Forearc magmatic evolution during subduction initiation: Insights from an Early Cretaceous Tibetan ophiolite and comparison with the Izu-Bonin-Mariana forearc
Forearc magmatic evolution during subduction initiation: Insights from an Early Cretaceous Tibetan ophiolite and comparison with the Izu-Bonin-Mariana forearc
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DOI：
10.1130/gsab.s.12670091
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发表时间：
2020-07
期刊：
影响因子：
--
通讯作者：
J. Dai;E. al.
中科院分区：
文献类型：
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作者：
J. Dai;E. al.
俯冲作用的启动是板块构造活动的一个关键过程。我们对俯冲起始过程中的熔融过程和岩浆演化的理解主要来自伊豆-小笠原-马里亚纳前弧的研究。许多俯冲带上的蛇绿岩与伊豆-小笠原-马里亚纳弧前序列相似。然而，是否有伊豆-小笠原-马里亚纳俯冲起始序列和俯冲带上蛇绿岩之间的差异仍然不清楚。在这里，我们报告现场地质，地球化学和地质年代学数据，从日喀则蛇绿岩（藏南）地幔和地壳部分的镁铁质和长英质岩石，从这条蛇绿岩和Izu-Bonin-Mariana前弧相同类型的已发表的数据进行了比较。蛇绿岩段被各种晚期岩脉侵入，包括辉长伟晶岩、闪长岩、玄武岩和斜长花岗岩。单斜辉石和角闪石的成分表明辉长岩伟晶岩形成于含水的高SiO2亏损熔体，而玄武质和闪长岩脉的全岩成分显示负Nb和Ta异常，表明亏损地幔的通量熔融。沿着镁铁质岩石，斜长花岗岩具有大致恒定的含量的La和Yb的SiO2含量的增加，这意味着镁铁质角闪岩的水化熔融。早期枕状玄武岩表现出与伊豆-小笠原-马里亚纳弧前玄武岩的地球化学相似性，但稍富集。综合区域地质背景，并与伊豆-小笠原-马里亚纳弧前岩浆活动相比较，我们认为，从洋中脊玄武岩类熔岩到与俯冲有关的基性岩脉和长英质岩脉的转变记录了拉萨造山带南翼早白垩世的俯冲起始事件。然而，日喀则蛇绿岩的地幔源区和岩浆演化比伊豆-小笠原-马里亚纳前弧的地幔源区和岩浆演化更多变。
Subduction initiation is a key process in the operation of plate tectonics. Our understanding of melting processes and magmatic evolution during subduction initiation has largely been developed from studies of the Izu-Bonin-Mariana forearc. Many suprasubduction zone ophiolites are analogous to the Izu-Bonin-Mariana forearc sequence. However, whether there are differences between Izu-Bonin-Mariana subduction initiation sequences and suprasubduction zone ophiolites remains unclear. Here, we report field geological, geochemical, and geochronological data from mafic and felsic rocks in the Xigaze ophiolite (southern Tibet) mantle and crustal section; the same types of published data from both this ophiolite and the Izu-Bonin-Mariana forearc are compiled for comparison. The ophiolite section is intruded by various late-stage dikes, including gabbroic pegmatite, diabase, basalt, and plagiogranite. The compositions of clinopyroxene and amphibole suggest that gabbroic pegmatite formed from hydrous high-SiO2 depleted melts, while whole-rock compositions of basaltic and diabase dikes show negative Nb and Ta anomalies, suggesting flux melting of depleted mantle. Along with the mafic rocks, plagiogranite has a roughly constant content of La and Yb with increasing SiO2 contents, implying hydrous melting of mafic amphibolite. Early-stage pillow basalts exhibit geochemical affinities with Izu-Bonin-Mariana forearc basalts, but they are slightly enriched. Synthesized with the regional geological setting and compared with Izu-Bonin-Mariana forearc magmatism, we propose that the transition from mid-ocean ridge basalt–like lavas to subduction-related mafic and felsic dikes records an Early Cretaceous subduction initiation event on the southern flank of the Lhasa terrane. However, the mantle sources and the magmatic evolution in the Xigaze ophiolite are more variable than those for the Izu-Bonin-Mariana forearc.