[SMP38-10] Petrologic evidence for ultrahigh-temperature metamorphism in lower crustal granulite xenoliths from Siberian kimberlite pipes
Keywords:Siberian craton, high-pressure granulite, ultrahigh-temperature metamorphism, xenolith, armalcolite
Solvus geothermometers to the integrated mode composition of exsolved pyroxenes and feldspars suggest that granulites have experienced a high-temperature metamorphism of at least 900oC, which is higher than previous temperature estimations. Pyroxene geothermometer also suggests that granulites have cooled to 600oC. Garnet contains abundant inclusions of Fe-Ti oxides and/or Ti-rich oxide minerals. It is noteworthy that our study newly discovered the occurrence of Fe-rich armalcolite. In pargasite-rich granulites, F contents of pargasite reach up to 0.8 wt% and ones of biotite reach up to 1.6 wt%, resembling to those minerals in some ultrahigh-temperature (UHT) metamorphic rocks in collisional orogenic belts. Applying in-situ petrographic thin-section U-Pb dating technique, we could confirm Paleoproterozoic age (1830 ± 18 Ma) for metamorphic zircon, textually in equilibrium with garnet and pargasite. Considering the exsolution features and the occurrence of Fe-rich armalcolite, the lower crust would have undergone a Proterozoic UHT metamorphism. Subsequently, it has cooled relatively fast and had stayed under the condition of amphibolite-facies for a considerable time until the Early Paleozoic kimberlite eruption.