Tectono-magmatic evolution of Tethyan oceanic lithosphere in supra subduction zone fore arc regime:Geochemical fingerprints from crust-mantle sections of Naga Hills Ophiolite

• 作者: Anisha Verencar Abhishek Saha Sohini Ganguly Manikyamba C
• 来源期刊: 地学前缘（英文版）
• 年，卷（期）: 2021,（3）
• 所属期刊栏目: Research Paper
• 页码范围: 19-41
• 页数: 23页
• 语种: 英文

摘要

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 ac-count for the mantle and crustal sections of NHO, northeastern India to address (i) the mantle processes and tec-tonic 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, Washelo 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 pro-files for the cumulate peridotite-olivine gabbro-gabbro assemblage constituting the crustal section of NHO show flat to depleted LREE 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 en-richment 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 interac-tions. 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) a depleted fore arc basalt (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 re-sidual upper mantle;(ii) the crust and upper mantle of the SSZ fore arc were progressively refertilised by boninitic melts generated in response to subduction initiation and slab-dehydration. The vestiges of Tethyan oce-anic 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 ex-tension and arc-continent accretion.