Petrological and Geochemical Studies of Lepidolite (LCT Type) and Non-Lepidolite Pegmatite’s from Chakrasila, Dhubri District, Assam, North East India

Lepidolite pegmatite occurs as intrusive within biotite gneiss and amphibolite of Assam Meghalaya Gneissic Complex (AMGC) or Precambrian Gneissic Complex in the Dhubri district, Assam. AMGC is the north western extension of the Proterozoic rocks of Meghalaya Plateau or Shillong plateau. In the field it occurs as small to large veins and scattered boulders. Lepidolite pegmatite is later intruded by non lepidolite pegmatite. Pegmatites are medium to coarse grained with quartz and K-feldspar. It also contains lepidolite, which occurs in the form of flakes and clusters varying from pink to purple in colour. Petrography of lepidolite pegmatite reveals lepidolite as major constituents with quartz, K-feldspar and muscovite as minor constituents. XRD analysis reveals lepidolite (muscovite) is major mineral phase with kaliophilite in minor amount. Geochemically, they are calc-alkaline to high calc-alkaline and per-aluminous in nature. On the basis of Alumina Saturation Index (ASI), these pegmatites resemble Lithium-Cesium-Tantalum (LCT) family and compositional affinity with S-type granites of orogenic environments. Trace element compositions (Rb, Sr, Ba) indicate crystal fractionations, variable degrees of fractionation, highly evolved nature of pegmatite’s and strongly differentiated granites protoliths as source. The different tectonic discrimination diagrams indicate S-type and I-type melt for pegmatite derivations. Therefore, both the studied pegmatites could be an evolved variety of granitic rocks that originated from the same magma. The REE is relatively low to moderate.