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Geochemical characteristics of mafic and felsic igneous rocks (1.9–1.75 Ga) in the Lesser Himalaya: Regional variation and its implications for tectonic setting
Island Arc ( IF 1.5 ) Pub Date : 2020-09-28 , DOI: 10.1111/iar.12369
Ryuichi Shinjo 1 , Tomoki Amuro 1 , Kohei Oura 1 , Kazuya Oshiro 1 , Shiro Tahara 1 , Harutaka Sakai 2
Affiliation  

Major and trace element and Nd–Hf isotopic compositions of Paleoproterozoic meta‐igneous rocks from the lower part of the Lesser Himalayan sequence provide insights into the nature of their source mantle and its tectonic setting. We observed bimodal (basaltic and rhyolitic) magmatic compositions in these rocks; this type of volcanism generally occurs in areas of extensional tectonics, particularly rifts. Two types of mafic rocks are identified: within‐plate type (subduction‐unrelated, enriched mantle derived) rocks and Nb–Ta‐depleted, continental arc‐like rocks. Within‐plate type mafic rocks are dominant in the eastern Nepal, though similar rocks are also found in western Nepal. Although models for the tectonic setting of these igneous rocks remain controversial (e.g. passive margin, continental arc, continental rift with possible hotspot), our new geochemical data support the rift model associated with mantle plume activity, which evolved to a passive margin. The Nb–Ta‐depleted, arc‐like rocks have lower εNd (t) (−1.7 to 1.7) and εHf (t) (0.9–4.4) than within‐plate type rocks; we consider that these rocks are formed from the interaction of mantle‐derived magmas with crustal components. All felsic rocks have similar or slightly lower εNd (t) (−5.2 to −0.1) and εHf (t) (−6.5 to 2.7) values relative to arc‐like mafic rocks, suggesting relatively limited crustal interaction. They may be derived from juvenile crust of underplated mafic magmas. Most of the felsic rocks have remarkably low Nb/Ta ratios (< 5), suggesting modification on their chemical composition by intensive sub‐solidus magmatic‐hydrothermal alteration.

中文翻译:

小喜马拉雅地区镁铁质和长英质火成岩(1.9-1.75 Ga)的地球化学特征:区域变化及其对构造环境的影响

小喜马拉雅层序下部的古元古代后火成岩的主要和微量元素以及Nd-Hf同位素组成提供了对其源地幔性质及其构造背景的认识。我们在这些岩石中观察到了双峰(玄武质和流纹岩)岩浆成分。这种类型的火山活动通常发生在伸展构造,特别是裂谷地区。识别出两种类型的镁铁质岩石:板内类型(俯冲无关,富集地幔衍生)岩石和Nb-Ta耗尽的大陆弧状岩石。板岩型镁铁质岩石在尼泊尔东部占主导地位,尽管在尼泊尔西部也发现了类似的岩石。尽管这些火成岩的构造环境模型仍然存在争议(例如,被动边缘,大陆弧,大陆裂谷和可能的热点),我们的新地球化学数据支持与地幔柱活动相关的裂谷模型,该模型演化为被动边界。Nb-Ta耗尽的弧状岩石比板内型岩石具有更低的εNd(t)(-1.7至1.7)和εHf(t)(0.9-4.4);我们认为这些岩石是由地幔源岩浆与地壳成分相互作用形成的。相对于弧状镁铁质岩石,所有长英质岩石的εNd(t)(−5.2至-0.1)和εHf(t)(−6.5至2.7)值均相似或略低,表明地壳相互作用相对有限。它们可能来自底层镁铁质岩浆的幼壳。大多数长英质岩石的Nb / Ta比值都非常低(<5),这表明强烈的亚固相岩浆-水热蚀变作用对其化学成分进行了改变。
更新日期:2020-10-15
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