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《Chinese Journal of Geology》 2003-04
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GEOCHEMISTRY OF EMEISHAN FLOOD BASALTS AT BINCHUAN AREA, SW CHINA: ROCK TYPES AND TEMPORAL EVOLUTION

Xiao Long 1 Xu Yigang 1 Mei Houjun 1 Sha Shaoli 2 (1. Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640; 2. The Third Geological Party, Bureau of Geology and Mineral Resources of Yunnan Province, Dali, Yunnan 671000)  
The Late Permian Emeishan flood basalts, with thickness over 5 000 meters at Binchuan area in western margin of the Yangtze craton, consisted mainly of tholeiitic basalts and minor alkaline basalts. Based on petrography, major and trace element data, the tholeiitic basalts could be classified into two major geochemical types: low Ti (LT) and high Ti (HT) basalts. The LT type lavas, occurred in the lower middle part of petrologic column, exhibited lower Ti/Y (500), ∑REE (120), (La/Yb) N (6), (Sm/Yb) N (2.0) and higher Mg # (0.48~0.67); whereas the HT type lavas on the top of the petrologic column had higher Ti/Y (500), ∑REE (150), (La/Yb) N (9), (Sm/Yb) N (3) and lower Mg # (0.39~0.53). The LT could be further divided into two subtypes: LT1 and LT2. They occurred in the lower and middle parts of the petrologic column respectively. LT1 lavas exhibited relatively higher Mg # (0.61~0.67) and lower Nb/U ratio than that of the LT2 type lavas, and had positive Th and U anomaly. The LT and HT lavas might be originated from different mantle source under various melting conditions and underwent distinct differentiation and contamination processes. The LT lavas, with significant "crustal source" signatures, might associate with the mobilization of enrichment component in lithosphere mantle. "Crustal source" signatures were getting less from the bottom (LT1) to the top (HT) of the petrologic column. LT lavas that might be formed at the main activity stage of the Emeishan plume, might be generated from the head of mantle plume with higher degree partial melting and then experienced weak garnet + clinopyroxene fractional crystallization. Late waning stage formed HT lavas experienced lower degree of partial melting, and then underwent strong plagioclase dominated fractional crystallization process in the upper crust and little crustal contamination.
【Fund】: 中国科学院知识创新项目 (KZCX2 1 0 1 );; 国家 973项目 (G1 9990 43 2 0 5 )的成果
【CateGory Index】: P588.145
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