Provenance of the Proterozoic Lesser Himalayan siliciclastics, northwest Himalaya, India: Implications to terrain accretion and crustal evolution

Abstract

We present petrographical and geochemical data for the Lesser Himalayan sedimentary succession from the Tons Valley section of the Garhwal Lesser Himalaya. The succession consists of the least metamorphosed shale-dominant Chandpur Formation overlain by coarser-grained sandstones of Nagthat Formation. Petrographic studies of the Jaunsar Group siliciclastics reveal that apart from quartz 84 ± 9%, feldspar forms 5 ± 4% (with PF > KF) and rock fragments (micro- to mega-crystalline chert, orthoquartzite, gneisses, phyllites, and schists) constitute the remaining 11 ± 8%. The occurrence and existence of the labile components like feldspars, and rock fragments such as phyllites and schists, indicate less weathering and a relatively short distance of transport and provenance from a clastic wedge with a recycled nature of sediments. The major element discriminant function analysis indicates that the Nagthat Formation samples plot in the quartzose sedimentary-felsic igneous field and the Lower Chandpur Formation plots in the intermediate igneous provenance, supporting derivation from an Upper Continental Crust-like source. The TiO2 vs Ni diagram indicates acidic source rocks. Trace and REE combined discriminant diagrams support significant mafic input and confirm mafic and ultramafic rocks in the source area. In the Eu/Eu* vs GdN/YbN plot, lower values of Eu/Eu* suggest that the igneous/metamorphic provenance was affected by intra-crustal differentiation. The GdN/YbN > 2 value indicates a contribution from older sources like the Archean, whereas Phanerozoic granites have a ratio <2. It gives a clear indication that the detrital derivation for the Jaunsar Group is a product of a mixed source, consisting of a major portion of the thick sediments and meta-sediments of the Aravalli-Delhi Supergroup, along with the basement granite and granite gneiss from the Banded Gneissic Complex (BGC) and Bundelkhand Granite (BG). During the deposition of the Jaunsar Group of sediments, there was no substantial change in the provenance; however, as evidence indicates, it can be inferred that as time passed and the erosion deepened and widened, the primary sources such as BG and BGC were exposed more and more gradually. Sedimentation continued in the NW sector all through the ages of the Columbian until the gradual breakup of the Rodinia supercontinent.