearing anatectic paragneisses from the central Grenville Province

Kyanite‐bearing paragneisses from the Manicouagan Imbricate Zone and its footwall (high‐P belt of the central Grenville Province) preserve evidence of partial melting with development of metamorphic textures involving biotite–garnet ± kyanite ± plagioclase ± K‐feldspar–quartz. Garnet in these rocks displays a variety of zoning patterns with respect to Ca. Pseudosection modelling in the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O (NCKFMASHTO) system using measured bulk rock compositions accounts for the textural evolution of two aluminous and two sub‐aluminous samples from the presumed thermal peak to conditions at which retained melt solidified. The prograde features are best explained by pseudosections calculated with compositions to account for melt loss. The intersection of isopleths of grossular content and Fe/(Fe + Mg) relating to large porphyroblasts of garnet provide constraints on the P–T conditions of the metamorphic peak. These P–T estimates are considered to be minima because of the potential for diffusional modification of the composition of garnet at high‐T and during the early stages of cooling. However, they are consistent with textural observations and pseudosection topology, with peak assemblages best preserved in rocks for which the calculated pseudosections predict only small changes in mineral proportions in the P–T interval, in which retrograde reactions are inferred to have occurred between the thermal peak and the solidus. Maximum P–T conditions (14.5–15.5 kbar and 840–890 °C) and steep retrograde P–T paths inferred for rocks from the Manicouagan Imbricate Zone are comparable with those determined for mafic rocks from the same area. In contrast, maximum P–T conditions of 12.5–13 kbar and 815–830 °C and flatter P–T paths are inferred for the rocks of the footwall to the Manicouagan Imbricate Zone. The general consistency between textures, mineral compositions and the topologies of the calculated pseudosections suggests that the pseudosection approach is an appropriate tool for inferring the P–T evolution of high‐P anatectic quartzo‐feldspathic rocks.     

Borosilicate- and phengite-bearing veins from the Grenville Province of Labrador: evidence for rapid uplift

Quartzo‐feldspathic veins emplaced within a migmatite terrane near Wilson Lake in the Grenville Province of central Labrador record a metamorphic event not evident in the host rocks. The discordant veins are undeformed and have undisturbed primary igneous/hydrothermal textures. Most of the veins contain euhedral kyanite, as well as aggregates of kyanite, K‐feldspar, phlogopite and minor dumortierite which are likely pseudomorphs after primary phengite. The reconstructed phengite compositions range from 3.1 to 3.2 Si per 11 oxygen formula unit. The pseudomorph assemblage is interpreted as the product of phengite + quartz melting under H₂O‐undersaturated conditions, which brackets P–T conditions of formation to about 9–16 kbar and 775–875 °C. A parallel vein that is likely of the same generation contains the borosilicate phases, dumortierite, prismatine and grandidierite, but no kyanite. The borosilicate assemblages constrain the P–T conditions of vein crystallization to ≥10 kbar and c. 750–850 °C. Vein emplacement is constrained to T ≤ 875 °C at the same pressures, which is well within the kyanite zone. Because the host rocks and veins must have experienced the same P–T history following vein emplacement, the presence of unreacted sillimanite in the host migmatites implies insufficient time for host rock equilibration. Slow reaction rates because of anhydrous conditions are not a likely explanation given the abundance of biotite and hornblende in the host rocks. The ductility implied by the breakdown of a hydrous phase (phengite) and the production of an H₂O‐undersaturated melt in the veins contrasts with the apparently brittle behaviour of the host rocks. The absence of deformation since the time of vein emplacement, even at temperatures above 750 °C, suggests that the deep crust in this part of Labrador had a very short residence time under conditions of the kyanite zone. Rapid decompression from those conditions is consistent with quartz + phengite melting and accounts for the relatively brittle behaviour of the terrane as it was uplifted.     

