Geology of Pulikonda and Dancherla alkaline complexes,Andhra Pradesh |
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Authors: | G Suresh R Ananthanarayana R C Hanumanthu Subhasish Ghosh A Anil Kumar K V S Reddy |
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Institution: | (1) Hydrogeology Laboratory, Department of Geology, Andhra University, Visakhapatnam 530 003, India |
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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. |
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