Geochemical evidence for forearc metasomatism of peridotite in the Xigaze ophiolite during subduction initiation in southern Tibet
Geochemical evidence for forearc metasomatism of peridotite in the Xigaze ophiolite during subduction initiation in southern Tibet
复制标题
藏南俯冲起始时期日喀则蛇绿岩中橄榄岩弧前交代作用的地球化学证据
DOI：
10.1016/j.lithos.2020.105896
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发表时间：
2021
期刊：
影响因子：
3.5
通讯作者：
Yong-Fei Zheng
中科院分区：
文献类型：
--
作者：
Mei-Shan Zhao;Yi-Xiang Chen;Yong-Fei Zheng
俯冲起爆是板块构造作用的关键过程之一。我们对这一过程的认识主要来源于对大陆边缘蛇绿岩中玄武岩的地球化学研究。由于弧前玄武岩的岩石学继承，获取弧前深部新生地幔楔流体交代的地球化学特征具有重要意义。蛇绿岩中的橄榄岩受蛇绿岩向大陆边缘构造侵位的影响最小。研究结果为揭示新生代印度-亚洲大陆碰撞新特提斯洋闭合前俯冲起始期的前弧交代作用提供了依据。尖晶石Cr#值高，CaO、al2o3和REE全岩富集，表明在海底扩张形成新特提斯洋中脊玄武岩的过程中，熔体萃取程度高（~10 ~ 20%）。微量元素分布模式呈LILE正异常，提示俯冲带流体交代作用。在一些哈尔茨伯基岩中还发现了透闪石和白云石，证明了这一点。根据碳酸盐矿物的存在与否，可以区分出两种交代流体。一是水溶液，导致透闪石的形成，并在斜辉石中富集LILE。二是含碳酸盐流体，形成白云石、透闪石和低Mg#（79 ~ 82）橄榄石。交代作用还导致辉石中结构羟基含量高，哈尔茨堡土的δ18O值升高。推断流体交代作用是在新特提斯洋中脊转化为海沟后不久，新特提斯幼年洋板向弧前深度的早期俯冲引起的。在这一俯冲起始期，流体是由下沉的脊壳向弧前深度加热脱水产生的。正是这些流体使上覆的新生地幔楔发生交代作用，导致地球化学富集特征叠加在蛇绿岩中岩石化学富集的哈尔茨伯尔岩上。因此，蛇绿岩中橄榄岩的弧前交代作用可作为俯冲起始的地球化学标志。日喀则蛇绿岩形成于中生代俯冲起始的脊沟转换，并于新生代侵位于亚洲大陆南缘。
Subduction initiation is one of the key processes in the operation of plate tectonics. Our understanding of this process is largely derived from geochemical studies of basaltic rocks in ophiolites at continental margins. Because of the petrological inheritance in forearc basalts, it is important to acquire geochemical signatures for fluid metasomatism of the nascent mantle wedge at forearc depths. This may be provided by the study of peridotite in ophiolites, which is least affected by tectonic emplacement of the ophiolites onto continental margins. For this purpose, we have carried out a comprehensive study of whole-rock and mineral geochemical compositions for harzburgites from the Xigaze ophiolites in the Yarlung Zangbo suture zone, southern Tibet. The results provide insights into the forearc metasomatism during subduction initiation before the closure of Neo-Tethyan Ocean for the India-Asia continental collision in the Cenozoic. The harzburgites show high spinel Cr# values and strong whole-rock depletion in CaO, Al2O3and REE, suggesting high degrees of melt extraction (~10–20%) during seafloor spreading to form the Neo-Tethyan mid-ocean ridge basalts. They exhibit positive LILE anomalies in trace element distribution patterns, indicating metasomatism by subduction zone fluids. This is supported by the occurrence of tremolite and dolomite in some of the harzburgites. Based on the presence or absence of carbonate minerals, two types of metasomatic fluids are distinguished. One is the aqueous solutions, leading to the formation of tremolite and the enrichment of LILE in clinopyroxene. The other is the carbonate-bearing fluids, resulting in the formation of dolomites, tremolite and low Mg# (79–82) olivine. The metasomatism also results in the high contents of structural hydroxyl in pyroxenes and elevated δ18O values for the harzburgites. It is inferred that the fluid metasomatism was induced by incipient subduction of the juvenile Neo-Tethyan oceanic slab to forearc depths shortly after conversion of the Neo-Tethyan mid-ocean ridge into the trench. During this period of subduction initiation, the fluids were produced by heating dehydration of the sinking ridge crust to the forearc depths. It is these fluids that would metasomatize the overlying nascent mantle wedge, leading to superimposition of the geochemically enriched signature on the lithochemically depleted harzburgites in the ophiolites. Therefore, the forearc metasomatism of peridotite in ophiolite can be used as a geochemical proxy for subduction initiation. Furthermore, the Xigaze ophiolites were produced by the ridge-trench conversion for subduction initiation in the Mesozoic and they were emplaced onto the southern margin of the Asian continent in the Cenozoic.