Petrology and Petrogenesis of Syenites from the Cuddapah Basin, Andhra Pradesh

Rare bodies of syenites of alkaline or subalkaline character occur as intrusions in the rocks of the Cuddapah Supergroup within the Cuddapah basin. The alkali syenites (found at Racherla, Idamakallu and Giddalur) are mainly composed of riebeckite (arfvedsonite), ilmenite, orthoclase and the subalkaline syenites (located at Gundlapalle) comprise hedenbergite, and microcline-perthite. Accessory quartz is invariably present in both the syenites. These intrusive bodies display contrasting mineralogical and geochemical characteristics. The quartz- and hypersthenenormative subalkaline syenite has the attributes of a ferrosyenite and has been subjected to hydrothermal alteration which not only resulted in the formation of secondary minerals like nontronite, amphibole, biotite and magnetite- all at the expense of hedenbcrgite, but also partly converted the Fe⁺² into Fe ⁺³. These subalkaline syenites are characterized by an extreme paucity of Mg, has low to moderate contents of Ba, Sr, Rb, Zr and REE. The quartz, hypersthene and acmite- normative alkali syenites (lusitanite) on the other hand are abnormally enriched in Ti; Rb is low but Ba, Sr, Ce, Nb, Nd, Ta, Sm, La, Zr am also the ratios K/Rb, Ti/Zr, Ce/Y and HREE/LREE are conspicuously high.

The syenites of the Cuddapah basin, seem to have crystallized under contrasting conditions. While the alkali syenites crystallized under high PH₂O, the crystallization of subalkaline syenite took place under dry arid reducing conditions signifying a closed system. It is envisaged that the subalkaline ferrosyenite of Gundlapalle was formed by low-pressure crystal fractionation of a continental tholeiitic basaltic magma. The alkali syenites, on the other hand, are postulated to have crystallized from a mantle-derived, hydrous, alkaline magma enriched in Large Ion Lithophile Elements and Light Rare Earth Elements.