Petrology and geochemistry of Bhanjada Bet phonolites, Kutch, Gujarat in Western Deccan Province: Possibility of a mantle-derived primary phonolite magma

Bhanjada Bet igneous rock lies to the east of Pachham Island and west of Khadir Island, well within the Great Rann of Kutch, Gujarat, western India. It is a small isolated hillock made up largely of phonolite with patches of trachyte and traversed by a mafic dyke. The phonolite is composed dominantly of sanidine, nepheline, aegirine, Ti-amphibole and glass. Sanidine occurs as phenocryst and nepheline as microphenocryst. Groundmass is composed of smaller alkali feldspar, aegirine, Ti-amphibole and glass. The lath-shaped feldspar in the groundmass defines excellent flow texture. Major element chemical composition indicates occurrence of two groups of phonolite-low silica (around 53%) and high silica (around 58%). The phonolite has higher abundance of MgO (2.21–3.5%) over FeO (2.43–2.6%) and very high Mg# (65–69 ) suggesting its primitive nature. Trace element abundances suggest an overall enrichment in LILE and selected HFSE, highly fractionated LREE pattern. High Mg# of the Bhanjada bet phonolite along with low values of Zr and Hf compared to phonolites of basanite phonolite suite where phonolite is an evolved member, is noted. The lower Zr and Hf values hint towards a less evolved primary phonolitic magma rather than phonolite as late differentiates of basanitic–phonolite suite. Experimental studies indicate low degree partial melting of a metasomatised lherzolite mantle at a pressure around 1–1.5 GPa can produce primary phonolite magma. Mantle xenoliths, found in alkali basalt of Kutch basin, show evidence of carbonatite metasomatism of lithospheric mantle. Bhanjada Bet phonolites are associated with layered mafic complex of Nir Wandh and other magmatic rocks of Pachham Island. Ages of Nir Wandh magmatic rocks and magmatic rocks of Pachham Island coincide with ages of early Deccan magmatism. Age of phonolite is not yet known. From field association and petro-mineralogy, we propose that Bhanjada Bet phonolite represents alkali magmatism of early Deccan age in Western Deccan Province.