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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Geochemical and petrological evidence for subduction–accretion processes in the Archean Eastern Indian Craton
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Geochemical and petrological evidence for subduction–accretion processes in the Archean Eastern Indian Craton

机译：太古代东印度克拉通俯冲增生过程的地球化学和岩石学证据

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摘要

The composition of sandstones often provides key evidence about the tectonic, weathering and transport processes operating on the surface at the time of deposition. Petrologic and geochemical analyses of little-metamorphosed middle–late Archean sandstones from the Eastern Indian Craton show that the sedimentary rocks were derived from dominantly 3.3-Ga-old amphibolites of the Older Metamorphic Group (OMG) and tonalites of the Older Metamorphic Tonalite Gneisses (OMTG), the two oldest lithologic units of this craton. Chondrite-normalized rare earth element (REE) patterns of the sandstones show a light REE-enriched signature with (La/Sm)_N varying from 5.2 to 6.7 with no Eu anomaly, while the heavy REEs display flat patterns with (Gd/Lu)_N values of 0.9–1.6. Primitive mantle-normalized incompatible and compatible trace element plots of these sandstones demonstrate an overall similarity with global Proterozoic–Archean sandstones, including strong Nb–Ta negative anomalies. In an f_(Sm/Nd) vs. ε_(Nd)(0) diagram, the sandstones plot precisely between the regional amphibolites and tonalites. We infer from the REE abundances and the f_(Sm/Nd) vs. ε_Nd(0) plot that the sandstones represent a bimodal mechanical mixture of OMG and OMTG. The low Ce/Pb ratios of these rocks of 1–4 indicate a variably Pb-enriched Archean crust and that the Ce/Pb ratio acquired the continental crustal signature, distinctly different from those of the bulk silicate earth and mantle values at least as early as mid-Archean. Strong Nb–Ta depletion relative to the primitive mantle suggests the sandstones were derived from subduction-related magmatic arc sources. This latter suggestion is strongly supported by the low Nb/Ta and high Zr/Sm ratios of these sandstones, identical to Archean tonalite–trondhjemites that require, based on recent trace-element partitioning results, their protoliths to have formed by subduction melting of low-magnesium amphibolites or metamorphosed hydrous basalts [1 and 2]. The average Nd model ages of the sandstones are greater than the Sm–Nd crystallization ages of the OMG and OMTG at ～3.3 Ga. The geochemical data presented here can be collectively interpreted to suggest the presence of subduction–accretion processes operational in the mid-Archean Eastern Indian Craton. Because the OMG and OMTG, the source rocks of the sandstones, formed in a subduction-related arc setting, the basement rocks upon which this arc was constructed must have been older. This observation and the depleted mantle Nd model ages (TDM) of the sandstones, ranging from 3.6 to 4.0 Ga, strongly indicate the presence of continental crust in this Eastern Indian craton older than 3.3 Ga and possibly as old as 4.0 Ga.
机译：砂岩的成分通常提供有关沉积时地表构造，风化和运移过程的关键证据。来自东印度克拉通的少量变质的中晚期太古宙砂岩的岩石学和地球化学分析表明，该沉积岩来源于较老的变质组（OMG）的占主导地位的3.3-Ga老角闪石和较老的变质方铅石片麻岩的同色岩（ OMTG），该克拉通的两个最古老的岩性单元。砂岩的球粒陨石归一化稀土元素（REE）模式显示出轻稀土元素富集特征，（La / Sm）_N在5.2至6.7之间变化，且没有Eu异常，而重稀土元素则显示出具有（Gd / Lu）平坦特征。 _N值为0.9-1.6。这些砂岩的原始地幔归一化不兼容和兼容的痕量图显示出与全球元古代—Archean砂岩总体相似，包括强Nb-Ta负异常。在f_（Sm / Nd）与ε_（Nd）（0）图表中，砂岩精确绘制在区域两性闪石和同色岩之间。我们从REE丰度和f_（Sm / Nd）与ε_Nd（0）曲线推断出，砂岩代表OMG和OMTG的双峰机械混合物。这些岩石的低Ce / Pb比1-4表示富集了不同程度的Pb的太古宙壳，并且Ce / Pb比获得了大陆地壳特征，至少在早期就与块状硅酸盐土和地幔值明显不同。作为中亚弓。相对于原始地幔，Nb-Ta的强烈消耗表明砂岩是从俯冲相关的岩浆弧源中提取的。这些砂岩的低Nb / Ta和高Zr / Sm比强烈支持后一种建议，这与太古代斜纹岩-长寿石一样，根据最近的痕量元素划分结果，它们的俯冲熔融形成了原岩。镁闪石或变质含水玄武岩[1和2]。在〜3.3 Ga时，砂岩的平均Nd模型年龄大于OMG和OMTG的Sm–Nd结晶年龄。这里提供的地球化学数据可以共同解释为在中晚期存在俯冲-增生过程。太古代的东印度克拉通。由于OMG和OMTG（砂岩的烃源岩）是在俯冲相关弧环境中形成的，因此构造此弧的基岩必须较旧。该观测结果和砂岩的耗尽地幔Nd模型年龄（TDM）为3.6至4.0 Ga，有力地表明该东印度克拉通年龄大于3.3 Ga，年龄可能高达4.0 Ga。

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