Geochemistry and Tectonic Implications of the Early Paleozoic Felsic to Intermediate Volcanic Rocks from Kaladawan Area,North Altyn

The Early paleozoic intermediate—felsic rock suit located in Kaladawan area consisting of mainly dacite,rhyolite and limited trachy dacite. They are geochemically characterized by high content of SiO2 ,high total alkali (Na2O+K2O=2.49%～9.74% ,Na2O/K2O =0.4～22.2) ,low but variable TiO 2 on the whole. Moreover Rittman values (δ) ranging from 0.2 to 5.52,alumina saturation index A/CNK (molecular proportion) ranging from 0.8 to 1.4 and corundum occurring in CIPW calculation which geochemically indicates that the dacite—rhyolite suite is attributed to low-K to med-K calc-alkaline rocks series and alkaline shoshonite series with a transition character that from metaluminous I-type to weakly—strongly peraluminous S-type granitics north—south through the Kaladawan aera. The remarkable enrichment of LREE,K,Ba,Th(U) ,Pb,Zr and distinct depletion of Nb(Ta) ,P,Ti,Sr,Rb inplys a closely subduction-related origin at active continental margin arcs. Petrological and geochemical features indicate that the dacites generated by dehydration partial melting of metaigeneous,the rhyolites derived from the dehydration partial melting of metapelites,and they could’nt be homologous magma evolution series. Combined with the field observational data,we presume that the intermediate—felsic rock suit is generated during syn-colllison and post-collision events of the late stage subduction in an active continental margin arc,they derived from partial melting at different depths of the grown crust below it which is due to low crustal (or lithospheric) delamination and asthenosphere mantle underplating caused by the thickening of continental crust during subduction and collision or stretching and thinning of the lithosphere in post-collision. Above all,the intermediate—felsic magmatic activity in Kaladawan area is a direct response to the southward subduction of ancient oceanic crust in Northern Altyn to the Milan—Jinyanshan Terrain during the Early Paleozoic period,and this also provides a definite evidence for confirming the polarity of Hongliugou—Lapeiquan ophiolite melange zone in the North Altyn Mountain.