Alkaline rocks of Samchampi-Samteran, District Karbi-Anglong, Assam, India

The Samchampi-Samteran alkaline igneous complex (SAC) is a near circular, p lug-like body approximately 12 km(2) area and is emplaced into the Precambr ian gneissic terrain of the Karbi Anglong district of Assam. The host rocks , which are exposed in immediate vicinity of the intrusion, comprise granit e gneiss, migmatite, granodiorite, amphibolite, pegmatite and quartz veins. The SAC is composed of a wide variety of lithologies identified as syenitic fenite, magnetite +/- perovskite +/- apatite rock, alkali pyroxenite, ijol ite-melteigite, carbonatite, nepheline syenite with leucocratic and mesocra tic variants, phonolite, volcanic tuff, phosphatic rock and chert breccia. The magnetite +/- perovskite +/- apatite rock was generated as a cumulus ph ase owing to the partitioning of Ti, Fe at a shallow level magma chamber (n ot evolved DI = O1). The highly alkaline hydrous fluid activity indicated b y the presence of strongly alkalic minerals in carbonatites and associated alkaline rocks suggests that the composition of original melt was more alka lic than those now found and represent a silica undersaturated ultramafic r ock of carbonated olivine-poor nephelinite which splits with falling temper ature into two immiscible fractions-one ultimately crystallises as alkali p yroxenite/ijolite and the other as carbonatite. The spatial distribution of varied lithotypes of SAC and their genetic relationships suggests that the silicate and carbonate melts, produced through liquid immiscibility, durin g ascent generated into an array of Lithotypes and also reaction with the c ountry rocks by alkali emanations produced fenitic aureoles (nephelinisatio n process). Isotopic studies (delta(18)O and delta(13)C) On carbonatites of Samchampi have indicated that the delta(13)C of the source magma is relate d to contamination from recycled carbon.