Petrogenetic implications of mineral chemistry and mode-based statistical studies of Sholayar alkaline syenite complex, Southern Granulite Terrane, India

A recent petrological study on the lesser-known Sholayar alkaline syenite complex (10°17′59.9′′N: 076°47′26.3′′E) accommodated in the high-grade rocks of the Southern Granulite Terrane reveals the presence of three distinct petrographic types namely leuco syenite, melanofelsic syenite and mela syenite. The syenites, in general, have the following constituent minerals, namely alkali feldspar (Or 76.8 Ab 23.2 –Or 93.4 Ab 6.6 ), plagioclase (An 4.1 –An 17.3 ), calcic amphibole (edenitic hornblende/ferropargasitic hornblende), quad pyroxene (diopside–hedenbergite), biotite and opaque minerals (magnetite and ilmenite). Mode-based statistical studies indicate that a highly significant correlation exists among certain mineralogical parameters including quartz%, total feldspar%, colour index, alteration index and hydration index. Isopleth plots of these parameters strongly suggest maintenance of a magmatic regime throughout, with increasing water content towards the end stage of crystallization. Several geothermobarometric methods point to shallow to moderate level emplacement (~9–23 km) of the alkaline/sub-alkaline syenite magma which was facilitated by a ‘hydration event’. The steep change in water gradients (in localized pockets) of the syenite intrusives has been corroborated by textural evidences. Research highlights First systematic petro-mineralogical report of Sholayar alkaline syenite complex from SGT. Involvement of single magmatic lineage with variable water content. Geothermobarometry suggests a moderate to shallow level of syenite emplacement.