Precipitation of rutile needles in garnet from sillimanite-bearing pelitic granulite from the Khondalite Belt, North China Craton

Abstract

Pelitic granulite from the Khondalite Belt, North China Craton, is composed of garnet, sillimanite, perthite, biotite and quartz. Oriented rutile needles in garnet are reported for the first time from the granulite. The rutile needles have been identified by optical microscopy and confirmed by in situ Raman spectroscopy, where they show the characteristic bands at 446–448 and 610 cm−1. Because of their very strong shape preferred orientation in 3–4 easily observable directions, these rutile needles are interpreted to have formed by precipitation during exhumation and cooling following peak metamorphism. The ternary feldspar geothermometer gives a peak metamorphic temperature of ~ 980 °C at 10 kbar. The presence of oriented rutile needles in garnet is suggested to be a valuable indicator of ultrahigh-temperature (UHT) metamorphism in the Khondalite Belt and therefore likely in other UHT terrains.