Dharwar stratigraphy revisited: new age constraints on the ‘oldest’ supracrustal rocks of western Dharwar craton, southern India

The timing of emplacement of the oldest supracrustal rocks represented by Sargur Group is significant in understanding crustal evolution processes in the western Dharwar craton. The age of ultramafic-mafic rocks of the Sargur Group is not well-constrained as the komatiitic and komatiitic basaltic rocks have always given unreliable ages due to larger errors attributed to their alteration. New Sm-Nd isotope data from relatively less-altered layered intrusive ultramafic-mafic rocks of Nuggihalli and Holenarsipur greenstone belts of Sargur Group, western Dharwar craton give a much precise age of 2934 ± 88 Ma. This is the youngest age obtained for the ultramafic-mafic rock complexes of Sargur Group and is indistinguishable from the oldest ages reported for the overlying Dharwar Supergroup rocks. Sargur Group rocks might merely represent older ultramafic equivalents of the Dharwar Supergroup. A clear temporal distinction does not exist to support the stratigraphic classification of these two groups separately. However, the sub-contemporaneous to older ages obtained for the tonalite-trondhjemite-granodiorite gneisses that surround these greenstone belts would imply that the narrow belts tucked within the gneisses are younger, and therefore, the greenstone belts, whose dismembered roots and parts are exposed alongside the gneisses, could represent an Archaean analogue of an ophiolite suite.

以萨尔古尔群（Sargur Group）为代表的最古老表壳岩（supracrustal rocks）的侵位时代，对于理解达尔瓦尔克拉通（Dharwar Craton）西部的地壳演化过程具有重要意义。萨尔古尔群的超镁铁质-镁铁质岩（ultramafic-mafic rocks）年龄尚未得到精准限定，这是由于科马提岩（komatiite）及科马提岩质玄武岩（komatiitic basaltic rock）因蚀变作用带来较大测年误差，其测年结果始终不可靠。本次研究获取了采自达尔瓦尔克拉通西部萨尔古尔群努吉哈利（Nuggihalli）与霍伦纳西普尔（Holenarsipur）绿岩带（greenstone belt）中蚀变程度相对较低的层状侵入型超镁铁质-镁铁质岩的钐-钕（Sm-Nd）同位素新数据，其测年结果为2934 ± 88 Ma，精度显著提升。该年龄是萨尔古尔群超镁铁质-镁铁质岩杂岩迄今获得的最年轻年龄，且与上覆达尔瓦尔超群（Dharwar Supergroup）岩石的已有最古老年龄无法区分。萨尔古尔群岩石或许仅仅代表了达尔瓦尔超群的古老超镁铁质对应岩系。目前尚无明确的时间界线来支持将这两个群单独进行地层划分。然而，环绕这些绿岩带的英云闪长岩-奥长花岗岩-花岗闪长岩片麻岩（tonalite-trondhjemite-granodiorite gneisses）所获得的近同期至更古老的年龄，意味着赋存于片麻岩中的狭窄绿岩带时代更晚；因此，那些与片麻岩伴露、具肢解残根与残块的绿岩带，或许可代表太古宙（Archaean）蛇绿岩套（ophiolite suite）的类似物。