Probing the basement of southern Tibet: evidence from crustal xenoliths entrained in a Miocene ultrapotassic dyke.

A variety of felsic and mafic granulites and ultramafic rocks occur as xenoliths within a 12.7 Ma ultrapotassic dyke intruding Xigaze flysch immediately to the north of the Yarlung-Tsangpo suture zone in southern Tibet. Garnet-clinopyroxene-plagioclase-quartz thermobarometry on mafic granulite xenoliths gives temperatures of 1130-1330 °C and pressures between 22 and 26 kbar indicating equilibration in the high-pressure and ultrahigh-temperature granulite field and defining a geotherm of c. 16 °C km-1. Ultramafic xenoliths consist mainly of hornblende and biotite, probably of restitic crustal rather than mantle origin, and attained peak metamorphic conditions of 920-1130 °C and 17-24 kbar, whereas felsic granulites equilibrated at 870-900 °C at an inferred pressure of 17 kbar. In situ U-(Th)-Pb laser ablation inductively coupled plasma mass spectrometry dating of zircons shows that protoliths may include Proterozoic basement rocks, Late Cretaceous calc-alkaline tonalites of the Gangdese batholith root and/or remnants of a Neo-Tethyan oceanic arc. Certain zircons from a felsic granulite and an ultramafic xenolith have mean 206Pb/238U ages of 16.8 ± 0.9 Ma and 15.6 ± 0.6 Ma, respectively, and monazites from a micaceous xenolith yielded a mean 208Pb/232Th age of 14.4 ± 0.4 Ma. These results show that the southern Tibet basement reached a thickness of c. 80 km by 17-14 Ma at the latest and has remained unchanged until the present day. [ABSTRACT FROM AUTHOR]