Nd-Sr-Os isotopic data of the Baishiquan mafic-ultramafic complex from East Tianshan, and implications for petrogenesis
Nd-Sr-Os isotopic data of the Baishiquan mafic-ultramafic complex from East Tianshan, and implications for petrogenesis
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作者：
He Jing
本文报道了产自东天山（中天山前寒武纪地块北段）并伴有铜镍矿床的白石泉基性-超基性杂岩的微量元素和Nd-Sr-Os同位素数据，以了解其幔源特征和幔壳相互作用过程。地球化学特征为LILEs富集，HFSEs耗竭。白石泉杂岩全岩Sr-Nd同位素组成变化显著，（87Sr/86Sr）i=0.7032~0.7066, eNd(t)=5.6~-0.9， Nd-Sr同位素比值呈负相关。配合物的Os同位素组成（187Os/188Os）i=0.295~1.18, Os=80×10-12~29×10-12。结果表明：(1)白石泉杂岩母岩浆在岩浆侵位过程中受到了明显的地壳污染，Nd、Sr同位素组成呈负相关且变化显著，Os同位素组成呈高放射性成因。这与白石泉杂岩在岩浆上升过程中没有受到地壳污染的流行模式不同；(2)白石泉杂岩体的母岩浆起源于古生代俯冲带流体/熔体交代的幼年岩石圈地幔。这需要重新考虑中天山前寒武纪地块的性质和分布。
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 (87Sr/86Sr)i=0.7032~0.7066 and eNd(t)=5.6~-0.9, and the Nd-Sr isotopic ratios are negatively correlated. The complex shows very radiogenic Os isotopic compositions, with (187Os/188Os)i=0.295~1.18 and Os=80×10-12~29×10-12. 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.