Rb/Sr-87Sr/86Sr Variations in Bombay Trachytes and Rhyolites (Deccan Traps): Rb-Sr Isochron, or AFC Process?

Abstract

A petrologically diverse suite of Deccan Trap rocks, comprising basalts, trachytes, rhyolites, and various alkaline rocks, was studied in the Bombay area along the western Indian rifted continental margin. A previous petrogenetic study of these trachytes and rhyolites suggested derivation through fractional crystallization of basaltic magmas, or partial melting of basaltic material at depth, without involvement of continental crust. A Rb-Sr isochron age for the rhyolites was also calculated, but this isochron in our view is fictitious and represents a mixing line instead. Although some of the evolved liquids may have formed by fractional crystallization of basaltic magmas or partial melting of earlyformed basalts, we show that Rb/Sr-87Sr/86Sr variations in the Bombay trachyte-rhyolite suite are consistent with a combined assimilation-fractional crystallization (AFC) process involving a parental trachyte magma (Sr 225 ppm, 87Sr/86Sr = 0.7040) and an Archaean basement gneiss (Sr 145 ppm, 87Sr/86Sr = 0.7690). Although the required amount of crustal contamination is small (0.27-3.15%), the effects are sufficiently large to make any isochron age determination unviable. The role of simple or complex mixing/contamination processes must be evaluated and rejected before age significance is ascribed to linear arrays in parent-daughter isotopic ratio plots.