Fluid inclusion study of pegmatite and aplite veins of Palaeoproterozoic basement rocks in Bangladesh: Implications for magmatic fluid compositions and crystallization depth

Abstract

We report the first fluid inclusion data on Palaeoproterozoic basement rocks in Bangladesh for the characterization of magmatic fluid compositions and determination of the crystallization pressure and temperature of the host rock. Fluid inclusions are present as primary and pseudosecondary types in quartz grains within pegmatite and aplite, which occur as veins in dioritic rocks. Both primary CO2-rich and H2O-rich inclusions are present, even in the same inclusion cluster, probably reflecting a single stage of fluid activity during the crystallization of the veins. The melting temperature (Tm) and homogenization temperature (Th) of the dominant carbonic inclusions are in the ranges of −56.6 °C to −58.1 °C and −6.8 °C to +30 °C, respectively; the Th values translate into densities of 0.59-0.97 g/cm3. Rare aqueous fluid inclusions have a final Tm value in the range of 0 °C to −10.8 °C and a Th value in the range of +209.8 °C to +405.5 °C, which corresponds to bulk densities of 0.52-0.97 g/cm3. Isochores of the inclusions and temperatures obtained from the zircon saturation thermometry of pegmatite indicate that the veins crystallized at ∼ 4.8 kbar and 660-670 °C (depth of ∼ 14 km). The results of this study will be useful in understanding the magmatism and metallogeny of felsic igneous rocks of Bangladesh, which are related to the formation of the Columbia supercontinent.