Petrogenetic Significance in the Relationship Between Nepheline Syenite and Shonkinite of Elchuru Alkaline Complex, Cuddapah Intrusive Province, Andhra Pradesh, India

The highly deformed eastern side of the Cuddapah basin led to the formation of numerous rifts, faults and major lineaments which in turn facilitated the Meso Proterozoic miaskitic alkaline magmatism. Co spatial association of mafic and felsic alkaline rocks within this zone is found in a row trending in NE-SW direction, all the plutons, constitute the Prakasam alkaline province later renamed as Cuddapah intrusive province. The above mentioned association appears pronouncedly at Elchuru, designated as Elchuru alkaline complex, where mafic and felsic (feldspathoidal) alkaline syenites i.e. shonkinite, malignite, nepheline syenite, lamprophyre are present in adequate representation to correlate the genesis of these rocks. Petrographically shonkinite shows the predominance of clinopyroxene, biotite and amphibole as mafics and on the other hand, nepheline and perthitic feldspar constitutes the felsics. This phenomenon is reverse in the case of nepheline syenites, which predominantly consists of subhedral nepheline along with the K-feldspar (perthitic) with subordinate aegirine augite, ferrohastingsite and discrete biotite. Predominance of mafics in the former and dearth of those in the later denote the fractionation of nepheline syenites from the shonkinitic magma. Geochemically, the percentages of FeO, MgO, and CaO are higher in shonkinite and low in nepheline syenite indicating the fractionational crystalisation process that was involved in the genesis of nepheline syenites. Subsequent increment of SiO₂, Na₂O and K₂O with the magmatic differentiation from the early formed shonkinites to later formed nepheline syenites is also noticed. Geochemical discrimination diagrams and the plots in the Petrogeny’s residua system at 1 Kb PH₂O clearly illustrates the evolution of nepheline syenites from the K-rich hydrous mafic magma. Enrichment of transition metals such as Cr, V, Ni, Zn and LFSE in the shonkinites and lack of them in the nepheline syenites further substantiate that the genesis of nepheline syenites is undoubtedly from an undersaturated shonkinitic magma at Elchuru alkaline complex.