Meso–Neoarchaean crustal evolution of the Bundelkhand Craton, Indian Shield: new data from greenstone belts

The Mauranipur and Babina greenstone belts of the Bundelkhand Craton are formed of the Central Bundelkhand greenstone complex (CBGC). This complex represents tectonic collage which has not been previously studied in depth. The purpose of this study is to contribute to the understanding of the main features of the Archaean crustal evolution of the Bundelkhand Craton. The CBGC consists of two assemblages: (1) the early assemblage, which is composed of basic-ultramafic, rhyolitic–dacitic, and banded iron formation units, and (2) the late assemblage, which is a felsic volcanic unit. The units and assemblages are tectonically unified with epidote–quartz–plagioclase metasomatic rocks formed locally in these tectonic zones.
The early assemblage of the Mauranipur greenstone belt is estimated at 2810 ± 13 Ma, from the U–Pb dating (SHRIMP, zircon) of the felsic volcanics. Also, there are inherited 3242 ± 65 Ma zircons in this rock. It is deduced that this assemblage is related to early felsic subduction volcanism during the Mesoarchaean that occurred in the Bundelkhand Craton.
Zircons extracted from metasomatic rocks in the early assemblage’s high-Mg basalts show a concordant age of 2687 ± 11 Ma. This age is interpreted as a time of metamorphism that occurred simultaneously with an early accretion stage in the evolution of the Mauranipur greenstone belt.
The felsic volcanism, appearing as subvolcanic bodies in the late assemblage of the Mauranipur greenstone belt, is estimated to be 2557 ± 33 Ma from the U–Pb dating (SHRIMP, zircon) of the felsic volcanic rocks. This rock also contains inherited 2864 ± 46 Ma zircons. The late assemblage of the Mauranipur greenstone belt corresponds with a geodynamic setting of active subduction along the continental margin during Neoarchaean.
The late assemblage Neoarchaean felsic volcanic rocks from the Mauranipur and Babina greenstone belts are comparable in age and geochemical characteristics. The Neoarchaean rocks are more enriched in Sr and Ba and are more depleted in Cr and Ni than the Mesoarchaean felsic volcanic rocks of the early assemblage.
Through isotopic dating and the geochemical analysis of the volcanic and metasomatic rocks of the CBGC, this study has revealed two subduction–accretion events, the Meso–Neoarchaean (2.81–2.7 Ga) and Neoarchaean (2.56–2.53 Ga), during the crustal evolution of the Bundelkhand Craton (Indian Shield).

班德勒坎德克拉通（Bundelkhand Craton）的毛拉尼普尔（Mauranipur）与巴比纳（Babina）绿岩带，均由中央班德勒坎德绿岩杂岩体（Central Bundelkhand greenstone complex，CBGC）构成。该杂岩体属于构造拼贴体，此前尚未得到深入研究。本研究旨在加深对班德勒坎德克拉通太古宙地壳演化主要特征的认识。CBGC包含两套组合：其一为早期组合，由基性-超基性岩系、流纹质-英安质岩系以及条带状铁建造单元组成；其二为晚期组合，即长英质火山岩单元。各单元与组合均属构造统一体，且在上述构造带内局部形成了绿帘石-石英-斜长石交代岩。
通过对长英质火山岩的U-Pb定年（SHRIMP，锆石）结果可知，毛拉尼普尔绿岩带的早期组合形成于2810±13 Ma。该岩石中还发现了年龄为3242±65 Ma的继承锆石。据此推断，该组合与班德勒坎德克拉通中太古代（Mesoarchaean）早期的长英质俯冲火山作用相关。
从早期组合高镁玄武岩内的交代岩中提取的锆石，其一致年龄为2687±11 Ma。该年龄被认为对应毛拉尼普尔绿岩带演化早期的增生阶段，同时发生了变质作用。
毛拉尼普尔绿岩带晚期组合中的长英质火山作用以次火山岩体形式产出，通过对长英质火山岩的U-Pb定年（SHRIMP，锆石）结果估算，其形成时代为2557±33 Ma。该岩石中同样含有年龄为2864±46 Ma的继承锆石。毛拉尼普尔绿岩带的晚期组合对应新太古代（Neoarchaean）大陆边缘活跃俯冲的地球动力学背景。
毛拉尼普尔与巴比纳绿岩带的新太古代晚期组合长英质火山岩，在年龄与地球化学特征上具有可比性。相较于早期组合的中太古代长英质火山岩，新太古代岩石的Sr、Ba元素更为富集，而Cr、Ni元素则更为亏损。
通过对CBGC火山岩与交代岩的同位素定年及地球化学分析，本研究揭示了班德勒坎德克拉通（印度地盾，Indian Shield）地壳演化过程中的两期俯冲-增生事件：中太古代-新太古代（2.81~2.7 Ga）与新太古代（2.56~2.53 Ga）。