Partial melting during tectonic exhumation of a granulite terrane: an example from the Larsemann Hills, East Antarctica

Anatectic migmatites in medium- to low-pressure granulite facies metasediments exposed in the Larsemann Hills, East Antarctica, contain leucosomes with abundant quartz and plagioclase and minor interstitial K-feldspar, and assemblages of garnet–cordierite–spinel–ilmenite–sillimanite. Qualitative modelling in the system K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O₂, in conjunction with various P–T calculations indicate that the high-grade retrograde evolution of the terrane was dominated by decompression from peak conditions of c. 7 kbar at c. 800 °C to 4–5 kbar at c. 750 °C. Extensive partial melting during decompression involved the replacement of biotite by the assemblage cordierite–garnet–spinel within the leucosomes. These leucosomes represent the site of partial melt generation, the cordierite–garnet–spinel–ilmenite assemblage representing the solid products and excess reactants from the melting reaction. The extraction and accumulation of this decompression-generated melt led to the formation of syntectonic pegmatites and extensive granitic plutons. Leucosome development and terrane decompression proceeded during crustal transpression, synchronous with upper crustal extension, during a progressive Early Palaeozoic collisional event. Subsequent retrograde evolution was characterized by cooling, as indicated by the growth of biotite replacing spinel and garnet, thin mantles of cordierite replacing spinel and quartz within metapelites, and garnet replacing orthopyroxene and hornblende within metabasites. P–T calculations on late mylonites indicate lower grade conditions of formation of c. 3.5 kbar at c. 650 °C, consistent with the development of late cooling textures.     

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

Due 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.     

High‐grade metamorphism and partial melting of basic and intermediate rocks

Rocks of basic and intermediate bulk composition occur in orogenic terranes from all geological time periods and are thought to represent significant petrological components of the middle and lower continental crust. However, the former lack of appropriate thermodynamic models for silicate melt, amphibole and clinopyroxene that can be applied to such lithologies at high temperature has inhibited effective phase equilibrium modelling of their petrological evolution during amphibolite‐ and granulite facies metamorphism. In this work, we present phase diagrams calculated in the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O₂ (NCKFMASHTO) compositional system for a range of natural basic and intermediate bulk compositions for conditions of 2–12 kbar and 600–1050 ^°C using newly parameterized activity–composition relationships detailed in a companion paper by Green et al. in this issue. Particular attention is given to mid‐ocean ridge basalt (MORB) and diorite protolith bulk compositions. Calculated subsolidus mineral assemblages in all basic and intermediate rock types are modally dominated by hornblende and plagioclase, with variable proportions of epidote, clinopyroxene, garnet, biotite, muscovite, quartz, titanite or ilmenite present at different pressures. The H₂O‐saturated (wet) solidus has a negative P?T slope and occurs between ～620–690 ^°C at mid‐ to lower‐crustal pressures of 5–10 kbar. The lowest‐T melts generated close to the wet solidus are calculated to have granitic major‐element oxide compositions. Melting at higher temperature is attributed primarily to multivariate hydrate‐breakdown reactions involving biotite and/or hornblende. Partial melt compositions calculated at 800–1050 ^°C for MORB show good correlation with analysed compositions of experimental glasses produced via hydrate‐breakdown melting of natural and synthetic basic protoliths, with Niggli norms indicating that they would crystallize to trondhjemite or tonalite. Diorite is shown to be significantly more fertile than MORB and is calculated to produce high‐T melts (>800 ^°C) of granodioritic composition. Subsolidus and suprasolidus mineral assemblages show no significant variation between different members of the basalt family, although the P?T conditions at which orthopyroxene stabilizes, thus defining the prograde amphibolite–granulite transition, is strongly dependent on bulk‐rock oxidation state and water content. The petrological effects of open‐ and closed‐system processes on the mineral assemblages produced during prograde metamorphism and preserved during retrograde metamorphism are also examined via a case‐study analysis of a natural Archean amphibolite from the Lewisian Complex, northwest Scotland.     

