13Liu et al.__IJES.pdfVIP

ORIGINAL PAPER Magma mixing revealed from in situ zircon U–Pb–Hf isotope analysis of the Muhuguan granitoid pluton, eastern Qinling Orogen, China: implications for late Mesozoic tectonic evolution Rui Liu ? Jian-Wei Li ? Shi-Jian Bi ? Hao Hu ? Mi Chen Received: 21 August 2012 / Accepted: 5 April 2013 / Published online: 12 May 2013  Springer-Verlag Berlin Heidelberg 2013 Abstract In this paper, we present zircon U–Pb age and Hf isotope data to document the significance of magma mixing in the formation of Late Jurassic granitoid intrusions in the eastern Qinling Orogen, China. The Muhuguan granitoid pluton from this orogen consists of monzogranite and lesser biotite granite and granodiorite, all containing abundant hornblende-rich cumulates, dioritic xenoliths, and mafic magmatic enclaves (MMEs). The monzogranite and granodiorite are intruded by a number of lamprophyre dykes. Both a cumulate and a dioritic xenolith samples have concordant zircon U–Pb ages of ca. 161 ± 1 Ma, but pos- sess contrasting Hf isotopic compositions. The cumulate has more radiogenic zircon Hf isotopes with negative eHf(t) values (-7.9 to -2.5) and TDM1 ages of 0.9–1.1 Ga, indicating its derivation likely from basaltic rocks of the Neoproterozoic to early Paleozoic Kuanping Group in the area. The dioritic xenolith has much lower zircon eHf(t) values of -19.5 to -8.8 and TDM2 ages of 2.4–1.7 Ga, consistent with a juvenile Paleoproterozoic crust source presumably represented by the metabasic rocks of the Qinling Group in the area. Individual samples of the monzogranite, MME, and a lamprophyre dyke have U–Pb ages of 150 ± 1, 152 ± 1, and 152 ± 1 Ma, respectively, demonstrating coeval mafic and felsic magmatism in the Late Jurassic. The lamprophyre dyke has homogeneous, highly negative zircon eHf(t) values (-29.8 to -24.8) and Archean TDM2 ages (3.0–2.7 Ga), and its genesis is inter- preted as partial melting of an ancient enriched subconti- nental mantle source. Zircons from the fine-grained MME