Geochemistry and Source Characteristics of Dykes Straddling the Panvel Flexure, India and Their Relationship to the Deccan-Aged Seychelles Intrusives

Several dykes straddle the Panvel flexure (PF) and intrude the compound pāhoehoe flows in the West Coast of India. Three geochemically distinct groups of dykes, that is, olivine normative tholeiites, quartz normative tholeiites with geochemical affinity to the Bushe, Poladpur and Ambenali formations and a nepheline normative alkali basalt are emplaced straddling the PF, striking parallel to the West Coast Fault (WCF) and the PF (NNW–SSE). The quartz normative tholeiites from the present study are restricted to the east of the PF axis and olivine normative tholeiites dominate west of the PF axis. The presence of olivine-bearing dykes west of the PF axis may represent the upper parts of a plumbing system that has tapped either a large basic magma chamber variably, that accumulated fairly large quantities of primary magma at the crust–mantle boundary or a distinct but linked magma reservoir that underwent different AFC processes to produce the olivine tholeiites straddling the flexure. Quartz tholeiite east of the PF axis represents differentiated products of these reservoirs. WCF and PF are imprints of the separation of Seychelles from India associated with the rifting subsidence of the crust and upliftment of the Western Ghats. Deccan age (69.1–73.5 Ma) dykes occurring in Praslin, Félicité and Mahé islands of Seychelles are quartz normative, olivine normative and nepheline normative, respectively, forming three distinct geochemical groups and are chemically akin to the Bushe and Khandala formations when plotted in bivariate ratio plots. Trace and REE-based models of intrusions straddling the PF and their counterparts in the Seychelles; indicate partial melting of dominantly garnet lherzolite source. Partial melting curves show olivine normative and quartz normative tholeiite dykes from Seychelles to dominantly have spinel lherzolite source with different degrees of partial melting. In contrast, the nepheline normative dyke has a spinel + garnet lherzolite source.