Zircon U-Pb dating and in-situ Hf isotopic analysis of Permian peraluminous granite in the Lhasa terrane, southern Tibet: Implications for Permian collisional orogeny and paleogeography

The Lhasa terrane has long been interpreted as a simple tectonic block rifted from Gondwana during the late Paleozoic and then drifted northward before finally amalgamating with the Qiangtang terrane during the Early Cretaceous. In this paper we document Permian peraluminous granites near Pikang in the southern margin of the central Lhasa terrane, close to the recently documented Songdo eclogite of Permian age. Zircon SHRIMP and LA-ICPMS U-Pb dating for a Pikang granite sample gives an identical crystallization age of about 263 Ma and a wide age range of inherited zircons (283-2141 Ma). In situ Hf isotopic analyses for 20 zircons of 263 Ma yielded epsilon(Hf(t)) values of -4.5 to +1.9. The Pikang granites have high A/CNK values ( >= 1.08) and high normative corundum (1.3-2.0%), indicative of peraluminous S-type granite. They are characterized by moderately negative Eu anomalies (Eu/Eu*=0.48-0.61), and strongly negative Ba, Nb, Sr, P and Ti anomalies. The granites have high epsilon(Nd(t)) values (-6.4 to -6.0) and low initial Sr-87/Sr-86 ratios (0.7082-0.7096) relative to melts derived from mature continental crust. These rocks are interpreted to have been generated by mixing between mantle melts and their induced melting of mature crustal materials. We interpret the Pikang peraluminous granite magmatism, the regional angular unconformity between the Middle and the Upper Permian and the eclogite of the same metamorphic age (similar to 262 Ma) from the same geotectonic location to represent different products of a common event in time and space. We tentatively term this common event as syncollisional orogeny, i.e., the Permian Gangdese Orogeny. We further hypothesize that the orogeny may be genetically associated with the collision between the Lhasa terrane and the northern margin of Australia, following the closure of the Paleo-Tethyan Ocean south of the Lhasa terrane. (C) 2009 Elsevier B.V. All rights reserved.