科研之友
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摘要
The intimate spatial relationship between the Jiajika Li-rich pegmatites (hosting the largest Li ore deposit in China) and the Majingzi granite pluton allows us to explore the origin of pegmatites and associated Li-mineralization mechanism by examining the trace elements and Sr-Nd-Li isotopes of the two rock units in eastern Tibetan plateau. The Jiajika Li-rich pegmatites show extremely low CaO, TFe2O3, MgO, Sr and Ba, and high Li and Rb when compared with the adjacent Majingzi two-mica granite, and their initial Sr isotopic ratios (0.7212-0.7249, obtained from apatite) are significantly higher than those of the granite and the surrounding Xikang Group metapelites (0.7128-0.7163). Whole rock Li isotopes analyses yield delta Li-7 values of + 0.3 to + 1.9 parts per thousand for the Jiajika Li-rich pegmatites, - 0.5 to - 0.8 parts per thousand for the Majingzi two-mica granite, and - 3.2 to + 2.4 parts per thousand for the Xikang Group metapelites, respectively. Modeling studies on trace elements and Li isotopes consistently demonstrate that the Jiajika Li-rich pegmatites are unlikely to have been originated from extreme differentiation of the Majingzi two-mica granite as traditionally thought. Instead, they could be directly generated by low degrees (5-20%) of muscovite-dehydration melting of a mixed source dominated by Li-rich claystones and subordinate Xikang Group metapelites under amphibolite facies conditions. We suggest that the existence of Li-rich claystone interlayers (probably accompanied by evaporates and carbonates) in the source is crucial to pegmatitic spodumene mineralization. This explains the abundance of fluxing components and Li mineralization in the Jiajika pegmatite, and the general observation that Li-rich pegmatites always show Li isotopic compositions lighter than the Li-poor counterparts in the same orogenic belt.
