Evolution of alkaline carbonatite complex of Samchampi Samteran area Karbi Anglong Assam

Abstract

The alkaline carbonatite complex of Samchampi-Samteran is a part of major alkaline-carbonatite belt extending from Meghalaya to Assam. The Complex is near circular ( 12km2 in area) occurs at the intersection of ENE and NNE lineaments. The litho units are magnetite +- apatite +- perovskite rock, pyroxenite, alkali pyroxenite- melteigite- ijolite, shonkinite, nepheline/foidal syenite, alkali syenite, phonolite and volcanic tuff. Carbonatites are intricately associated with these rocks. Fenites and syenitic fenites are the most dominant rock types in the area. Phosphatic-chert breccia, secondary in nature, is also present in the Complex. The alkaline mafic - ultramafic are predominantly agpaitic and silica undersaturated, while the late fractionates are miaskitic and distinctly alkaline. The carbonatites are calcio, magnesio and ferro-carbonatites. The silicates are the fractional crystallization differentiation of parental magma which through process of liquid immiscibility caused the development of carbonatite melt. The parental magma as deduced for the alkaline complex is calcium rich ultramafic composition charged with volatiles, which has high Mg* values and also peralkaline. The fenitization includes both sodic and potassic types and formed by both carbonatites and mafic alkaline rocks into the gneissic country rocks. The complex shows enrichment in some incompatible elements like K, P, Zr, Nb, Rb, Ba, SR and Th. The similar rations of Zr/Y and Nb/Zr and Log ( Zr/Ti02) indicate the rocks of the complex are co-magmatic. The pristine carbonatites have 0 = 7.2% SMOW and C = -3.84%. PDB indicating their temperature of formation between 7000C and 11000C and are mantle derived.