An experimental study of grain scale melt segregation mechanisms in two common crustal rock types

Creation of pathways for melt to migrate from its source is the necessary first step for transport of magma to the upper crust. To test the role of different dehydration‐melting reactions in the development of permeability during partial melting and deformation in the crust, we experimentally deformed two common crustal rock types. A muscovite‐biotite metapelite and a biotite gneiss were deformed at conditions below, at and above their fluid‐absent solidus. For the metapelite, temperatures ranged between 650 and 800 °C at P_c=700 MPa to investigate the muscovite‐dehydration melting reaction. For the biotite gneiss, temperatures ranged between 850 and 950 °C at P_c=1000 MPa to explore biotite dehydration‐melting under lower crustal conditions. Deformation for both sets of experiments was performed at the same strain rate (ε.) 1.37×10^?5 s^?1. In the presence of deformation, the positive ΔV and associated high dilational strain of the muscovite dehydration‐melting reaction produces an increase in melt pore pressure with partial melting of the metapelite. In contrast, the biotite dehydration‐melting reaction is not associated with a large dilational strain and during deformation and partial melting of the biotite gneiss melt pore pressure builds more gradually. Due to the different rates in pore pressure increase, melt‐enhanced deformation microstructures reflect the different dehydration melting reactions themselves. Permeability development in the two rocks differs because grain boundaries control melt distribution to a greater extent in the gneiss. Muscovite‐dehydration melting may develop melt pathways at low melt fractions due to a larger volume of melt, in comparison with biotite‐dehydration melting, generated at the solidus. This may be a viable physical mechanism in which rapid melt segregation from a metapelitic source rock can occur. Alternatively, the results from the gneiss experiments suggest continual draining of biotite‐derived magma from the lower crust with melt migration paths controlled by structural anisotropies in the protolith.     

Phase equilibria modelling and zircon age dating of pelitic granulites in Zhaojiayao,from the Jining Group of the Khondalite Belt,North China Craton

The Jining Group occurs as the eastern segment of the Khondalite Belt, North China Craton and is dominated by a series of granulite facies rocks involving ‘normal’ pelitic granulites recording peak temperatures of ~850 °C and ultrahigh‐temperature (UHT) pelitic granulites recording peak temperatures of 950–1100 °C. The P–T paths and ages of these two types of granulites are controversial. Three pelitic granulite samples in the Jining Group comprising two sillimanite–garnet gneiss samples (J1208 and J1210) and one spinel–garnet gneiss sample (J1303) were collected from Zhaojiayao, where ‘normal’ pelitic granulites occur, for determination of their metamorphic evolution and ages. Samples J1208 and J1210 are interpreted to record cooling paths from the T_max stages with P–T conditions respectively of ~870–890 °C/7–8 kbar and >840 °C/>7.5 kbar constrained from the stability fields of the observed mineral assemblages and the isopleths of plagioclase, garnet and biotite compositions in pseudosections. Sample J1303 is interpreted to record pre‐T_max decompression from the kyanite‐stability fields to the T_max stage of 950–1020 °C/8–9 kbar and a post‐T_max cooling path revealed mainly from the stability field of the observed mineral assemblage, the plagioclase zoning and the biotite composition isopleth in pseudosections. The post‐T_max cooling stage can be divided into suprasolidus and subsolidus stages. The suprasolidus cooling may not result in an equilibrium state at the solidus in a rock. Therefore, different minerals may record different P–T conditions along the cooling path; the inferred maximum temperature is commonly higher than the solidus as well as different solidi being recorded for different samples from the same outcrop but experiencing different degrees of melt loss. Plagioclase compositions, especially its zoning in plagioclase‐rich granulites, are predicted to be useful for recording the higher temperature conditions of a granulite's thermal history. The three samples studied seem to record the temperature range covering those of the ‘normal’ and UHT pelitic granulites in the Jining Group, suggesting that UHT conditions may be reached in ‘normal’ granulites without diagnostic UHT indicators. LA‐ICP‐MS zircon U–Pb data provide a continuous trend of concordant ²⁰⁷Pb/²⁰⁶Pb ages from 1.89 to 1.79 Ga for sample J1210, and from 1.94 to 1.80 Ga for sample J1303. These continuous and long age spectrums are interpreted to represent a slow cooling and exhumation process corresponding to the post‐T_max cooling P–T paths recorded by the pelitic granulites, which may have followed the exhumation of deeply buried rocks in a thickened crust region resulted from a collision event at c. 1.95 Ga as suggested by the pre‐T_max decompression P–T path.