Textural and compositional variation of carbonate minerals in the Newania carbonatites, Rajasthan, India

Abstract

The Newania carbonatite complex (~1473 Ma) in northwestern India is one of the oldest carbonatitic occurrence in India. It is a plutonic complex, comprising of two types of carbonatites and fenitized aureole within host Untala granite of Banded Gneissic Complex. The two types of carbonatites present include: (1) magnesiocarbonatite, and (2) ferrocarbonatite. This study illustrates textural and compositional variation of carbonate phases present in magnesiocarbonatite. Magnesiocarbonatite is principally composed of dolomite-ankerite series carbonates and accessory magnesite-siderite series carbonates. Dolomite-ankerite series carbonates occur as coarse-to-medium-grained (>200 µm) subhedral-to-anhedral crystals, displaying three types of deformation textures: (1) equigranular mosaic fabric with frequent triple junctions, thick-to-thin twin lamellae and absence of strain-derived deformational changes; (2) elongated medium-grained crystals, oriented in one direction, having high aspect ratio and twin lamellae density; and (3) coarse-grained crystals, mostly subhedral, with straight-to-lobate-to-serrated grain boundaries; surrounded by numerous fine-grained crystals which at places, impregnates the coarse carbonate crystals and form finger-like and island structures. Texture (1) is typical of plutonic carbonatites in extensional intraplate setting and texture (2) and (3) represents deformational changes as a result of high-strain and low-T conditions to high-T dynamic recrystallization, respectively. However, deformation has minor to none influence on the liquidus composition of dolomite-ankerite series carbonates. These carbonates, irrespective of texture, correspond compositionally with dolomite and ferroan dolomite, having Fe2+/Fe2++Mg=0.12–0.43. Concomitant increase in Fe and Mn and decreasing Mg at relatively restricted Ca content indicate magmatic origin and compositional evolutionary trend. Magnesite-siderite series carbonates are medium-grained (<300µm) subhedral to anhedral discrete crystals. These are magmatic in origin and are crystallized simultaneously with surrounding dolomite-ankerite series carbonates. Compositionally, these are ferroan magnesite and magnesian siderite with restricted Mg:Fe ratio of 0.96–1.12. The presence of magmatic magnesite-siderite series carbonates and the evolutionary trend displayed by dolomite-ankerite series carbonates attest for magmatic origin of the Newania carbonatites. Experimental work has demonstrated that dolomite and magnesite are stable up to the mantle depths of 80–110 kms and >110 kms, respectively. As the pressure approaches 32 kBar, the melts produced via. partial melting of phlogopite-bearing peridotite becomes more enriched in its magnesian content as the carbonate mineralogy changes from dolomite to magnesite. Such mantle-derived carbonatitic melts are inferred to be parent of the Newania carbonatites.