Newly discovered ca. 1.6 Ga K-bentonite layers from the Lakharpata Group in Nepal Lesser Himalaya: Implications for basin evolution during the Columbia breakup

Abstract

• Zircon U-Pb age for newly discovered K-bentonite (ca. 1.6 Ga) constrained the deposition time of the Lakharpata Group. • The Lakharpata Group in Nepal Lesser Himalaya is coeval sediment with the Lower Vindhyan in India. • The Lakharpata Group was deposited in a passive continental margin basin during the Columbia breakup. The Lakharpata Group, a set of marine sediments with a thickness of over 5000 m, occupies the middle part of the Lesser Himalayan sequence and records important tectono-sedimentary evolution in the northern margin of the Indian continent. However, the duration of deposition and tectonic background of the Lakharpata Group are poorly defined. This study presents zircon U-Pb ages and Hf isotopes, as well as petrochemical data for the newly discovered K-bentonite layers in the Aru Formation, the upper part of the Lakharpata Group. Integrating the detrital zircon ages of the underlying metasedimentary rocks and the sedimentation rate of dolomite in the South China Block, we suggest that the Lakharpata Group was deposited during the late Paleoproterozoic to early Mesoproterozoic (ca. 1680–1580 Ma) rather than from Mesoproterozoic to Early Paleozoic as previously presumed. Stratigraphic correlations show the Lakharpata Group in Nepal Lesser Himalaya correlates to the Lower Vindhyan Supergroup, as well as to the Deoban and Rautgara formations in Kumaun Lesser Himalaya and the Baxa Group in Sikkim Lesser Himalaya, India. The Lakharpata Group comprises two depositional sub-sequences that record two transgression-regression sedimentary processes, which are comparable to the Lower Vindhyan. Geochemical and sedimentological studies suggest that the Lakharpata Group may represent the deposition of a passive continental margin basin during the breakup of the Columbia supercontinent.