Tectonic evolution of the Madawara greenstone belt, southern Bundelkhand craton, central India: a window to see the Meso- to Neoarchaean geodynamics

The Palaeo- to Neoarchaean Bundelkhand Craton (BkC) in central India has received attention due to its felsic crust evolution. There are two linear patches of relict greenstone belts in the BkC, and the central greenstone belt has been the focus of extensive research for the past two decades. In contrast, the southern greenstone belt has received inadequate attention. At the southern BkC, the E-W Madawara Greenstone Belt is represented by patches of remnant mantle serpentinites and metasedimentary rocks that are tectonically juxtaposed with Archaean basement TTG gneisses and Proterozoic Bijawar Group of rocks. This study highlights key findings from fields, geological maps, petrography, and a large number of whole rock chemical analyses of serpentinites and spinel chemistry to unravel the tectonic evolution story. We identified two serpentinites with contrasting petrological and geochemical properties: spinel peridotite (SP) and talc-tremolite schist (TTS). SP is primarily dunite to harzburgite with relatively high MgO (33 wt%), enriched Yb, and shows strong re-fertilization. TTS, on the other hand, is lherzolitic and less refractory (MgO = 25 wt%). Chemically, these two serpentinites were mantle wedge and subducted peridotites, respectively. The unaltered cores of the chromium spinels represent a fore-arc setting. These findings, in conjunction with prior research, paint a compelling picture of a Neoarchaean (~2.5 Ga) subduction system once present in the southern Bundelkhand Craton. The serpentinites represent relict ophiolites – from a northerly dipping trench. It also explains a Mesoarchaean rift basin system between the two cratonic nuclei of southern Bundelkhand, which amalgamated during the Kenorland Supercontinent formation.

位于印度中部的古-新太古代本德尔坎德克拉通（Bundelkhand Craton，BkC）因其长英地壳演化历程而备受关注。该克拉通内分布两处线性残留绿岩带，其中中部绿岩带在过去二十年间已成为广泛研究的焦点，而南部绿岩带则未得到足够重视。在本德尔坎德克拉通南部，东西向的马德瓦拉绿岩带以残留地幔蛇纹岩与变沉积岩斑块为特征，这些岩石与太古代基底TTG片麻岩（Tonalite-Trondhjemite-Granodiorite gneisses）及元古代比贾瓦尔岩群构造叠置。本研究依托野外调查、地质填图、岩石学分析以及针对蛇纹岩的大量全岩化学分析与尖晶石化学数据，旨在厘清该区域的构造演化脉络。研究识别出两类岩石学与地球化学特征迥异的蛇纹岩：尖晶石二辉橄榄岩（SP，spinel peridotite）与滑石-透闪片岩（TTS，talc-tremolite schist）。尖晶石二辉橄榄岩主要为纯橄岩至方辉橄榄岩，氧化镁含量相对较高（33 wt%），富集镱（Yb），并表现出显著的再富集作用；而滑石-透闪片岩则属于二辉橄榄岩质岩石，难熔程度更低（氧化镁含量为25 wt%）。地球化学特征表明，两类蛇纹岩分别对应地幔楔与俯冲方辉橄榄岩。铬尖晶石的未蚀变核部指示其形成于弧前环境。结合已有研究，上述发现为本德尔坎德克拉通南部曾存在新太古代（约2.5 Ga）俯冲系统提供了有力佐证。该区域的蛇纹岩代表了北倾俯冲带的残留蛇绿岩。本研究同时解释了本德尔坎德南部两个克拉通核体之间的中太古代裂谷盆地系统，该盆地系统在肯诺兰超大陆形成过程中发生了拼合。