Arc magmatism associated with steep subduction: Insights from trace elements and Sr-Nd-Hf-B isotope systematics
Arc magmatism associated with steep subduction: Insights from trace elements and Sr-Nd-Hf-B isotope systematics
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与陡峭俯冲相关的弧岩浆作用：微量元素和 Sr-Nd-Hf-B 同位素系统学的见解
DOI：
10.1002/2016jb013289
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发表时间：
2017
期刊：
影响因子：
--
通讯作者：
Lin Zhengfan
中科院分区：
文献类型：
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作者：
Zhang Yunying;Yuan Chao;Sun Min;Long Xiaoping;Wang Yunpeng;Jiang Yingde;Lin Zhengfan
俯冲带是板块脱水、地幔交代和熔融等元素循环的主要场所。然而，与陡峭的俯冲有关的板片流体的性质在很大程度上仍然未知。为了阐明这一问题，我们对北山造山带晚古生代（318- 312 Ma）的中间岩墙进行了综合研究。脉岩主要由闪长质和花岗闪长质岩石组成。闪长岩脉具有典型的俯冲改造特征，具有较高的Mg#、Nd（t）和Hf（t），以及较低的初始Sr同位素，表明其可能起源于俯冲改造地幔。花岗闪长质岩墙具有高Mg#、高Sr/Y、La/Yb和Na 2 O/K2 O比值、低Y和Yb含量、洋中脊玄武岩样Sr-Nd同位素和高锆石Hf（t）等特征，与板状埃达克岩相似，表明它们可能是俯冲洋壳部分熔融形成的。同时代的埃达克质和正常闪长岩脉反映了一种热异常，可能是由俯冲洋板回滚造成的。闪长岩脉的B-11值为-7.7 ~-6.4，埃达克岩脉的B-11值相对较高，为-6.9 ~-4.4。埃达克岩脉的B-11值低于典型蚀变洋壳的值，这与俯冲洋板在早期俯冲过程中B-11的损失相一致。混合模型的结果表明，地幔源的闪长岩脉已杂交的B-11亏损的流体从一个高度脱水的板片在深部，由于俯冲洋板的高角度倾角。
Subduction zones are the major sites for elemental cycling via slab dehydration and subsequent mantle metasomatism and melting. However, the nature of slab fluids associated with steep subduction remains largely unknown. To clarify this issue, we present an integrated study for Late Paleozoic (318-312Ma) intermediate dykes from the Beishan orogenic collage, NW China. The dykes consist mainly of dioritic and granodioritic rocks. The dioritic dykes exhibit typical subduction-like geochemical signatures, together with relatively high Mg#, high epsilon(Nd)(t) and epsilon(Hf)(t), and low initial Sr isotopes, suggesting that they originated probably from a subduction-modified mantle. The granodioritic dykes exhibit high Mg#, high Sr/Y, La/Yb, and Na2O/K2O ratios, low Y and Yb contents, and mid-ocean ridge basalt-like Sr-Nd isotopes and high zircon epsilon(Hf)(t), similar to slab-derived adakite, indicating that they were likely formed by partial melting of subducted oceanic crust. The coeval adakitic and normal dioritic dykes reflect a thermal anomaly that was probably caused by rollback of subducted oceanic slab. The dioritic dykes have B-11 values from -7.7 to -6.4, whereas the adakitic dykes have relatively high B-11 values from -6.9 to -4.4. The B-11 values of adakitic dykes are lower than those of typical altered oceanic crust, in agreement with the expected loss of B-11 from subducted oceanic slab during early subduction. Results of a mixing model suggest that the mantle source of the dioritic dykes has been hybridized by B-11-depleted fluids expelled from a highly dehydrated slab at deep depth, owing to the high-angle dip of the subducting oceanic slab.