Geochemistry and Ar-40/Ar-39 geochronology of the Nandurbar-Dhule mafic dyke swarm: Dyke-sill-flow correlations and stratigraphic development across the Deccan flood basalt province

摘要

The ENE-WSW-trending Nandurbar-Dhule swarm is the best developed tholeiitic dyke swarm in the Deccan Traps. We obtained Ar-40/Ar-39 ages of 67.06 +/- 0.60, 67.49 +/- 0.89, and 63.43 +/- 0.48 Ma (2 sigma internal errors) on three of its largest dykes (36-79 km long), indicating that swarm emplacement spanned >= 2.5 million years under regional crustal extension. Our Sr-Nd-Pb isotopic data, combined with previously available geochemical data, identify multiple magma injections in some dykes and also identify probable feeder dykes of some lavas in the lower Western Ghats sequence and in Saurashtra, each similar to 200 km away. Several dykes are compositionally distinct from hitherto analysed lavas; >50% of the analysed Nandurbar-Dhule dykes are isotopically like the Mahabaleshwar and Panhala formations of the upper Western Ghats sequence, covering a very narrow isotopic range, but have the distinctive chemical signatures of the high-TiO2 Kolhapur Unit of the southernmost Western Ghats. These dykes thus possess a unique combination of isotopic and chemical characteristics not hitherto known in Deccan tholeiites, cross-combining features of different eruptive units in the Wai Subgroup of the Western Ghats. This new, "Nandurbar-type" chemical-isotopic flavour is however frequently observed in dykes, sills, and lavas in the Pachmarhi, Shahdol, and Mandla areas 450-600 km to the east, and in Deccan-age dykes cutting through the Early Cretaceous Rajmahal Traps of eastern India. Varied geochemical evidence indicates that the northern and northeastern Deccan lava stratigraphy (such as the Pavagadh section and the Pachmarhi-Shahdol-Jabalpur-Mandla areas) is largely independent of the Western Ghats lava stratigraphy.