Probing the basement of southern Tibet: evidence from cr

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• journal_title：Journal of the Geological Society
• Contributor：G.H.-N. Chan ; D.J. Waters ; M.P. Searle ; J.C. Aitchison ; M.S.A. Horstwood ; Q. Crowley ; C.-H. Lo ; J.S.-L. Chan
• Publisher：Geological Society of London
• Date：2009-
• Format：text/html
• Language：en
• Identifier：10.1144/0016-76492007-145
• journal_abbrev：Journal of the Geological Society
• issn：0016-7649
• volume：166
• issue：1
• firstpage：45
• section：Research Article

摘要

<p id="p-1">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 m>cm>. 16 °C kmp>−1p>. 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. m>In situm> 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 p>206p>Pb/p>238p>U ages of 16.8 ± 0.9 Ma and 15.6 ± 0.6 Ma, respectively, and monazites from a micaceous xenolith yielded a mean p>208p>Pb/p>232p>Th age of 14.4 ± 0.4 Ma. These results show that the southern Tibet basement reached a thickness of m>cm>. 80 km by 17–14 Ma at the latest and has remained unchanged until the present day. p>