Alkaline Magmatism in the Eastern Ghat Belt - A Critique

Almost all the feldspaihoidal alkaline complexs of the Eastern Ghat granulite belt are virtually restricted to its western margin, arid are significantly confined to the junction zone between the catonic and mobile belt regions. The presence of subordinate amounts of quartz syenite in preponderantly nephe1ine syenite plutons is ascribed, among other causes, to the dominant role of crustal contamination by initially undersaturated magmas. The not-so-common ferrosye nites, though cospatial, are not comagmatic with nepheline syenites. White ferrosyenites represent the late stage derivatives of tholeiitic magmas, nepheline syenites correspond to the end stage differentiation products of malignites/melteigites. The so-called "alkalic stamp" for rocks purported to be gabbros, in the vicinity of alkaline plutons, is realised as merely a myth. The longevity or propensity for rejuvenation of alkaline magmatism and carbonatite emplacements of mantle origin, in abyssal faults, is fairly well established. The possibility of the presence of isotopically distirict subcontinental mantle reservoirs in a part of the belt is discussed. Some general characteristics of the feldspathoidal complexes are summarized the need for studying them on modern lines to understand their precise petrogenetic schemes and exact crystallization histories is outlined.