The Deccan nephelinite conundrum - derivation from a metasomatized sub-continental lithospheric mantle or a mantle transition zone?

Nephelinite, the earliest derivation of the rift-related magmatism, is distributed across diverse global tectonic settings, including the end-Cretaceous Deccan Large Igneous Province, India. Two contrasting global models are in vogue, favouring the genesis of the nephelinites, viz. (i) low-degree partial melting of a volatile-rich, metasomatized subcontinental lithospheric mantle or (ii) mantle transition zone. We compile all the available Deccan-nephelinites data and compare it with the global datasets. Our observation shows – i) a substantial latitudinal trend in the chemical variation of the Deccan nephelinites from north to south; ii) the nephelinites of the Sarnu-Dandali and the Kutch alkaline complexes show the most primitive signature and match with that of the eastern China variants, while the remaining fraction shows evolved character; iii) a conspicuous similarity in the trace element patterns between Mesoproterozoic kimberlites and HIMU type OIB. We therefore conclude that, despite being a by-product of the contemporaneous Deccan magmatism, a distinct difference exists amongst the earliest derivative of the Reunion plume caused by Deccan magmatism. We further highlight that the source heterogeneity existed in the earliest stages beneath the western Indian subcontinental lithospheric mantle. A much deeper origin, along with a carbonated peridotite source, can be invoked for the genesis of these Deccan nephelinites. Deccan LIP nephelinites are temporally connected but chemically disconnected.Gradual variation in chemistry with respect to the northern to southern geographical disposition of nephelinites in the western Indian shield.Mantle Transition Zone source has been invoked and similarity with kimberlites and HIMU-OIB.LAB beneath the western Indian shield varies even during the minuscule earliest phase. Deccan LIP nephelinites are temporally connected but chemically disconnected. Gradual variation in chemistry with respect to the northern to southern geographical disposition of nephelinites in the western Indian shield. Mantle Transition Zone source has been invoked and similarity with kimberlites and HIMU-OIB. LAB beneath the western Indian shield varies even during the minuscule earliest phase.