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Authors
Affiliations
1 Centre for Earth Science Studies, Trivandrum-695031, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 70, No 2 (2007), Pagination: 194-206
Abstract
Profuse Proterozoic basic magmatism is represented by mafic dykes all around the Cuddapah Basin and by lava flows/sills at different stratigraphic levels of sedimentary sequences within the basin. However, the genetic links between magmatism all around and within the basin are rather obscure. Available geochronological data have been examined in the light of our geochemical and palaeomagnetic results. The geochemical results clearly show that the dykes are sub-Alkalic tholeiitic basalts in composition and no distinctions are visible in the elemental trends with respect to field orientations or discrete age groups. However, the LIL element chemistry has been variably affected by different degrees of alteration. Such an alteration seems to have resulted in wide range in their ages, mainly by the Rb-Sr, K-Ar and Ar-Ar methods. Using palaeomagnetic constraints we recalculated the Sm-Nd isotopic data of three dykes of Mahabubnagar-Wanparti sector and estimated c. 2.4 Ga and 2.1 Ga age for the early dyke magmatism around the Cuddapah basin. Two other discrete dyke magmatic activities are assigned 1.8 and 1.65 Ga based on palaeomagnetic correlations. Thus, the sub-alkalic tholeiitic basic magmatism within lower Cuddapah Basin and around is confined to the Palaeoproterozoic times and the younger ages are attributed to disturbed isotopic systems. An interesting observation is that the dykes in the Anantapur-Gooty sector are predominantly rich in MgO, Ni and Cr and low in Zr and are dominated by olivine cumulus mineralogy. This feature evidently suggests that the dykes in this region solidified in much closer proximity to the subsurface magma chamber, retaining early-formed dense phases like olivine still as cumulates/phenocrysts. Such an interpretation provides geochemical support to the inference from Bouguer gravity anomaly for subsurface magma underplating in the southwest Cuddapah Basin. This is the first study from India to provide support from the geochemistry of dykes for subsurface magmatic body inferred from gravity anomaly.
Keywords
Mafic Dykes, Age, Petrology, Geochemistry, Proterozoic, Cuddapah Basin.
