Although many recent reviews emphasize a uniformity in granulite pressure-temperature (P-T) conditions and paths, granulites in reality preserve a spectrum of important petrogenetic features which indicate diversity in their modes of formation. A thorough survey of over 90 granulite terranes or occurrences reveals that over 50% of them record P-Tconditions outside the 7.5+ 1 kbar and 8OO + 5O°C average granulite regime preferred by many authors. In particular, an increasing number of very high temperature (900-1000 °C) terranes are being recognized, both on the basis of distinctive mineral assemblages and geothermobarometry. Petrogenetic grid and geothermobarometric approaches to the determination and interpretation of P-T histories are both evaluated within the context of reaction textures to demonstrate that the large range in P-Tconditions is indeed real, and that both near-isothermal decompression (ITD) and near-isobaric cooling (IBC) P-T paths are important. Amphibolite-granulite transitions promoted by the passage of CO 2 -rich fluids, as observed in southern India and Sri Lanka, are exceptional and not representative of fluid-related processes in the majority of terranes. It is considered, on the contrary, that fluid-absent conditions are typical of most granulites at or near the time of their recorded thermal maxima.ITD granulites are interpreted to have formed in crust thickened by collision, with magmatic additions being an important extra heat source. Erosion alone is not, however, considered to be the dominant post-collisional thinning process. Instead, the ITD paths are generated during more rapid thinning (l-2mm/yr exposure) related to tectonic exhumation during moderate-rate or waning extension. IBC granulites may have formed in a variety of settings. Those which show anticlockwise P-T histories are interpreted to have formed in and beneath areas of voluminous magmatic accretion, with or without additional crustal extension. IBC granulites at shallow levels ( < 5 kbar) may also be formed during extension of normal thickness crust, but deeper-level IBC requires more complex models. Many granulites exhibiting IBC at deep crustal levels may have formed in thickened crust which underwent very rapid (5 mm/yr) extensional thinning subsequent to collision. It is suggested that the preservation of IBC paths rather than ITD paths in many granulites is primarily related to the rate and timescale of extensional thinning of thickened crust, and that hybrid ITD to IBC paths should also be observed.Most IBC granulites, and probably many ITD granulites, have not been exposed at the Earth's surface as a result of the tectonic episodes which produced them, but have resided in the middle and lower crust for long periods of time (100-2000 Ma) following these events. The eventual exhumation of most granulite terranes only occur through their incorporation in later tectonic and magmatic events unrelated to their formation.
Ultra-high-temperature (UHT) metamorphism occurs when the continental crust is subjected to temperatures of greater than 900°C at depths of 20-40 km. UHT metamorphism provides evidence that major tectonic processes may operate under thermal conditions more extreme than those generally produced in numerical models of orogenesis. Evidence for UHT metamorphism is recorded in mineral assemblages formed in magnesian pelites, supported by high-temperature indicators including mesoperthitic feldspar, aluminous orthopyroxene and high Zr contents in rutile. Recent theoretical, experimental and thermodynamic data set constraints on metamorphic phase equilibria in FMAS, KFMASH and more complex chemical systems have greatly improved quantification of the P-T conditions and paths of UHT metamorphic belts. However, despite these advances key issues that remain to be addressed include improving experimental constraints on the thermodynamic properties of sapphirine, quantifying the effects of oxidation state on sapphirine, orthopyroxene and spinel stabilities and quantifying the effects of H 2 O-CO 2 in cordierite on phase equilibria and reaction texture analysis. These areas of uncertainty mean that UHT mineral assemblages must still be examined using theoretical and semi-quantitative approaches, such as P(-T)-l sections, and conventional thermobarometry in concert with calculated phase equilibrium methods. In the cases of UHT terranes that preserve microtextural and mineral assemblage evidence for steep or Ônear-isothermalÕ decompression P-T paths, the presence of H 2 O and CO 2 in cordierite is critical to estimates of the P-T path slopes, the pressures at which reaction textures have formed and the impact of fluid infiltration. Many UHT terranes have evolved from peak P-T conditions of 8-11 kbar and 900-1030°C to lower pressure conditions of 8 to 6 kbar whilst still at temperature in the range of 950 to 800°C. These decompressional P-T paths, with characteristic dP ⁄ dT gradients of 25 ± 10 bar°C )1 , are similar in broad shape to those generated in deep-crustal channel flow models for the later stages of orogenic collapse, but lie at significantly higher temperatures for any specified pressure. This thermal gap presents a key challenge in the tectonic modelling of UHT metamorphism, with implications for the evolution of the crust, sub-crustal lithosphere and asthenospheric mantle during the development of hot orogens.
Highly magnesian and aluminous migmatitic gneisses from Mather Peninsula in the Rauer Group, Eastern Antarctica, preserve ultrahigh temperature (UHT) metamorphic assemblages that include orthopyroxene+sillimanite±quartz, garnet+sillimanite±quartz and garnet+orthopyroxene±sillimanite. Garnet that ranges up to X Mg of 71.5 coexists with aluminous orthopyroxene that shows zoning from cores with 7.5-8.5 wt% Al 2 O 3 to rims with up to 10.6 wt% Al 2 O 3 adjacent to garnet. Peak P-T conditions of 1050°C and 12 kbar are retrieved from Fe-Mg-Al thermobarometry involving garnet and orthopyroxene, in very good agreement with independent constraints from petrogenetic grids in FeOMgO-Al 2 O 3 -SiO 2 and related chemical systems. Sapphirine, orthopyroxene and cordierite form extensive symplectites and coronas on the early phases. The specific reaction textures and assemblages involving these secondary phases correlate with initial garnet X Mg , with apparent higher-pressure reaction products occurring on the more magnesian garnet, and are interpreted to result from an initial phase of ultrahigh temperature near-isothermal decompression (UHT-ITD) from 12 to 8 kbar at temperatures in excess of 950°C. Later textures that involved biotite formation and then partial breakdown, along with garnet relics, to symplectites of orthopyroxene+cordierite or cordierite+spinel may reflect hydration through back-reaction with crystallizing melts on cooling below 900-850°C, followed by ITD from 7 to 8 kbar to c. 5 kbar at temperatures of 750-850°C. The tectonic significance of this P-T history is ambiguous as the Rauer Group records the effects of Archean tectonothermal events as well as high-grade events at 1000 and 530 Ma. Late-stage biotite formation and subsequent ITD can be correlated with the P-T history preserved in the Proterozoic components of the Rauer Group and hence with either 1000 or 530 Ma collisional orogenesis. However, whether the preceding UHT-ITD history reflects a temporally unrelated event (e.g. Archean) or is simply an early stage of either the late-Proterozoic or Pan-African tectonism, as recently deduced for similar UHT rocks from other areas of the East Antarctica, remains uncertain.
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