Carboniferous-Permian Rift-Related Volcanism and Mantle Plume in the Tianshan, Northwestern China

The Tianshan rift-related volcanic rocks make up a large igneous province erupted during the Carboniferous-Early Permian period in northwestern China.The volcanic successions comprise thick piles of basaltic lavas and subordinate intermediate and silicic lavas and pyroclastics.Based on petrographic,major and trace element,and Sr-Nd-Pb isotope data,the Tianshan basalts can be classified into two major magma types.These are:(1) a high-Ti/Y(HT) type that exhibits high Ti/Y(>500),Ce/Y(>3),and relatively low Nb/Zr(< 0.11) and ε_Nd(t)(-1.15);(2) a low-Ti Y(LT) that has low Ti/Y(<500).The L T lavas can be futher divided in to two sub types.LT 1 lavas exhibit lower Nb/Zr(< 0.15) and higher ε_Nd(t)(+3.1 to + 9.7);LT 2 lavas have higher Nb/Zr(> 0.16) and lower ε_Nd(t)(=-0.98 to -2.91).Elemental and isotopic data suggest that the chemical variations of the H T and L T lavas can not be explained by crystallization from a common parental magma.In stead,they most likely originatedfroman oceanic-sland-basalt-like mantle source(⁸⁷Sr/⁸⁶Sr(t)≈0.7045,ε_Nd(t)≈+4,²⁰⁶Pb/²⁰⁴Pb(t)≈18.35,²⁰⁷Pb/²⁰⁴Pb(t)≈15.66,²⁰⁸Pb/²⁰⁴Pb(t)≈38.25,La/Nb≈0.7) under various melting conditions andunderwent distinct differentiation and contamination processes.