Ultra-depleted melt product preserved in the Ladong ophiolitic peridotites of the North Qilian Orogenic Belt, Northern Tibet
Ultra-depleted melt product preserved in the Ladong ophiolitic peridotites of the North Qilian Orogenic Belt, Northern Tibet
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DOI：
10.1016/j.lithos.2022.106985
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发表时间：
2022-12
期刊：
影响因子：
3.5
通讯作者：
Xiang Zhou;Jian-Ping Zheng;Zengbao Huang;Zhi-Yong Li;Wei-Qi Zhang;Hong-Da Zheng;Q. Xiong;H. Dai
中科院分区：
文献类型：
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作者：
Xiang Zhou;Jian-Ping Zheng;Zengbao Huang;Zhi-Yong Li;Wei-Qi Zhang;Hong-Da Zheng;Q. Xiong;H. Dai
在地球上地幔中，迁移岩浆和固体残留物的组成基本上受源物质和熔融行为的控制，并与构造动力学有关。因此，蛇绿岩岩性记录的罕见的地球化学特征可能反映了独特的来源或特定的熔融历史。本文报道了北祁连造山带拉东蛇绿岩杂岩中方辉橄榄岩、方辉橄榄岩和橄榄辉橄榄岩的全岩和矿物组成。尖晶石方辉橄榄岩具有高的尖晶石Cr#值(65~70)和轻稀土元素(LREE)亏损模式(LaN/YBN=0.04~0.12)，类似于20%或更多熔体提取后的残留物。方辉橄榄岩中的微辉橄榄岩透镜体表现出极度亏损的LREE配分模式(全岩Lan/YBN=0.04-0.05)，单斜辉石中的Na和Ti含量较低。它们被解释为寄主方辉橄榄岩与软流圈减压熔融产生的超贫化熔体反应的产物。橄榄辉橄榄岩露头具有辉石颗粒和高镁橄榄石之间平衡良好的三重结点结构(Fo=0.85-0.86；块状岩石Mg=0.93)，与堆积成因一致。在橄榄辉橄榄岩中，单斜辉石中LREE的亏损浓度(In/YBN=0.05-0.18)表明其贫化程度低于形成辉石的熔体。因此，拉东蛇绿岩记录了两次熔体活动，产生了亏损到超亏损的岩浆，可能是在东原特提斯带俯冲开始时产生的。
In Earths upper mantle, the compositions of both migrating magmas and solid residues are basically controlled by source materials and melting behavior which are related to tectonic dynamics. Thus, infrequent geochemical signatures recorded by ophiolitic lithologies may reflect a unique source or specific melting history. Here, we report whole-rock and mineral compositions of the harzburgite, wehrlite, and olivine websterite in the Ladong ophiolite complex of the North Qilian Orogen Belt (northern Tibet). The dominant spinel harzburgite has high spinel Cr# values of 65–70 and light rare earth element (LREE) depleted patterns (LaN/YbN= 0.04–0.12), resembling the residua after 20% or more of melt extraction. Wehrlite lenses within the harzburgite display extremely depleted LREE patterns (whole-rock LaN/YbN= 0.04–0.05) and low Na and Ti contents in clinopyroxene. They are interpreted as products of reaction between the host harzburgite and an ultra-depleted melt produced by asthenospheric decompression melting. Outcrops of olivine websterite have well-equilibrated triple junction textures between pyroxene grains and high-Mg olivine (Fo = 85–86; bulk rock Mg# = 93), consistent with a cumulate origin. In the olivine websterite, the depleted LREE concentrations (LaN/YbN= 0.05–0.18) in clinopyroxene suggest a less depleted nature than the wehrlite-forming melt. Thus, the Ladong ophiolite records two episodes of melt activity that produced depleted to ultra-depleted magmas, which may have been generated during subduction initiation in eastern Proto-Tethyan belt.