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首页> 外文期刊>Geoscience frontiers >Geochemical evolution of the Mangalwar Complex, Aravalli Craton, NW India: Insights from elemental and Nd-isotope geochemistry of the basement gneisses
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Geochemical evolution of the Mangalwar Complex, Aravalli Craton, NW India: Insights from elemental and Nd-isotope geochemistry of the basement gneisses

机译：印度西北部Aravalli Craton，Mangalwar Complex的地球化学演化：地下片麻岩的元素和Nd同位素地球化学的见解

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The Banded Gneissic Complex (BGC) of the Aravalli Craton is?divided into BGC-I and BGC-II; the BGC-II (central Rajasthan) is comprised of the Sandmata Complex and the Mangalwar Complex. We report elemental and Nd-isotope geochemistry of basement gneisses of the Mangalwar Complex and constrain its origin and evolution. Geochemically, the basement gneisses have been classified as low-SiO 2 gneisses (LSG) and high-SiO 2 gneisses (HSG). Both the LSG and HSG are potassic, calc-alkaline and peraluminous in nature. The LSG are enriched in incompatible (K, Sr, Ba, large ion lithophile elements) and compatible elements (MgO, Cr, and Ni). They display fractionated rare earth element patterns (avg. La N /Yb N ?=?12.1) with small Eu-anomaly ( δ Eu?=?0.9), and exhibit negative anomalies of Nb and Ti in primitive mantle-normalized multi-element diagram. In terms of Nd-isotope geochemistry, the LSG are characterized by ε Nd ( t )?=??4.2 and depleted mantle model age of 3.3?Ga. To account for these geochemical characteristics we propose a three-stage petrogenetic model for the LSG: (1) fluids released from dehydration of subducting slab metasomatised the mantle-wedge; (2) the subducting slab underwent slab-breakoff causing upwelling and decompression melting of the asthenosphere during waning stage of subduction; and (3) upwelling asthenosphere provided the requisite heat for partial melting of the metasomatised mantle-wedge leading to generation of the LSG parental magma. Asthenospheric upwelling also contributed in the LSG petrogenesis which is evident from its high Mg# (avg. 0.53). The LSG formed in this way are contemporary and chemically akin to sanukitoids of the BGC-I and Archean sanukitoids reported elsewhere. This provides a basis to consider the LSG as a part of the BGC-I. Contrary to the LSG, the HSG are depleted in compatible elements (MgO?=?avg. 1.1?wt.%; Cr?=?avg. 8?ppm; Ni?=?avg. 6?ppm) but enriched in incompatible elements (Sr?=?avg. 239?ppm, Ba?=?avg. 469?ppm). Its ε Nd ( t ) values vary from??9.5 to??5.4. These chemical features of the HSG are akin to potassic granitoids found elsewhere. In this backdrop, we propose that the HSG suite of the Mangalwar Complex was derived from re-melting (partial) of an older crust (TTG?) occurring within the BGC-II. Graphical abstract Display Omitted Highlights ? First attempt to study geochemistry of the reworked BGC-II. ? Studied gneisses classified into low- and high-SiO 2 gneisses. ? Three-stage petrogenesis suggested for low-SiO 2 gneisses (sanukitoids). ? Proposed crustal partial re-melting process for generation of high-SiO 2 gneisses. ? Aravalli Craton is a result of terrane accretion of BGC-I and BGC-II.
机译：Aravalli Craton的带状片麻岩复合体（BGC）分为BGC-I和BGC-II。 BGC-II（拉贾斯坦邦中部）由Sandmata建筑群和Mangalwar建筑群组成。我们报告了Mangalwar复杂的基底片麻岩的元素和Nd同位素地球化学，并限制了其起源和演化。在地球化学上，基底片麻岩被分类为低SiO 2片麻岩（LSG）和高SiO 2片麻岩（HSG）。 LSG和HSG均为钾，钙碱性和铝质。 LSG富含不相容的元素（K，Sr，Ba，大离子亲石元素）和相容的元素（MgO，Cr和Ni）。它们显示了具有少量Eu异常（δEuδ=？0.9）的稀有稀土元素模式（平均La N / Yb Nα=？12.1），并且在原始地幔归一化多元元素中显示了Nb和Ti的负异常。图。就Nd同位素地球化学而言，LSG的特征是εNd（t）δ=≥4.2，地幔模型年龄为3.3≤Ga。为了考虑这些地球化学特征，我们为LSG提出了一个三阶段成岩模型：（1）俯冲板脱水脱水后释放出来的流体与幔幔交界。 （2）俯冲板块发生俯冲破坏，在俯冲阶段逐渐减弱的过程中，导致软流圈的上涌和减压融化; （3）上升流的软流圈为交化后的幔楔的部分融化提供了必要的热量，从而导致了LSG母岩浆的产生。软流圈上升也导致了LSG的成岩作用，这从其高Mg＃（平均0.53）可以明显看出。以这种方式形成的LSG具有当代性，并且在化学上类似于BGC-I的类瘤菌体和其他地方报道的太古代类瘤菌体。这为将LSG视为BGC-I的一部分提供了基础。与LSG相反，HSG消耗了兼容元素（MgO3 =平均1.1％wt％; Cr =平均8％ppm; Ni =平均6µppm），但富含不兼容元素（Sr 2 ＝平均239ppm，Ba 2 ＝平均469ppm）。它的εNd（t）值在9.5至5.4之间变化。 HSG的这些化学特征类似于在其他地方发现的钾质类花岗岩。在这种背景下，我们建议将Mangalwar Complex的HSG套件衍生自BGC-II中发生的较旧地壳（TTG？）的重熔（部分）。图形摘要显示省略的突出显示？首次研究重制的BGC-II的地球化学。 ？研究的片麻岩分为低和高SiO 2片麻岩。 ？建议低SiO 2片麻岩（sanukitoids）的三阶段成岩作用。 ？拟议的地壳部分重熔工艺，用于生产高SiO 2片麻岩。 ？ Aravalli Craton是BGC-I和BGC-II的地形积聚的结果。

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