Petrogenesis of the Angadimogar Syenite, Kerala and its Taphrogenic Affiliation

Abstract

A syenite pluton occurring ncar Angadimogar in Cannanore district. northern Kerala and emplaced close to the continental margin and along a NW-SE trending faultlineament is reported. Alkali feldspar constitutes the dominant mineral with subordinate albitic plagioclase and quartz. corresponding to nordmarkitic composition. The syenite shows unique assemblage of accessory minerals. namely hornblende, riebeckite. crocidolite and acmite with diopside, bi.otite. zircon, sphene and calcite. High alkali content, with dominance of Na₂0 over K₂0, moderate transition dement contents and low Rb values are characteristic. Petrochemical features suggest melt equilibration from a K-rich, Rb-depleted mantle source, with subsequent amphibole and alkali feldspar fractionation. Significant increase in fO₂, fH₂O and peralkalinity towards the residual stage is deduced. The syenite belongs to the group of Late Precambrian-Early Palaeozoic alkaline intrusives of the region, with its generation and emplacement correlatable with the taphrogenesis of the western continental margin of lndia.