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

XIA Lin-qi, LI Xiang-min, XIA Zu-chun, XU Xue-yi, MA Zhong-ping, WANG Li-she(Xi'an Institute of Geology and Mineral Resources, Xi'an 710054, Shaanxi, 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 LT lavas can be futher divided into two subtypes. LT1 lavas exhibit lower Nb/Zr (0.15) and higher ε_ Nd (t) (+3.1to+9.7); LT2 lavas have higher Nb/Zr ( 0.16) and lower ε_ Nd (t) (=-0.98to-2.91). Elemental and isotopic data suggest that the chemical variations of the HT and LT lavas can not be explained by crystallization from a common parental magma. Instead, they most likely originated from an oceanic-island-basalt-like mantle source (~ 87 Sr/~ 86 Sr (t)≈0.7045, ε_ Nd (t)≈+4,~ 206 Pb/~ 204 Pb(t) ≈18.35,~ 207 Pb/~ 204 Pb(t)≈15.66,~ 208 Pb/~ 204 Pb(t)≈38.25, La/Nb≈0.7) under various melting conditions and underwent distinct differentiation and contamination processes. The predominantly HT alkaline lavas were generated by low degrees of partial melting in the garnet stability field of the mantle source. The chemical variation of the HT lavas is controlled by a clinopyroxene (Cpx) [±olivine (Ol)] fractionation. In contrast, parental magmas of the LT type were generated by lower degree of partial melting for the LT2 (alkaline) subtype and by higher degree of partial melting for the LT1 (predominantly tholeiitic) subtype of the mantle source around the spinel-garnet transition zone. These magmas were then subjected to shallow level gabbroic fractionation, which led to larger chemical variation. The Tianshan basalts may result from a starting mantle plume. The petrogenesis of both the HT and LT magmas was further complicated by contamination of crust and lithospheric mantle. Our data show that spatial petrogeochemical variations exist in the volcanic rocks of the Tianshan large igneous province. The location of the thickest LT1 volcanic succession, which may record the main episode of the basalt emplacement, in the eastern Tianshan may have been centered over the mantle plume or mantle melting anomaly. In contrast, the less abundant HT and LT2 basalts may imply a weakening of the influence of the mantle plume activity. In fact, the HT and LT2 basalts are the dominant magma type in the periphery of the province. The lower degrees of mantle melting of the HT and LT2 lavas may be the result of a relatively thicker lithosphere and lower geotherm.
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