Petrology, geochemistry and tectonic settings of the mafic dikes and sills associated with the evolution of the Proterozoic Cuddapah Basin of south India |
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| Authors: | Nilanjan Chatterjee Somdev Bhattacharji |
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| Institution: | (1) Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 02139 Cambridge, Massachusetts, USA;(2) Department of Geology, Brooklyn College, City University of New York, 11210 Brooklyn, New York, USA |
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| Abstract: | In this article we summarize the petrological, geochemical and tectonic processes involved in the evolution of the Proterozoic
intracratonic Cuddapah basin. We use new and available ages of Cuddapah igneous rocks, together with field, stratigraphic,
geophysical and other criteria, to arrive at a plausible model for the timing of these processes during basin evolution. We
present petrological and geochronological evidence of dike emplacement along preferred lineament directions around the basin
in response to stresses, which may have been responsible for the evolution of the basin itself. Basaltic dike intrusion started
on the south Indian shield around 2400 Ma and continued throughout the Cuddapah basin evolution and sedimentation. A deep
mantle perturbation, currently manifested by a lopolithic cupola-like intrusion under the southwestern part of the basin,
may have occurred at the onset of basin evolution and played an important role in its development. Paleomagnetic, gravity
and geochronological evidence indicates that it was a constant thermal source responsible for dike and sill emplacement between
1500 and 1200 Ma both inside and out-side the basin. Lineament reactivation in the NW-SE and NE-SW directions, in response
to the mantle perturbation, intensified between 1400 and 1200 Ma, leading to the emplacement of several cross cutting dikes.
Fe-Mg partition coefficients of olivine and augite and Ca-Na partition coefficient of plagioclase, calculated from the composition
of these minerals and bulk composition of their host rocks, indicate that the dikes outside the Cuddapah basin are cumulates.
The contemporary dikes may be related by fractional crystallization as indicated by a positive correlation between their plagioclase
Ca# (atomic Ca/Ca+Na]) and augite Mg# (atomic Mg/Mg+Fe]). A few NW-SE and NE-SW cross cutting dikes of the period between
1400 and 1200 Ma, preserve petrographic evidence of episodic magmatic intrusive activity along preferred directions. Petrological
reasoning indicates that a magmatic liquid reacted with a set of cross cutting dikes, intruding into one that was already
solidified and altering the composition of the magma that produced the other dike.
The Cuddapah basin tholeiites may be related by fractional crystallization at 5 kb and 1019-1154‡ C, which occurred in the
lopolithic cupola near the southwestern margin of the basin. Xenolith bearing picrites, which occur near the periphery of
the cupola, originated by the accumulation of xenoliths in the tholeiites. This is indicated by the composition of the olivine
in the xenoliths (Fo78.7-81.9), which are closely similar to calculated olivine compositions (Fo77.8-78.3) in equilibrium with the tholeiites under the sameP-T conditions. It is inferred that fractionation in the cupola resulted in crystals settling on its walls. Hence, the xenolith-bearing
sills occur at the periphery of the lopolithic body.
The tholeiites both inside and outside the basin are enriched in incompatible elements compared to mid oceanic ridge basalts.
The Ba, Rb and K contents of the Cuddapah and other Proterozoic Gondwana tholeiites indicate that a widespread metasomatic
enrichment of the mantle source may have occurred between R∼2.9 and R∼2.7Ga. There may be local heterogeneity in the source
of the Cuddapah tholeiites as indicated by different Ba/Rb, Ti/Zr, Ti/Y, Zr/Nb and Y/Nb in samples inside and outside the
basin. Large-scale differences such as the low P2O5-TiO2 and high P2O5-TiO2 basaltic domains of the Jurassic Gondwana basalts, however, did not exist during the Proterozoic time period under consideration.
Although we are beginning to understand the tectono-magmatic processes involved in the evolution of the Cuddapah basin, much
work remains to be done to obtain a complete picture. Future research in the Cuddapah basin should focus on obtaining accurate
ages of the igneous rocks associated with the evolution of the basin. |
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| Keywords: | Cuddapah proterozoic basalt geochemistry mafic dike mafic sill petrology |
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