Trace element and Nd-isotopic evidence for sediment sources in the mid-Proterozoic Vindhyan Basin, central India

Abstract

Ancient sediments reflect the compositions of old eroded continental crustal material and preserve geochemical signatures of paleo-tectonic and volcanic events. Geochemical study of such sediments provides insight into tectonic events that shaped the face of the Earth. Nd isotopes are best suited for such studies because rare earth elements (REE), specifically Sm and Nd are not fractionated during post-depositional processes such as erosion, transportation and diagenesis of organic-poor sediments and hence reflect the source composition of these sediments. Sedimentation in the Proterozoic Vindhyan Basin of central India with a ∼4 km thick succession of unmetamorphosed and mostly undeformed sediments lasted over 1000 Ma from ∼1630 Ma up to the Precambrian–Cambrian boundary. These sediments provide an ideal opportunity to study the evolution of ancient continental crust as well as the tectonic evolution of Peninsular India. The occurrence of porcellanites in the lower part of the basin indicates active volcanism during deposition of the Vindhyan sediments. These tuffs show large ion lithophile element (LILE) enrichment as well as depletions in Nb and Ta in primitive mantle-normalized plots, characteristic of an arc volcanic source. In combined trace element ratio plots, these volcanic rocks indicate mixing between mantle-derived melts and the continental crust and overlap with the composition of global arcs. Their initial ɛ Nd at 1630 Ma, ranging from −5.7 to −0.9, is similar to those observed in ‘Andean-type’ continental margin arcs. Trace element concentration patterns as well as Nd isotopic compositions of the clastic and non-clastic sediments of the Vindhyan Basin, from one of the type-localities of the Vindhyan stratigraphy in the Son valley, are vastly different from those of the Bundelkhand granite, which is the basement for these sediments, indicating that these granitic rocks were not a major source of sediments for the Vindhyan Basin. Our Nd isotopic study clearly indicates that sedimentation in the Vindhyan Basin, which lasted over ∼1 Ga, was not continuous. The boundaries between the Semri and Kaimur Groups as well as the Rewa and Bhander Groups are characterized by abrupt changes in the Nd isotopic compositions indicating changes in provenance, with sediments in the Kaimur and Bhander Groups being derived from more juvenile sources compared to the Semri and Rewa Groups, respectively. This inference is also supported by changes in T DM and f Sm/Nd for the sediments across these boundaries. The strikingly different trace element patterns of the Rewa Group sediments compared to the underlying Kaimur sediments indicates that sediment sources also changed across the Kaimur–Rewa boundary. Multiple changes in provenance in the Vindhyan sediments indicate that this basin was tectonically active during the entire span of sedimentation. While there is some uncertainty in determining the exact sources of sediments for the Vindhyan Basin, we argue on the basis of our geochemical, in particular Nd-isotopic ratios data, and available paleocurrent data that majority of the sediments were derived from a now-extinct Andean-type arc source in the south that resulted from a southerly dipping subduction prior to the collision of the Bhandara and Bundelkhand cratons. According to this reconstruction, the Vindhyan Basin was initiated as a foreland Basin after the collision of these two cratons.