Preservation of glacial and interglacial phases in Tethys Himalaya: evidence from geochemistry and petrography of Permo-Carboniferous sandstones from the Spiti region, Himachal Pradesh, India

Abstract

Through a multidisciplinary approach, including petrology and geochemistry, the sedimentary provenance and paleo-weathering of the Permo-Carboniferous Spiti sandstones of Tethys Himalaya are investigated. The Spiti region consists of texturally immature to mature sandstones composed of unsorted to sorted and subangular to subrounded clastic grains dominated by variable amounts of quartz and feldspar accompanied by lithic fragments (mostly metasedimentary, sedimentary, and plutonic grains). They are characterized by moderate to high SiO2 contents, moderate K2O/Na2O ratios, but relatively low mafic contents. Uniform rare earth element (REE) patterns similar to upper continental crustal (UCC) with light REE (LREE) enrichment (LaN/SmN = 3.91), flat heavy REE (HREE; GdN/YbN = 1.21–2.5), and negative Eu anomalies with variable amounts of ΣREE and Eu anomalies (0.4–0.8) suggest that hydraulic sorting played a significant role. The striking similarities of the multi-elemental spider diagrams of the Spiti sandstones and the Himalayan granitoids indicate that sediments are sourced from the Proterozoic and Cambro-Ordovician orogenic belts of the Himalayan region. The nature of the feldspar observed in thin sections from most altered to euhedral pristine minerals corresponding to Carboniferous to lower Permian sandstones strongly indicates a change in climate from most favorable conditions for rapid feldspar alteration (humid) to conditions where negligible alteration is possible (arid and glacial). It is found that the chemical index of alteration (CIA) values of these sandstones accorded with inferences based on modified chemical index of alteration (CIX) and αCa values, and sedimentologic and paleontological evidence, discriminating well between warm-humid (indicated by high CIA values) and arid-glacial (representing low CIA) conditions in the Spiti basin. Thus, these results document a complete record of glacial and interglacial phases in the Permocarboniferous Spiti sandstones, and the interpretations are consistent with other such studies on the Phanerozoic glaciation events on Gondwana supercontinent.