Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

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Abstract

Geological studies on saturated to oversaturated and subsolvus aegirine-riebeckite syenite bodies of the Pulikonda alkaline complex and Dancherla alkaline complex were carried out. The REE distribution of the Dancherla syenite shows a high fractionation between LREE and HREE. The absence of Eu anomaly suggests source from garnet peridotite. The Pulikonda syenite shows moderate fractionation between LREE and HREE as reflected by enrichment of HREE and moderate enrichment of LREE. The negative Eu anomaly indicates role of plagioclase fractionation.Three distinct co-eval primary magmas i.e. mafic syenite-, felsic syenite- and alkali basalt magmas - all derived from low-degrees of partial melting of mantle differentiates and enriched metasomatised lower crust played a major role in the genesis and emplacement of the syenites into overlying crust along deep seated regional scale trans-lithospheric strike-slip faults and shear zones following immediately after late-Archaean calc-alkaline arc magmatism at different time-space episodes i.e. initially at craton margin and later on into the thickened interior of the Eastern Dharwar craton. The ductile sheared and folded Pulikonda alkaline complex was evolved dominantly from the magmas derived from partial melting of lower crust and minor juvenile magmas from mantle. Differentiation and fractionation by liquid immiscibility of mafic magma and commingling-mixing of intermediate and felsic magmas followed by fractionational crystallisation under extensional tectonics during waning stages of calc-alkaline arc magmatism nearer to the craton margin were attributed as the main processes for the genesis of Pulikonda syenite complex. Commingling and limited mixing of independent mantle derived mafic and felsic syenitic magmas and accompanying fractionation resulting into soda rich and potash rich syenite variants was tentatively deduced mechanism for the origin of Dancherla, Danduvaripalle, Reddypalle syenites and other bodies belonging to Dancherla alkaline complex at the craton interior. The Peddavaduguru syenite was formed by differentiation of alkali mafic magma (gabbro to diorite) and it's simultaneous mingling with fractionated felsic syenitic magma under incipient rift. Vannedoddi and Yeguvapalli syenites were derived due to desilicification and accompanying alkali feldspar mestasomatism of younger potash rich granites along Guntakal-Gooty fault and along Singanamala shear zone respectively.

Dancherla syenite body has yielded a six point Rb-Sr whole rock isochron age of 2211±110 Ma with low initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7004±0.00046; MSWD = 5.8. The low initial Sr_(i) ratio suggests mantle derived juvenile additions to the crust. The Pulikonda syenite body yielded a six point whole rock isochron age of 1500 ±100 Ma with initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7085±0.0047 and MSWD = 2%. The higher initial Sr_(i) value for this body suggests metasomatised source. Resetting of Rb-Sr systematics has resulted younger age for Pulikonda syenite though it is supposed to be older as per the geological setting and this anomaly is interpreted interms of repeated deformation and reactivation of shear zone, emplacement of alkali granite into Pulikonda syenite and the role of Rudravaram shear zone (basement tectonics) in the formation of the Nallamali fold belt of the Cuddapah Basin. Alkaline magmatism was initially triggered along microplate margin tectonic setting and later confined to the intraplate tectonic setting.

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