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Tectono-magmatic evolution of Tethyan oceanic lithosphere in supra subduction zone forearc regime: Geochemical fingerprints from crust-mantle sections of Naga Hills Ophiolite
Geoscience Frontiers ( IF 8.9 ) Pub Date : 2020-11-18 , DOI: 10.1016/j.gsf.2020.10.003
Anisha Verencar , Abhishek Saha , Sohini Ganguly , C. Manikyamba

The Naga Hills Ophiolite (NHO) belt in the Indo-Myanmar range (IMR) represents a segment of Tethyan oceanic crust and upper mantle that was involved in an eastward convergence and collision of the Indian Plate with the Burmese Plate during the Late Cretaceous-Eocene. Here, we present a detailed petrological and geochemical account for the mantle and crustal sections of Naga Hills Ophiolite (NHO), Northeastern India to address (i) the mantle processes and tectonic regimes involved in their genesis and (ii) their coherence in terms of the thermo-tectonic evolution of Tethyan oceanic crust and upper mantle. The NHO suite comprises well preserved crustal and mantle sections discretely exposed at Moki, Ziphu, Molen, Weshello and Lacham areas. The ultramafic-mafic lithologies of NHO are mineralogically composed of variable proportions of olivine, orthopyroxene, clinopyroxene and plagioclase. The primary igneous textures for the mantle peridotites have been overprinted by extensive serpentinisation whereas the crustal section rocks reflect crystal cumulation in a magma chamber. Chondrite normalised REE profiles for the cumulate peridotite-olivine gabbro-gabbro assemblage constituting the crustal section of NHO show flat to LREE depleted patterns consistent with their generation from depleted MORB-type precursor melt in an extensional tectonic setting, while the mantle peridotites depict U-shaped REE patterns marked by relative enrichment of LREE and HREE over MREE. These features collectively imply a dual role of depleted MORB-type and enriched arc-type mantle components for their genesis with imprints of melt-rock and fluid-rock interactions. Tectonically, studied lithologies from NHO correspond to a boninitic to slab-proximal Island Arc Tholeiite affinity thereby conforming to an intraoceanic supra subduction zone (SSZ) fore-arc regime coherent with the subduction initiation process. The geochemical attributes for the crustal and mantle sections of NHO as mirrored by Zr/Hf, Zr/Sm, Nb/Ta, Zr/Nb, Nb/U, Ba/Nb, Ba/Th, Ba/La and Nd/Hf ratios propound a two-stage petrogenetic process: (i) adepleted Fore arc basalts (FAB) type tholeiitic melt parental to the crustal lithologies was extracted from the upwelling asthenospheric mantle at SSZ fore-arc extensional regime thereby rendering a refractory residual upper mantle; (ii) the crust and upper mantle of the SSZ fore arc was progressively refertilised by boninitic melts generated in response to subduction initiation and slab-dehydration. The vestiges of Tethyan oceanic lithosphere preserved in NHO represent an accreted intra-oceanic fore-arc crust and upper mantle section which records a transitional geodynamic evolution in a SSZ regime marked by subduction initiation, fore arc extension and arc-continent accretion.



中文翻译:

前俯冲带上特提斯洋岩石圈的构造-岩浆演化:纳迦山蛇绿岩地壳幔剖面的地球化学指纹

印缅范围内的纳迦丘陵蛇绿岩(NHO)带代表了特提斯洋壳和上地幔的一部分,参与了白垩纪-始新世晚期印度板块与缅甸板块的东向汇聚和碰撞。 。在这里,我们介绍了印度东北部纳迦丘陵蛇绿岩(NHO)的地幔和地壳剖面的详细岩石学和地球化学描述,以解决(i)与其成因有关的地幔过程和构造机制,以及(ii)在特提斯洋壳和上地幔的热构造演化。NHO套件包括保存完好的地壳和地幔部分,分别在Moki,Ziphu,Molen,Weshello和Lacham地区裸露。NHO的超镁铁质岩性岩性在矿物学上由可变比例的橄榄石组成,邻苯二酚,斜环和斜长石。大量蛇纹岩化作用覆盖了地幔橄榄岩的主要火成岩纹理,而地壳剖面岩石则反映出岩浆室内的晶体堆积。构成NHO地壳部分的累积橄榄岩-橄榄石辉闪长辉岩组合的球粒陨石归一化REE剖面显示出平坦至LREE的贫化模式,这与它们在伸展构造环境中从贫化的MORB型前驱体熔体生成的现象一致,而地幔橄榄岩则描述了U-以MREE上LREE和HREE相对富集为特征的REE形。这些特征共同暗示了贫化的MORB型和富集的弧型地幔组分在其成因上具有熔岩和流体-岩相互作用的烙印的双重作用。从构造上讲,NHO研究的岩性对应于一个板状至板状的近岛弧型Tholeiite亲和力,从而符合与俯冲引发过程相一致的洋上超俯冲带(SSZ)的前弧型。Zr / Hf,Zr / Sm,Nb / Ta,Zr / Nb,Nb / U,Ba / Nb,Ba / Th,Ba / La和Nd / Hf比值反映了NHO地壳和地幔剖面的地球化学属性提出了一个两阶段的成岩过程:(i)从SSZ前弧伸展带上的软流圈地幔中提取出与地壳岩性有关的前弧玄武岩(FAB)型熔融熔体,从而形成了难熔的残留上地幔;(ii)SSZ前弧的地壳和上地幔被俯冲作用引发和板坯脱水反应所产生的贝尼特熔体逐渐溶入。

更新日期:2020-11-18
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