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

Abstract

A variety of felsic and mafic granulites and ultramafic rocks occur as xenoliths within a 12.7million-year-old 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 22-26 kbar indicating equilibration in the high-pressure and ultrahightemperature granulite field and defining a geotherm of ca. 16 °C/km. 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 LA-ICP-MS 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 206 Pb/ 238 U ages of 16.8 ± 0.9 and 15.6 ± 0.6 Ma respectively, and monazites from a micaceous xenolith yielded a mean 208 Pb/ 232 Th age of 14.4 ± 0.4 Ma. These results show that the southern Tibet basement reached ca. 80 km thickness by 17-14 Ma at the latest and that the crust remained as thick to the present day.

Speculations on the formation of cratons and cratonic basins

“…Fig. 8a shows a number of estimates of P, T from such nodules, from Hacker et al (2000Hacker et al ( , 2005, Ducea et al (2003), Jolivet et al (2003), Ding et al (2007), Chan et al, (2009), andGordon et al (2012). In agreement with Fig.…”

“…The estimates are from the metamorphic xenolith locations shown in Fig. 2b, marked with the same three letter codes, from Hacker et al (2000Hacker et al ( , 2005, Ducea et al (2003), Jolivet et al (2003), Ding et al (2007), Chan et al, (2009), andGordon et al (2012). (b) shows the same curves for shortening occurring 3 Ga ago.…”

Crustal–lithospheric structure and continental extrusion of Tibet

“…In the Karakoram terrane (western equivalent of the Qiangtang block) numerous lamprophyric dykes (mainly biotite-bearing minettes and amphibole-bearing vogesites, with similar Oligocene-early Miocene ages intrude the northern Karakoram terrane (Searle et al 1992(Searle et al , 2010a. Chan et al (2009) sampled both mafic UHT-UHP granulite and hornblende + biotite restitic xenoliths formed at 1130-1330 8C and 22-26 kbar as well as felsic granulites formed at 870-900 8C and 17 kbar from a 12.7 Ma shoshonitic dyke in SW Tibet. U-Pb zircon and monazite ages from the granulite xenoliths span 14.4 AE 0.4 to 16.8 AE 0.9 Ma.…”

“…We are reliant on lower crustal xenoliths entrained in Miocene ultrapotassic volcanic rocks and dykes to infer the composition of the lower crust. Chan et al (2009 described high-pressure granulite xenoliths from southern Tibet and the Qiangtang terrane respectively. P-T conditions of these Miocene granulites indicate a thick (60-80 km), hot (.900-1000 8C) and dry lower crust.…”

“…The composition of the lower crust and upper mantle can directly be known only from xenoliths entrained in ultrapotassic or potassic (shoshonitic) volcanic rocks that have sampled the lower crust on their way up to the surface (e.g. Hacker et al 2000Hacker et al , 2005Ducea et al 2003;Chan et al 2009). …”