中国科学院地球化学研究所柏中杰团队使用熔融地球动力学方法，揭示了中国西藏普朗蛇绿岩的俯冲成因。这一研究成果于2024年4月9日发表在国际顶尖学术期刊《地球化学学报》上。

本研究利用岩石学、矿物学和地球化学资料，对雅鲁藏布缝合带(YZSZ)西段普朗蛇绿岩的地幔橄榄岩进行熔融地球动力学模拟，探讨其构造环境。普朗二辉橄榄岩的矿物组成以原粒状结构为特征，具有类似深海橄榄岩的矿物成分，包括尖晶石中较低的Cr^#(20 ~ 30)和TiO₂含量(＜0.1wt%)，斜方辉石中较高的Al₂O₃(2.9wt%-4.4wt%)和CaO(1.9wt%-3.7wt%)，以及单斜辉石中轻稀土元素(LREE)的亏缺。这些二辉橄榄岩的成分可以用约11%的DMM源动态熔融模拟，其中一小部分熔融物(约0.5%)被包裹在岩内，熔融过程与典型的深海橄榄岩相似。

普朗方辉橄榄岩具有斑岩碎屑结构，并具高耐火矿物成分，如，高尖晶石Cr^#(40-68)、低斜方辉石Al₂O₃ (<2.2wt%)和CaO (<1.1wt%)。这些方辉橄榄岩中的单斜辉石富集Sr(高达6.0ppm)和LREE[(Ce)_N= 0.02-0.4]，但贫Ti(平均200ppm)和HREE[(Yb)_N<2]。更重要的是，越贫化的样品往往具有较高的单斜辉石Sr和LREE含量。观测结果发现，随着板块流体的不断涌入，在俯冲带存在一个开放体系的含水熔融过程。

模拟结果显示，这些方辉橄榄岩可由19％-23％的含水熔融作用形成，板状流体的供给率为0.1%-1%。方辉橄榄岩中的单斜辉石的V/Sc比值低于二辉橄榄岩，表明方辉橄榄岩的熔融系统处于高氧化阶段，这与方辉橄榄岩的水合物熔融环境相一致。因此，普朗蛇绿岩经历了从MOR到SSZ构造环境的转变。

据介绍，关于青藏高原南部的雅鲁藏布江蛇绿岩(YZO)究竟形成于洋中脊(MOR)环境，还是超俯冲带(SSZ)的争议至今仍无定论。

附：英文原文

Title: Melting geodynamics reveals a subduction origin for the Purang ophiolite, Tibet, China

Author: Ruan, Tao, Bai, Zhong-Jie, Zhu, Wei-Guang, Zheng, Shi-Ji

Issue&Volume: 2024-04-09

Abstract: The debate regarding whether the Yarlung–Zangbo ophiolite (YZO) on the south of the Qinghai-Tibet Plateau, formed in a mid-ocean ridge (MOR) or a supra-subduction zone (SSZ) setting has remained unresolved. Here we present petrological, mineralogical, and geochemical data associated with modeling melting geodynamics of the mantle peridotites from the Purang ophiolite in the western segment of the Yarlung–Zangbo Suture Zone (YZSZ) to explore its tectonic environment. The Purang lherzolites are characterized by the protogranular texture and have abyssal-peridotite-like mineral compositions, including low Cr^# (20–30) and TiO₂ contents (<0.1wt%) in spinel, high Al₂O₃ (2.9wt% – 4.4wt%) and CaO (1.9wt% – 3.7wt%) contents in orthopyroxene and LREE-depletion in clinopyroxene. Compositions of these lherzolites can be modeled by~11% dynamic melting of the DMM source with a small fraction of melt (~0.5%) entrapped within the source, a similar melting process to typical abyssal peridotites. The Purang harzburgites are characterized by the porphyroclastic texture and exhibit highly refractory mineral compositions such as high spinel Cr^# (40–68), low orthopyroxene Al₂O₃ (<2.2wt%) and CaO (<1.1wt%) contents. Clinopyroxenes in these harzburgites are enriched in Sr (up to 6.0 ppm) and LREE [(Ce)_N=0.02–0.4], but depleted in Ti (200 ppm, on average) and HREE [(Yb)_N<2]. Importantly, the more depleted samples tend to have higher clinopyroxene Sr and LREE contents. These observations indicate an open-system hydrous melting with a continuous influx of slab fluid at a subduction zone. The modeled results show that these harzburgites could be formed by 19% – 23% hydrous melting with the supply rate of slab fluid at 0.1%–1%. The lower clinopyroxene V/Sc ratios in harzburgites than those in lherzolites suggest a high oxidation stage of the melting system of harzburgites, which is consistent with a hydrous melting environment for these harzburgites. It is therefore concluded that the Purang ophiolite has experienced a transformation of tectonic setting from MOR to SSZ.

DOI: 10.1007/s11631-024-00687-1

Source: https://link.springer.com/article/10.1007/s11631-024-00687-1