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Geochronological and Geochemical Constraints on Lizhigou Middle Triassic Felsic Volcanic Rocks in Hainan and Its Tectonic Implications

Gou Qiyu;Qian Xin;He Huiying;Zhang Yuzhi;Wang Yuejun;Guangdong Provincial Key Laboratory of Geodynamics and Geohazards,School of Earth Science and Engineering,Sun Yat-sen University;  
Hainan Island,which is located between the Indochina and South China blocks,has undergone multiphased structural overprinting and complex tectonic evolution.However,the Late Paleozoic tectonic regime and amalgamation process of the South China with Indochina blocks are still controversial.The newly-identified felsic volcanic rocks in the Lizhigou area are important for understanding the Early Mesozoic tectonic evolution in Hainan.LA-ICP-MS zircon U-Pb dating results show that the representative rhyolite samples yield a weighted mean ~(206)Pb/~(238)U age of 241±6 Ma(MSWD=0.9).These samples contain SiO_2 ranging from 72.34%to 77.83%,Al_2O_3 from 10.51%to 13.53%,K_2 O from 2.85%to 4.85%and Na_2 O from 1.75%to3.79%.The A/CNK values range from 0.99 to 2.07.These samples have REE concentrations of 87×10~(-6) to 177×10~(-6),and Eu/Eu*of 0.49-0.65.They are characterized by subparallel right-sloping REE pattern and enrichment in LREEs and LILEs,and depletion in HFSEs,similar to those of arc volcanics.Theirε_(Nd)(t)values range from-12.1 to-11.3.These felsic volcanic rocks are the products of partial melting of metasediments with significant fractionation crystallization.In combination with other available data,it is inferred that the Lizhigou felsic volcanic rocks can be compared with the Middle-Late Triassic igneous rocks along the Jinshajiang-Ailaoshan-Songma zone,and formed during the assemblage of the South China with Indochina blocks related to East Paleotethyan consumption.
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