Petrography, Geochemistry and Origin of the Carbonatite Veins of Mer Pluton, Mundwara Igneous Complex, Rajasthan

Alvikite veins, genetically related to Mer alkali pluton arc located in the peripheral Erinpura granites of Mundwara Igneous Complex. These veins vary in length from 7 to 70 metres and are lenticular with discontinuous lenses arranged en echelon., Fluidal, pegmatitic, porphyritic, vesicular and frothy textures, chilled marginal effects, fenitization and carbonatization are observed. Their mineralogy and textures indicate that they are pneumatolytic hydrothermal veins.

Trace element content (in ppm) is Y-53, La-163, Ce-74, Sc-18, Rb-4, Ta-5, Th-5, U-3, Sr-997, Ba-684, Pb-36, V-26. Cr-15, Co-6 and Cu-4. The mean values and the range of trace element content (in %) of pure calcites of these veins are, Sr-0.605(0.71-0.41); Ba-0.059 (0.07-0.03); Ce-O.038 (0.047-0.033); La-0.029 (0.034-0.021); Y-0.0069 (0,0081-0.005); Mn-O.4 (0.47-0.32); Mg-0.05 (0.14-nil); P-0.05 (0.19-nil) ; Sr, Ba, Mn and Mg contents are well within the range of values reported for other carbonatites, though they are on the lower side of the average values. Lower values may be due to the hydrothermal nature of these veins.

Mer Pluton occupying a small 2 Sq. Km. area, presents a complete suite of differentiated rock series, pyroxenite, banded gabbro, mela gabbro, theralite, essexite, nepheline syenite, sodalite syenite and the parent chilled basalt, basanite. High oxygen fugacity conditions of formation of this rock suite are indicated by the crystallization of magnetite in considerable quantities from the earliest formed pyroxenite, conversion of titanomagnetite to titanohematite, ilmenite to titanohematite and rutile in the chilled basalt, and extensive maghemitization of magnetite in almost all other rock types. Petrochemical studies indicate that the carbonated basanitic magma, following the calc-alkaline trend of fractional crystallization, has given rise to the miascitic suite of Mer pluton. It is inferred that the carbonatites are formed from tile residual carbothermal liquids. Na, Fe-rich vapour phase, in equilibrium with other phases in carbothermal solutions has fenitized the surrounding rocks.