科研之友
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摘要
We report trace elements and Nd-Sr-Os isotopic data for the Baishiquan mafic-ultramafic complex that occurs in the East Tianshan Mountains ( in the northern part of the Mid-Tianshan Precambrian block) and is accompanied by Cu-Ni deposits, to understand its mantle source characteristics and processes of mantle-crustal interaction. Geochemically,, the complex is characterized by enrichment of LILEs and depletion of HFSEs. The whole-rock Sr-Nd isotopic compositions of the Baishiquan complex vary significantly, with (Sr-87/Sr-86), = 0.7032 similar to 0.7066 and epsilon(Nd) (t) = 5.6 similar to -0.9, and the Nd-Sr isotopic ratios are negatively correlated. The complex shows very radiogenic Os isotopic compositions, with (Os-187/Os-188)(i) = 0.295 similar to 1.18 and Os = 80 x 10(-12) similar to 29 x 10(-12). Based on these data, we conclude that (1) the parent magma of the Baishiquan complex experienced significant crustal contamination during magma emplacement, as is revealed by the negatively correlated and significantly varied Nd and Sr isotopic compositions and the highly radiogenic Os isotopic compositions. This is different from a popular model that the Baishiquan complex received no crustal contamination during magma ascent; (2) the parent magma of the Baishiquan complex originated from a juvenile lithospheric mantle that was previously metasomatized by subduction zone fluids/melts in the Paleozoic. This requires reconsideration of the nature and distribution of the Mid-Tianshan Precambrian block.
