The compositional variability of the sandstones leads to insight about the controlling sedimentary processes and plate tectonic environments. The geochemical composition of the Lower Gondwana sandstones exposed along the Main Boundary Thrust in parts of East Siang and West Siang districts of Arunachal Pradesh, India was determined to deduce their provenance and tectonic setting governing their deposition. The overall analyses of the samples from the study area reveals the chemically coherent nature of the sediments and derivation from rocks of acidic and intermediate compositions. Trace element concentrations of the rocks of the study area are in concurrence with average Upper Continental Crust (UCC) whereas the Rare Earth Element (REE) values indicate felsic source rocks. Discrimination of tectonic setting using oxide data indicates passive margin setting for the sediments. The Chemical Alteration Index (CIA) was calculated and indicates medium to high chemical weathering during sedimentation of the basin.