GEOLOGY AND GEOCHEMISTRY OF THE DEGANA PLUTON - A PROTEROZOIC RAPAKIVI GRANITE IN RAJASTHAN, INDIA

The Degana pluton hosts one of the few known tungsten deposits in Indi a. It is an epizonal, moderately high silica pluton emplaced during th e Proterozoic in a post-tectonic setting. Though homogeneous in compos ition, it displays textural heterogeneity from coarse-grained hypidiom orphic to fine-grained porphyritic to hypabyssal granite porphyry. Gen etically related rhyolites are also present. Coherency of geochemical and mineralogical attributes in the Degana pluton can be explained by fractional crystallisation. Complex variety of hydrothermal and pneuma tolytic features is also present. At shallow depths, emanation differe ntiation has led to progressive enrichment of Li, Rb, and W. Both the plutonic and volcanic phases of the magma show development of rapakivi texture and other diagnostic characteristics of the rapakivi granites . The Degana granite is a ''specialised granite'' and classified as an A-type intraplate anorogenic granite of mantle plume origin. The mine ralogy and chemistry of the Degana pluton compares well with the vario us rapakivi granites of south-eastern Fennoscandia. Chemical and textu ral characteristics of the Degana pluton provide a constraint on the f ormation of the rapakivi texture when interpreted in terms of experime ntally determined phase equilibria. The mantling process is interprete d as a result of pressure fluctuations due to escape and recharging of volatiles (e.g., H2O and F) accompanying the emplacement of the magma .