Presence of detrital olivine and serpentine minerals in the Dihing unit of upper Assam: Implication towards the source

Abstract

A significant occurrence of olivine and serpentine (antigorite, lizardite) are reported for the first time in the Dihing sedimentary unit of upper Assam. Petrography of the Dihing Formation documents olivine, serpentine (antigorite, lizardite), pyroxene and major framework grains (quartz, feldspar and variety of lithic fragments). Based on modal composition, the Dihing unit samples are classified as lithic arenite. Heavy mineral analysis in addition to the above, provides evidence of other minerals such as zircon, tourmaline, rutile, garnet, epidote, amphibole, sphene, aluminosilicates, apatite, spinel, staurolite, chloritoid, chlorite and mica. The heavy mineral assemblage is dominated by olivine (36.9%) and pyroxene (16.5%). The low (10.7%) zircon–tourmaline–rutile (ZTR) index infers mineralogically immature nature of sediments. Raman spectroscopy confirms the presence of antigorite and electron microprobe analysis corroborates the existence of olivine, pyroxene, serpentine and chrome-spinel in the analyzed samples. The imbricated clasts and cross-bedding sedimentary structures indicate SSW to SW paleo-current direction. The peridotite and serpentinite rocks of the Tidding suture zone from the Lohit–Dibang valleys are suggested as the likely sediment provenance for olivine and serpentine minerals. We advocate that the Lohit and Dibang rivers earlier flowed close to the Indo-Burman Ranges (IBR) depositing the Plio-Pleistocene Dihing sediments before merging with the Brahmaputra river.