First occurrence of melilite, potassic richterite and tetraferriphlogopite in Deccan Trap-related alkaline rocks, and its petrogenetic significance: The Rajpuri ijolite-nephelinite intrusion, Murud, Mumbai area, India

The Deccan-Traps-related “ijolite” intrusion of Rajpuri, Murud, south of Mumbai, is formed by dominant diop- side ± forsteritic olivine, set in a holocrystalline, fine-grained matrix rich in nepheline, opaque oxides, perovskite, phlogo- pite and other phases, grading a more felsic facies rich in phenocrysts of altered nepheline and less clinopyroxene (i.e., a “nephelinite”). The holocrystalline samples have clinopyroxene crystals hosting rounded microcrystals of melilite close to the rims. The presence of melilite is unique among the Deccan Trap-related alkaline intrusions in India, and implies that a batch of larnite-normative, extremely silica undersaturated silicate magma filled the Murud intrusion, and that crystalliza- tion took place in a low-pressure environment. Interstitial Mg-rich alkali amphibole (potassic richterite), growing poikiliti- cally around nepheline, oxides and tetraferriphlogopite also suggest that many clinopyroxene phenocrysts of the Rajpuri intrusion, and their host melilite grains, crystallized in magmas different from the last they were entrained. Melilite and potassic richterite are not phases that can crystallize together in sodic-alkaline magmas. The unusual geochemical and iso- topic composition of the Rajpuri intrusion, indicating magmas derived from old, enriched lithospheric mantle, and the pres- ence of melilite, are strong indication that this is one of the most peculiar alkaline intrusions related to the Deccan Traps.