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 Giddalu r) are mainly composed of riebeckite (arfvedsonite), ilmenite, orthocl ase and the subalkaline syenites (located at Gundlapalle) comprise hed enbergite, and microcline-perthite. Accessory quartz is invariably pre sent in both the syenites. These intrusive bodies display contrasting mineralogical and geochemical characteristics. The quartz- and hyperst henenormative subalkaline syenite has the attributes of a ferrosyenite and has been subjected to hydrothermal alteration which not only resu lted in the formation of secondary minerals like nontronite, amphibole , biotite and magnetite- all at the expense of hedenbergite, but also partly converted the Fe+2 into Fe+3. These subalkaline syenites are ch aracterized by an extreme paucity of Mg, has low to moderate contents of Ba, Sr, Rb, Zr and REE. The quartz, hypersthene and acmite- normati ve alkali syenites (lusitanite) on the other hand are abnormally enric hed in Ti; Rb is low but Ba, Sr, Ce, Nb, Nd, Ta, Sm, La, Zr and also t he ratios K/Rb, Ti/Zr, Ce/Y and HREE/LREE are conspicuously high. The syenites of the Cuddapah basin, seem to have crystallized under contra sting conditions. While the alkali syenites crystallized under high PH 2O, the crystallization of subalkaline syenite took place under dry an d reducing conditions signifying a closed system. It is envisaged that the subalkaline ferrosyenite of Gundlapalle was formed by low-pressur e crystal fractionation of a continental tholeiitic basaltic magma. Th e alkali syenites, on the other hand, are postulated to have crystalli zed from a mantle-derived, hydrous, alkaline magma enriched in Large I on Lithophile Elements and Light Rare Earth Elements.