There are debates on the tectonic setting of the acid volcanic rocks of Xiaotikanlike Formation exposed along Kuqa River in Southwest Tianshan,and precise U-Pb ages of these rocks have not been reported yet. 7 geochemical samples and 2 samples for zircon U-Pb dating from this volcanic suit are analyzed. Geochemical data indicate that the Xiaotikanlike Formation volcanic rocks are mainly rhyolites. They are characterized by high alkali(Na2O+K2O=6.89%～8.46%),potassium(K2O/Na2O=1.6～2.1)and Al2O3(12.83%～14.78%)content,while low CaO(1.06%～2.51%),MgO(0.27%～0.44%)and Fe2O3(1.29%～2.68%)content. They are strongly enriched in LREE and depleted in HREE,and have medium negative Eu anomaly(δEu=0.46～0.54)in the chondrite-normalized REE-patterns diagram. They are also enriched in LILE elements like Rb,Th,U,K,Pb,and depleted in Sr,Nb,Ta,P and Ti. Based on the geochemical features and CaO/Na2O,(La/Yb)N ratios,we suggest that the rhyolites were derived from partial melting of metagreywackes. The residual minerals were with garnet,hornblende and plagioclase. In situ U-Pb LA-ICP-MS dating on magmatic zircons from the two samples yield ages of 288.3 Ma and 290.3 Ma respectively. Integrating previous studies,especially the extensive distributed post-orogenic granites of Permian in southwest Tianshan,we conclude that the Xiaotikanlike Formation volcanic rocks have the features of peraluminous S-type granite formed in a syn-collisional setting,and we believe that the southwest Tianshan Ocean has been closed at least in Early Permian.
. Geochemistry and geochronology of Early Permian acid volcanic rocks along Kuqa River and its tectonic implication in the southern margin of South Tianshan Orogen, Xinjiang[J]. Chinese Journal of Geology, 2014, 49(1): 242-258.