Abstract Back-arc magmatism is perhaps extremely rare in a Paleoproterozoic mobile tectonic regime. The present study provides a detailed account on the petrogenesis of basalts that are interleaved with marine sedimentary rocks in the Agori Formation, which overlies the Chitrangi Formation in the ~2.1 Ga Mahakoshal supracrustal belt within the Central India Tectonic Zone. The basalts are sub alkaline, tholeiitic in nature. They have moderate concentrations of TiO2 (~0.9 wt%), Nb (2.6–5.2 ppm), and Zr (59–105 ppm), Zr/Nb (18–24) and Zr/Y (3.1–3.6) ratios, mildly enriched chondrite normalized rare earth element patterns (LaN/YbN = 1.6–2.8), and negative Nb and Ti anomalies on a primitive mantle normalized trace element variation diagram. Post-magmatic low grade metamorphism, alteration and trace element mobility, and/or contamination by Archean continental crust, if any, cannot relate to these patterns. The positive initial eHf (2.1 Ga) + 2.24 to +5.15 and eNd(2.1 Ga) + 1.02 to +1.54 isotopic compositions in these basalts are variable, yet consistent with a juvenile mantle source. We ascribe the isotopic variability in the Agori basalts to reflect the general isotopic variability in the depleted mantle at 2.1 Ga. Consequently, the basalts were produced by decompression partial melting in the shallow upper mantle in a back-arc setting. Subduction zone magmatic processes produced the arc volcanics in the upper stratigraphic unit (Agori), whereas, a plume source generated the ferropicrites in the underlying Chitrangi Formation. The evolution of the Mahakoshal supracrustal belt predates the collision and interlocking of North and South India cratonic blocks, which resulted in the formation of Central India Tectonic Zone. Hence, it represents an allochthonous plume-arc accreted terrain in the CITZ.

中文翻译：

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约 2.1 Ga Mahakoshal 上地壳带：印度中部构造带的异地地形

摘要 弧后岩浆活动在古元古代移动构造体系中可能极为罕见。本研究详细说明了在 Agori 组中与海相沉积岩交错的玄武岩的岩石成因，Agori 组位于印度中部构造带内~2.1 Ga Mahakoshal 上地壳带的 Chitrangi 组上。玄武岩是亚碱性的拉斑性玄武岩。它们具有中等浓度的 TiO2 (~0.9 wt%)、Nb (2.6–5.2 ppm) 和 Zr (59–105 ppm)、Zr/Nb (18–24) 和 Zr/Y (3.1–3.6) 比率，温和富集球粒陨石归一化稀土元素模式（LaN/YbN = 1.6-2.8），以及原始地幔归一化微量元素变化图上的负 Nb 和 Ti 异常。岩浆后低品位变质作用、蚀变和微量元素迁移，和/或太古代大陆地壳的污染，如果有的话，与这些模式无关。这些玄武岩中的正初始 eHf (2.1 Ga) + 2.24 至 +5.15 和 eNd(2.1 Ga) + 1.02 至 +1.54 同位素组成是可变的，但与年轻地幔源一致。我们将 Agori 玄武岩的同位素变异性归因于反映 2.1 Ga 耗尽地幔的一般同位素变异性。因此，玄武岩是由弧后环境中浅层上地幔的减压部分熔融产生的。俯冲带岩浆作用在上层地层单元（Agori）产生了弧火山岩，而在下伏的 Chitrangi 组中，羽流源产生了铁质岩。Mahakoshal 上地壳带的演化早于北印度和南印度克拉通块体的碰撞和联锁，这导致了印度中部构造带的形成。因此，它代表了 CITZ 中的异地羽状弧增生地形。