Refining the P–T records of UHT crustal metamorphism S. L. HARLEY Grant Institute of Earth Science, School of GeoSciences, University of Edinburgh, Kings Buildings, West Mains Road, Edinburgh EH9 3JW, UK (simon.harley@ed.ac.uk) ABSTRACT 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. Key words: cordierite; fluid activity; Granulites; pseudosections; P–T paths; ultra-high-temperature. INTRODUCTION In recent years, the recognized pressure–temperature (P–T) domain of metamorphism has been vastly ex- panded as the extremes of metamorphism have become more clearly documented in the geological record. With respect to extremes of temperature, ultra-high- temperature (UHT) metamorphism is now recognized as a fundamental aspect of the Earth system (Harley, 1998a, 2004; Kelsey et al., 2003a, 2005; OÕBrien & Ro¨ tzler, 2003; Pattison et al., 2003; Osanai et al., 2004). Crustal rocks undergoing UHT metamorphism are subjected to temperatures of 900–1100 °C at only moderate pressures (7–13 kbar), on regional scales (>10 3 km 2 : Ellis, 1980; Ellis et al., 1980; Harley, 1989, 1998a). Hence, UHT metamorphism provides clear evidence that major tectonic processes affecting the continental crust may progress under thermal condi- tions that greatly exceed those generally produced in numerical models of collisional orogenesis (Beaumont et al., 2001, 2006; Jamieson et al., 2004; Burg & Gerya, 2005). The extreme thermal conditions have implica- tions for rheological models describing the deforma- tion of the deep crust (e.g. Lund et al., 2006) and thermotectonic models that examine the nature of coupling between the crust, sub-crustal lithosphere and asthenospheric mantle. It is therefore important to obtain accurate quantitative P–T and P–T–time path records for UHT terranes, as these, along with rates and time scale data, provide key constraints on the thermotectonic models. A variety of indicators (Fig. 1; e.g. Ellis et al., 1980; Harley, 1998a; Hokada, 2001; Kelsey et al., 2003a; Fonarev et al., 2006) including mineral assemblages in magnesian pelites, aluminous orthopyroxene, meso- perthitic feldspar, metamorphic pigeonite in ironstones and high Zr contents in rutile (Zack et al., 2004; Watson et al., 2006; Tomkins et al., 2007), quantified using calculated phase diagram methods, provide conclusive evidence for UHT metamorphism in J. metamorphic Geol., 2008, 26, 125–154 doi:10.1111/j.1525-1314.2008.00765.x Ó 2008 Blackwell Publishing Ltd 125