40Ar/39Ar geochronology of phlogopites and its mineralization in the Mithapukur kimberlites: New Lower Cretaceous (∼109–115 Ma) kimberlitic occurrences in Bangladesh

Abstract

Phlogopites in a suite of ultramafics from Bangladesh have been analyzed to ascertain the mineralization and the timing of cooling of phlogopites and reveal their emplacement tectonics. In recent times, the 40Ar/39Ar dating method is incredibly popular due to having phlogopite in most kimberlitic rocks. The 40Ar/39Ar age dating of phlogopites yields ideal plateau ages of 108.5 ± 1.3 Ma, 110.8 ± 0.9 Ma, 113.7 ± 1.1 Ma, 114.8 ± 1.0 Ma and an upper-staircase release pattern age of 111.7 ± 1.1 Ma. It is the first report on the Lower Cretaceous kimberlites (109–115 Ma) occurrence in Bengal Basin, emplaced during the break-up of India and Antarctica-Australia. The studied phlogopite chemistry and petrographic data suggest that the rocks are typically orangeites, comparable with Indian orangeites, Antarctica and some African transitional or hypabyssal kimberlites. These phlogopites are invariably consistent with most of the kimberlites of the world and those described as a clear sign of a metasomatized source in the mantle. The tectonic activities of the studied ultramafics suggest the development of enormous stress gradients in the lithosphere, creating a path of melt migration from the transitional or lower mantle. These multiple tectono-thermal processes were the key driver for the present kimberlites’ emplacement within Paleoproterozoic basement rocks.