No abstract

The granite in Samingiri - Dilsekgiri area occurs as discordant, isolated pluton within the migmatitic terrain of West Garo Hills district, Meghalaya. The pluton is exposed over 140 sq km (18 km x 8 km). It exhibits structures of solid state and piecemeal stoping effect proximal to the contact and enveloped by a contact metamorphic aureole of albite-epidote-hornfels facies. Modally, it is biotite-monzogranite and biotite-syenogranite with minor biotite, chlorite, epidote and sericite and accessories like zircon, apatite, allanite, pyrite, magnetite and sphene. Geochemically, it is marked by restricted composition (69-76 wt% SiO2), high alkalies, low Ca, metaluminous to strongly peraluminous (Molar Al2O3/CaO+Na2O+K2O = 0.95 - 1.54), high FeO/MgO, high Ga/Al, high contents of Rb, Sr, Ba, Y, Zr and Ce and depleted in Ti and P. The field observation, mineralogical and geochemical aspects indicate the post-tectonic nature of West Garo pluton more like as A-type granite formed by partial melting of lower crustal blocks followed by low to moderate degree of fractional differentiation. Low Ca, alkaline nature and peraluminous character point to A-type nature of West Garo granite significantly different from other granites of Meghalaya Plateau. Rb-Sr age (616±86 Ma) of granite, however, corresponds to widespread Middle to Upper Pan African activity, a thermal event prevailed during Late Proterozoic - Early Palaeozoic (500-800 Ma) period, manifested in the form of several granitic intrusions in the basement gneissic complex and the overlying Proterozoic metasediments of the Shillong Group in Meghalaya Plateau.

Geologically, the Marlagalla area represents the southern part of NNE-SSW trending Nagamangala schist belt, which is a member of ancient supracrustals of Sargur Group and situated in the southern part of Western Dharwar Craton. The area is mostly soil cover with limited exposure of amphibolite/hornblende schist and intrusive pegmatite. Pegmatites occur as lensoidal bodies within the amphibolite.Microgravity surveys are attempted for the first time to detect pegmatites within the amphibolite. The density between pegmatite (Density= 2.58-2.61 gm/cc) and host rock amphibolite (Density=3.09-3.14 gm/cc) gives detectable contrast to identify pegmatite within amphibolite. Gravity data are acquired over a grid of 5m x 30m using CG-5 Autograv gravimeter which has 1 μGal sensitivity. Elevation data are acquired using SP80 post processing DGPS instrument which has 5 mm elevation accuracy. Analysis of residual gravity and first vertical derivative (FVD) filters facilitated in delineating the pegmatite bodies. The pegmatite bodies show low gravity anomalies with an amplitude range of -5 to -200 μGal. FVD map of the residual gravity indicated four major N100E trending linear anomalies of -36 to 3.0 μGal/m corresponding to pegmatite bodies of varying strike (100-200m) and width (10-15m).2D-Forward modeling of microgravity data helped in deriving the subsurface geometry of the pegmatites. Borehole drilled subsequently based on the results of microgravity surveys intercepted pegmatites within the amphibolite. Thus, the technique has proved for exploration of pegmatite in contrasting environment of amphibolite. Pegmatite are known for rare metals and lithium minerals. Indirectly the model proposed here will help in exploration of rare metals and Li bearing pegmatite.