Mantle evolution beneath a back-arc basin: Highly siderophile elements and Os isotope of Dongco ophiolitic peridotites, Tibet

IF 3.6 2区地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS

Chemical Geology Pub Date : 2025-03-28 DOI:10.1016/j.chemgeo.2025.122757

Xiao-Ni Li , Wei-Qi Zhang , Tong Liu , Chang Zhang , Yin-Zheng Lin , Zhen Zhang , Jia-Ning Zhu , Hong-Yu Li , Chuan-Zhou Liu

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Abstract

Coexistence of mantle peridotites with distinct evolutionary histories is commonly observed in global ophiolites, especially in supra-subduction zone (SSZ) settings. The Dongco ophiolite in central Tibet has been interpreted to form within a Jurassic slow-spreading back-arc basin. This study presents comprehensive geochemical data of two types of peridotites from the Dongco ophiolite to investigate processes that occurred in the asthenosphere beneath a back-arc basin. Type-1 peridotites contain 5–9 vol% clinopyroxene and have spinel Cr# values <0.21, which are more fertile in compositions compared to Type-2 peridotites. Type-1 peridotites are characterized by relatively flat whole-rock REE patterns, but variably LREE-depleted patterns for pyroxenes. Their HSE patterns are slightly fractionated. Type-1 peridotites are estimated to have experienced 2–8 % degrees of melting under water-poor conditions, similar to mid-ocean ridge (MOR) peridotites. In contrast, Type-2 peridotites contain less than 2 vol% clinopyroxene and have spinel Cr# >0.55. They display U-shaped whole-rock REE patterns and LREE-enriched patterns for pyroxenes. Their HSE patterns are highly fractionated, with positive platinum anomalies and supra-chondritic Pt/Pd and Ru/Ir ratios. Type-2 peridotites are estimated to have undergone 20–24 % degrees of melting under hydrous conditions, resembling typical SSZ peridotites. Both types of Dongco peridotites have similar ranges of ¹⁸⁷Os/¹⁸⁸Os ratios, yielding Re depletion ages (T_RD) of 0.2–1.0 Ga. We interpret the Dongco peridotites represent residues of distinct mantle sources generated at different stages during the evolution of a back-arc basin. Type-2 peridotites represent residues of mantle wedge after hydrous melting and thus represent SSZ- peridotites; Type-1 peridotites are relics of the asthenosphere after low degrees of melting that were negligibly affected by slab-released fluids or hydrous melts, and belong to MOR- peridotites. The tectonic juxtaposition of SSZ- and MOR-type peridotites in the Dongco ophiolite supports the hypothesis of partial replacement of sub-arc mantle by the upwelling asthenosphere during back-arc spreading.

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弧后盆地地幔演化：西藏洞洞蛇绿橄榄岩的高亲铁元素和Os同位素

在全球蛇绿岩中，特别是在超俯冲带（SSZ）环境中，普遍存在具有不同演化历史的地幔橄榄岩共存现象。西藏中部东错蛇绿岩被解释为形成于侏罗纪慢扩张弧后盆地内。本文利用东沟蛇绿岩中两类橄榄岩的综合地球化学资料，探讨了弧后盆地软流圈的演化过程。1型橄榄岩含斜辉石5-9 vol%，尖晶石Cr值<；0.21，其组成比2型橄榄岩更肥沃。1型橄榄岩具有相对平坦的全岩REE模式，而辉石质则具有不同的lree亏缺模式。它们的HSE模式略有分化。据估计，1型橄榄岩在缺水条件下经历了2 - 8%的融化度，与洋中脊（MOR）橄榄岩相似。2型橄榄岩斜辉石含量小于2 vol%，尖晶石Cr = 0.55。辉石岩呈现u型全岩REE模式和lree富集模式。它们的HSE模式是高度分馏的，具有正铂异常和超线粒体的Pt/Pd和Ru/Ir比率。2型橄榄岩估计在含水条件下经历了20 - 24%的熔融程度，与典型的SSZ橄榄岩相似。两类东洞橄榄岩的187Os/188Os比值范围相似，Re枯竭年龄（TRD）为0.2 ~ 1.0 Ga。东沟橄榄岩代表了弧后盆地不同演化阶段不同地幔源的残余。2型橄榄岩为地幔楔块熔融后的残余物，为SSZ型橄榄岩；1型橄榄岩是软流圈低熔融后的遗迹，受板释流体或含水熔体的影响可忽略不计，属于more -橄榄岩。东沟蛇绿岩中SSZ型和莫尔型橄榄岩的构造并置支持弧后扩张过程中上升流软流圈部分取代弧下地幔的假说。

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来源期刊

Chemical Geology 地学-地球化学与地球物理

CiteScore

7.20

自引率

10.30%

发文量

374

审稿时长

3.6 months

期刊介绍： Chemical Geology is an international journal that publishes original research papers on isotopic and elemental geochemistry, geochronology and cosmochemistry. The Journal focuses on chemical processes in igneous, metamorphic, and sedimentary petrology, low- and high-temperature aqueous solutions, biogeochemistry, the environment and cosmochemistry. Papers that are field, experimentally, or computationally based are appropriate if they are of broad international interest. The Journal generally does not publish papers that are primarily of regional or local interest, or which are primarily focused on remediation and applied geochemistry. The Journal also welcomes innovative papers dealing with significant analytical advances that are of wide interest in the community and extend significantly beyond the scope of what would be included in the methods section of a standard research paper.