Subduction-induced mantle heterogeneity beneath Eastern Tianshan and Beishan: Insights from Nd-Sr-Hf-O isotopic mapping of Late Paleozoic mafic-ultramafic complexes

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• 作者：Ben-Xun Su^a ; ^b ; ^{subenxun@mail.igcas.ac.cn} ; Ke-Zhang Qin^a ; ^{kzq@mail.igcas.ac.cn} ; He Sun^a ; Dong-Mei Tang^a ; Patrick Asamoah Sakyi^c ; Zhu-Yin Chu^b ; Ping-Ping Liu^d ; Qing-Hua Xiao^a
• 关键词：Central Asian Orogenic Belt ; Mafic&ndash ; ultramafic complex ; Mantle heterogeneity ; Nd&ndash ; Sr&ndash ; Hf&ndash ; O isotopes ; Subduction
• 刊名：Lithos
• 出版年：2012
• 期刊代码：54_00244937
• 类别：ear
• 出版时间：March, 2012
• 卷：134-135
• 期：Complete
• 页码：41-51
• 文件大小：1933 K

摘要

Extensive late Paleozoic mafic-ultramafic complexes in the Eastern Tianshan and Beishan regions in the southern Central Asian Orogenic Belt (CAOB) represent an ideal opportunity to assess the characteristics of the mantle in this area. Whole-rock Nd-Sr and zircon Hf-O isotopic data from the mafic-ultramafic complexes of the Jueluotage Belt, Middle Tianshan Massif and Beishan Rift were compiled for isotopic mapping. 蔚_Nd(t) and (⁸⁷Sr/⁸⁶Sr)_i values range from + 5.5 to + 9.5 and 0.702 to 0.706 respectively in the Jueluotage Belt, and from 鈭?#xA0;2 to + 11 and 0.703 to 0.710 respectively in the Middle Tianshan and Beishan. Zircon 蔚_Hf (t) and 未¹⁸O values are in the range of + 6.0-+ 17.0 and 4.14鈥扳€?.00鈥?respectively in the Jueluotage Belt, + 1-+ 8 and 5.17鈥扳€?.30鈥?respectively in the Middle Tianshan, and show large variations of 鈭?#xA0;1-+ 17 and 4鈥扳€?2鈥?respectively in the Beishan. Spatially, the (⁸⁷Sr/⁸⁶Sr)_i ratios of mafic-ultramafic complexes increase from the Jueluotage Belt and Beishan to the Middle Tianshan, while the 蔚_Nd(t) values show an opposite trend and reflect a mixing character between depleted mantle and EM-II reservoirs. Zircon 蔚_Hf (t) and 未¹⁸O values exhibit an apparent southward decreasing and increasing trends respectively, from the Jueluotage Belt, through Middle Tianshan, to Beishan Rift. Their corresponding Hf model ages increase continuously from north to south. These observations demonstrate that the mantle sources beneath the Eastern Tianshan and Beishan are compositionally heterogeneous, which presumably result from significant and variable degrees of subduction-related modification. We therefore suggest that the late Paleozoic subcontinental lithospheric mantle beneath the Jueluotage Belt was modified by slab-derived melts and fluids during the subduction of the Junggar Ocean, whereas the mantle beneath the Beishan Rift was influenced mainly by slab-derived fluids during the subduction of the South Tianshan Ocean. The unique similarities of the Middle Tianshan mafic-ultramafic complexes reflect a two-stage modification of their mantle source, namely the sequential subduction of the early South Tianshan Ocean, followed by subduction of the Junggar Ocean. Thus, the subduction-induced mantle heterogeneity beneath the CAOB is probably regional in scope.