Petrogenesis of Caboniferous-Early Permian Rift-Related Volcanic Rocks in the Tianshan and its Neighboring Areas, Northwestern China

The Tian shan Carboniferous-Early Permian riftrelated volcanism in northw estern China represents anewly-recognized largeigneous province extending over at least 1.7×10⁶km².The volcanic successions comprise thick piles of basaltic lavas and subord inate intermediate and silicic lavas and pyroclastics,and are in terpreted to result fromamantle plume with component of ε_Nd(t)≈+5,(⁸⁷Sr/⁸⁶Sr(t)≈0.704 and La Nb≈0.9.On the basis of petrogeochemical data,the Carboniferous-Early Permian basic lavas can be classified in to high-Ti/Y(HT,Ti/Y>500)and low-Ti/Y(LT,Ti/Y<500)magma types.The LT lavas can be further divided in to four sub types:LT1,LT2,LT3 and LT4.The chemical evolution of the LT1,LT2(in central Tian shan and Baishan of Gansu),LT4(in western Tian shan, Jungar and Baishan of Xinjiang)and LT3,HT(Tarim)lavas is controlled by an olivine(ol)+clinopy roxene(cpx)fractionation,but gabbroic fractionation accounts for the chemical variation of the LT4 lavas from eastern Tianshan.Elemental and isotopic data suggest that the chemical variation of the Carboniferous-Early Permian riftrelated basic lavas in the Tianshan and their neighboring areas can not be explained by crystallization from a commonparental magma.The Sr-Nd isotopic variation of the crustally contaminated LT3 and LT4 lavas is related to the nature of lithosp here through which the plume-derived melts have erupted.