Geochemistry and petrogenesis of syenites and associated rocks of the Elagiri complex, Southern Granulite Terrane, India

Abstract

The Elagiri complex (12°31′N, 78°35′E) in the Southern Granulite Terrane (SGT) of the Indian shield comprises syenites along with lenses of gabbroic rocks. Lamprophyres and carbonatites represent the later phases which intrude both the magmatic units of the Elagiri complex and the surrounding host rocks. The alkaline rocks in the Elagiri suite include syenites, quartz syenites, alkali feldspar syenites, quartz alkali feldspar syenites, monzonites and quartz monzonites. The gabbroic suite includes pyroxene–hornblende gabbronorite. The lamprophyre dyke rocks correspond to minette and monchiquite varieties. The carbonatite dykes are composed of igneous carbonate minerals with little altered feldspar and opaque oxides. We present major and trace element chemistry of the various rocks in the Elagiri suite which indicates a broadly overlapping chemistry for the syenites and related group. The gabbro – hornblende gabbro members have chemical characters which are distinct from those of syenites. The minette variety of lamprophyre is silica-oversaturated (quartz normative) whereas the monchiquite variety reveals olivine normative character. The geochemical variations in the Elagiri suite clearly suggest the role of magmatic differentiation in generating the syenite–gabbro (hornblende gabbro)–lamprophyre association. The syenitic rocks of Elagiri are marked by more or less uniform rock/chondrite pattern yielding similar Ce/Yb ratios in the absence of Eu-anomaly suggesting negligible or no role of plagioclase fractionation. The rock/chondrite REE patterns of gabbro–hornblende gabbro are typically much more enriched and can be attributed to liquid immiscibility from an alkaline basic magma. The geochemical and tectonic features of the Elagiri suite suggest a syn-collisional or arc setting in a subduction controlled milieu, which is in accordance with the recent models on the Neoproterozoic convergent margin in southern India and the subduction-tectonics associated with the closure of the Mozambique Ocean during the final assembly of the Gondwana supercontinent.