Petrography, geomagnetism, and rare-earth element abundances of the Rajahmundry lavas, eastern India

Abstract

Closely jointed, veined and locally sheared basalts some 15–20 m thick are exposed in quarries at Gauripatna and Kateru on the banks of the Godavari River, near Rajahmundry, Andhra Pradesh, India (17°N 21°E). The clinopyroxene in these basalts is variably replaced by saponite, which locally preserves the primary phenocryst shape. At Gauripatna, the amygdales have dolomite in the centre and saponite in the rim; opaque phases occur only as dendritic aggregates within saponite. At Kateru, dolomite is absent and large and hypidiomorphic magnetite mantles fresh clinopyroxene. Measured rare-earth element (REE) concentrations indicate the occurrence of light REE-enriched basalts, most probably derived from a single mantle source region by different degrees of partial melting. Comparison with published data indicates that the Rajahmundry basalts have REE abundances similar to lavas from the Deccan Traps, western India. Fairly intense brittle deformation of the Rajahmundry lavas, at both the scale of outcrop and hand specimen, is evident in the deeper parts of the quarries. A pilot study of the magnetic fabric shows a large apparent spread in azimuth. Palaeopole positions derived from such deformed lavas are likely to be unreliable, due to replacement of primary iron oxides.