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Xue-Yi Xu , Xian Institute of Geology and Mineral Resources (China)
Lin-Qi Xia, Xian Institute of Geology and Mineral Resources (China)
Zu-Chun Xia, Xian Institute of Geology and Mineral Resources (China)
Xiang-Min Li, Xian Institute of Geology and Mineral Resources (China)
Zhong-Ping Ma, Xian Institute of Geology and Mineral Resources (China)
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The Neoproterozoic (846∼776Ma) Bikou Group volcanic rocks distributed in Yangtze craton in southwestern China. The volcanic succession comprises a thick pile of mafic volcanic rocks , subordinate silicic and rare intermediate volcanic rocks. Based on detailed field observations and petrological studies, the volcanic rocks of the Bikou group can be divided into three volcanic cycles, and each cycle consists of basic volcanic rocks (spilite or spilitic tuff) in lower part and acid volcanic rocks (quartz keratophyre or quartz keratophyric tuff) in the upper part. The basic volcanic rocks of the first cycles belong to the alkali basalt magma series and those of the second and the third cycles the tholeiitic magma series; the acid volcanic rocks are the product of the anatexis of the crust and the volcanic rocks of the Bikou Group are of the typical bimodal volcanic rocks formed in a continental rift and is the product of continental extension on the north margin of the Neoproterozoic Yangtze block.
On the basis of geochemical data, the Bikou Group rift-related mafic lavas generally belong to a low Ti/Y (500) magma type. Elemental and isotopic data suggest that the chemical variations of the Bikou Group mafic lavas cannot be explained by crystallization from a common parental magma. The Bikou Group mafic lavas most likely originated from a mantle plume source (εNd(t)≈+3, 87Sr/86Sr≈0.704, La/Nb≈0.7). Crustal contamination has also contributed significantly to the formation of the Bikou Group rift-related basic lavas. Our data show that spatial geochemical variations exist in the Bikou group rocks. The Bikou mafic lavas are dominantly tholeiitic in composition in the eastern Hongyangou and Xintianba-Heimulin area, and were generated by high degrees of partial melting in the garnet stability field. In contrast, in the western Baiyang-Bikou area the parental magmas were generated by lower degrees of partial melting for the alkaline lavas and by higher degrees of partial melting for the tholeiitic lavas from a mantle source in the spinel-garnet transition zone. These magmas were then subjected to shallow level gabbroic (cpx+plag+ol) fractionation, which led to larger chemical variations.
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