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Journal of the Geological Society; 2009; v. 166; issue.1; p. 45-52;
DOI: 10.1144/0016-76492007-145
© 2009 Geological Society of London

Research Article

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

G.H.-N. Chan1,2, D.J. Waters1, M.P. Searle1, J.C. Aitchison3, M.S.A. Horstwood4, Q. Crowley4, C.-H. Lo5 & J.S.-L. Chan3

1 Department of Earth Sciences, Parks Road, Oxford University, Oxford OX1 3PR, UK
2 SRK Consulting, 10 Richardson Street, West Perth, 6005 WA, Australia
3 Department of Earth Sciences, University of Hong Kong, Hong Kong, P.R. China
4 NERC Isotope Geosciences Laboratory, Keyworth, Nottingham NG12 5GG, UK
5 Department of Geosciences, National Taiwan University, Taipei, Taiwan

*Corresponding author (e-mail: dave.waters{at}earth.ox.ac.uk)

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.

Supplementary material: Mineral analyses used for pressure–temperature estimations and results of 40Ar/30Ar laser single-grain fusion experiments are available at http://www.geolsoc.org.uk/SUP18325.





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