Abstract
Mafic dikes and dike swarms associated with tholeiitic flows of the continental Deccan flood basalt province are studied to understand the timing of lithospheric extension, rifting, and rift activation related to the eruption of the Deccan basalts. Integrated geophysical, tectonic, and geochemical studies and and K-Ar ages of these mafic dikes and associated Deccan flows along the intraplate Narmada-Tapti rift and western continental margin rift point to flood basalt eruptions between 67 and 64 Ma (peaking at ~65 Ma) with concurrent lineament reactivation into rifts and dominant N-S and E-W extension of the Indian lithosphere. Correlation of Bouger gravity, shallow and deep seismic, and heat flow anomalies show penetration of elongated and discontinuous mafic bodies between 5 and 6 km deep along the central axis and flanks of the intraplate rift and a long, elongated, and anomalous mantle body into the upper crust along the continental margin rift with upward inflexion of the Moho toward the intersection of the two rifts. Crustal thinning along the rifts ranges between 8 and 25 km. Correlated geophysical and field-structural data show listric faulting and high concentrations of mafic dikes over rift-oriented geophysical anomalies. Field and geochemical relations, and age (67-64 Ma) similarities of many mafic dikes and basal flows, indicate their comagmatic nature and establish many rift-oriented mafic dikes as primary feeders. Geochemical and petrological evidence indicates that the majority of the lower Deccan tholeiites evolved in local and multiple magma chambers close to the surface up to a depth of 7 km, which is consistent with the geophysical evidence. An asthenospheric plume origin under a thinned lithosphere for the parental olivine-tholeiites is possible. Liquid line of descent calculations shown on clinopyroxene-olivine-silica and clinopyroxene-plagioclase-olivine pseudoternary plots and variation diagrams involving MgO and various oxides and elements suggest that the tholeiites of dikes and flows are related by fractional crystallization. The dikes and flows also show evidence of contamination (mainly crustal), which diminishes from the lower Deccan to the Poladpur and the Ambenali flows. Geochemical-stratigraphic relations indicate that the basal flows and dikes in Narmada-Tapti rift region and basal flows in the Gujarat region are older than the Igatpuri flows in the Western Margin rift region. A mantle plume model of laterally spreading magma with propagation of dikes from magma chambers and formation of magma chambers and eruptive centers along lithospheric weak trends is presented to explain copious and rapid eruptive activity over a widely spread area, crustal contamination during ascent, and magma evolution in shallow crustal chambers in the Deccan province.
Published Version
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