ABSTRACT

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.

摘要

以萨古尔群为代表的最古老上壳岩的就位时机对于理解西达尔瓦克拉通的地壳演化过程具有重要意义。Sargur 群超基性-基性岩的年龄没有得到很好的限制，因为科马提质和科马提质玄武质岩石由于其蚀变导致的较大误差而总是给出不可靠的年龄。来自达尔瓦克拉通西部 Sargur 群 Nuggihalli 和 Holenarsipur 绿岩带的相对较少改变的层状侵入超镁铁质岩的新 Sm-Nd 同位素数据给出了 2934 ± 88 Ma 的精确年龄。这是 Sargur 群超基性-基性岩复合体获得的最年轻年龄，与上覆 Dharwar 超群岩石报道的最古老年龄无法区分。Sargur Group 岩石可能仅代表 Dharwar Supergroup 的较旧的超镁铁质等价物。不存在明确的时间区别来分别支持这两个组的地层分类。然而，从这些绿岩带周围的闪长岩-长闪长岩-花岗闪长岩片麻岩获得的亚同期到更老的年代，意味着夹在片麻岩内的狭窄带更年轻，因此，绿岩带，其被肢解的根和部分暴露在外与片麻岩一起，可以代表蛇绿岩套件的太古代类似物。