On ultrahigh temperature crustal metamorphism:Phase equilibria,trace element thermometry,bulk composition,heat sources,timescales and tectonic settings

Ultrahigh temperature （UHT） metamorphism is the most thermally extreme form of regional crustal metamorphism, with temperatures exceeding 900 ℃. UHT crustal metamorphism is recognised in more than 50 localities globally in the metamorphic rock record and is accepted as ＇normal＇ in the spectrum of regional crustal processes. UHT metamorphism is Wpically identified on the basis of diagnostic mineral assemblages such as sapphirine ＋ quartz, orthopyroxene ＋ sillimanite ＋ quartz and osumilite in Mg-Al- rich rock compositions, now usually coupled with pseudosection-based thermobarometry using internally-consistent thermodynamic data sets and/or Al-in-Orthopyroxene and ternary feldspar thermobarometry. Significant progress in the understanding of regional UHT metamorphism in recent years includes： （1） development of a ferric iron activity-composition thermodynamic model for sapphirine, allowing phase diagram calculations for oxidised rock compositions; （2） quantification of UHT conditions via trace element thermometry, with Zr-in-rutile more commonly recording higher temperatures than Ti-in-zircon. Rutile is likely to be stable at peak UHT conditions whereas zircon may only grow as UHT rocks are cooling. In addition, the extent to which Zr diffuses out of rutile is controlled by chemical communication with zircon; （3） more fully recognising and utilising temperature-dependent thermal properties of the crust, and the possible range of heat sources causing metamorphism in geodynamic modelling studies; （4） recognising that crust partially melted either in a previous event or earlier in a long-duration event has greater capacity than fertile, unmelted crust to achieve UHT conditions due to the heat energy consumed by partial melting reactions; （5） more strongly linking U-Pb geochronological data from zircon and monazite to P-T points or path segments through using Y ＋ REE partitioning between accessory and major phases, as well as phase diagrams incorporating Zr and REE; and （6） improved insight into the settings and factors responsible for UHT metamorphism via geodynamic forward models. These models suggest that regional UHT metamorphism is, principally, geodynamically related to subduction, coupled with elevated crustal radiogenic heat generation rates.

Cenozoic ultrahigh-temperature metamorphism in pelitic granulites from the Mogok metamorphic belt, Myanmar

Ultrahigh-temperature(UHT)metamorphism is critical for understanding the most extreme thermal evolution of continental crust.However,UHT metamorphism predominantly occurred in the Precambrian and is rarely observed in the modern Earth.Here,we report the discovery of~25 Ma UHT granulites from the Mogok metamorphic belt(MMB)in Myanmar via a combined study of petrology and geochronology.The studied pelitic granulites well preserve a peak mineral assemblage of garnet+sillimanite+plagioclase(antiperthite)+K-feldspar+quartz+Ti-rich biotite+rutile+ilmenite.Pressure(P)-temperature(T)pseudosections and conventional geothermobarometry data only constrain the P-T conditions of the peak stage to<12 kbar and 780–890°C.However,high Zr contents in the matrix rutile(3005–4308 ppm)and high Ti contents(up to 9.2 wt% TiO_(2))in the biotite demonstrate that the Mogok granulites may have experienced UHT metamorphism.The Zr-in-rutile thermometer and X_(Grs) isopleth in the pseudosections yield peak P-T conditions of~12 kbar and>900°C.In situ SIMS and LAICP-MS U-Pb dating and trace element analyses show that both metamorphic zircon cores and rims have flat heavy rare earth element(HREE)patterns with negative Eu anomalies.The metamorphic zircon rims show the lowest HREE contents and yield ^(206)Pb/^(238)U ages of 24.9±0.5 and 25.4±0.6 Ma,respectively,representing the timing of UHT metamorphism.Our results indicate that the central MMB underwent~25 Ma UHT metamorphism,which is possibly induced by continental rifting along the thinned orogenic lithosphere.Our data,as well as reported Cenozoic UHT events,further suggest that UHT metamorphism can be produced in the modern plate tectonic regime by lithospheric extension.