Magmatic record of Prototethyan evolution in SW Yunnan, China: Geochemical, zircon U-Pb geochronological and Lu-Hf isotopic evidence from the Huimin metavolcanic rocks in the southern Lancangjiang zone

摘要

The Huimin metavolcanic rocks, consisting mainly of basalt, andesite, and dacite, are exposed predominantly in the Huimin area of the southern Lancangjiang zone, SW Yunnan. Three metavolcanic rock samples yielded zircon U-Pb ages of 456 +/- 3 Ma, 456 +/- 7 Ma and 459 +/- 14 Ma, suggesting that the eruption occurred in the Late Ordovician. These metavolcanics are characterized by sub-alkaline rocks, with high Al2O3 (12.45-17.68 wt.%) and low TiO2 (0.60-0.96 wt.%). The rocks are enriched in LREEs and LILEs with weakly negative Eu anomalies (Eu/Eu* = 037-0.99) and depleted in HFSEs, which is geochemically similar to typical subduction-related arc volcanic rocks. Zircons with Late Ordovician magmatic ages (similar to 456 Ma) have homogeneous Lu-Hf isotopic compositions with in-situ epsilon(Hf) (t) values of -4.36-3.30 and Hf model ages of 1.51-1.55 Ga, indicating that the Huimin metavolcanic rocks might have originated from a metasomatic mantle source. Our data provide reliable evidence for the presence of Ordovician magmatism in the Sanjiang area, represent the magmatic record of Prototethyan evolution in SW Yunnan, and identify a hitherto unknown early subduction event at the northeastern Gondwana margin. (C) 2014 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.