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.

The present study deals with the petrogenesis and age implication of anorogenic peralkaline granitoid rocks exposed along the North Puruliya Shear Zone (NPSZ) in Jhalda area of Puruliya district, West Bengal. Alkali granite consists of quartz, alkali feldspar, aegirine, riebeckite, arfvedsonite and biotite. These granitoid rocks have a high range of silica, very high total alkali content and are poor in CaO, Al 2 O 3 , FeO and MgO content. Geochemically, they are ferroan, alkalic, reduced and peralkaline granitoid rocks and have many similarities with A-type granites. Crystallisation temperatures of these granitoid rocks are greater than 900°C. U–Pb isotopic ages of zircon indicate a major age cluster ~966.7 ± 7.0 Ma. The oldest lower crustal rocks in and around Jhalda area are charnockite, khondalite, garnetiferous granite gneiss, which might have acted as source rocks. Trace element model indicates that a moderate degree partial melting (5–20%) of charnockite + khondalite source rock followed by ~30% fractional crystallisation of plagioclase feldspar is responsible to generate parent magma of alkali granite. Similar and overlapping crystallisation ages of 966.7 ± 7.0 Ma of the per-alkaline anorogenic/post-orogenic granites of present study with already reported orogenic I-type granites from Jhalda and S-type granites from nearby Raghunathpur area (age 1000 Ma) may indicate origin and emplacement of post-orogenic granites of Jhalda during orogeny–anorogeny transition at the time of waning stage of orogenic activity. Mantle upwelling in late to post-orogenic stage provides additional heat to initiate partial melting of lower crustal source rocks.

The evidence of a long‐lived Grenvillian orogeny around 1.0 Ga is preserved in the eastern part of India in the form of large‐scale syn‐collisional granitic activity and high‐grade metamorphism in the Chotanagpur Granite Gneissic Complex (CGGC). Geochemically, granitoid rocks of CGGC were classified as ferroan, per‐aluminous, and calc‐alkalic to alkali‐calcic S‐type granites which were generated from the anatexis of meta‐sedimentary protolith (khondalite) in a volcanic‐arc to syn‐collisional setting. Relatively high crystallization temperatures (800–850°C for garnetiferous granite gneiss, 770–830°C for megacrystic granite gneiss, and 639–725°C for pink granite) of these rocks were calculated by geothermometric calculations. The REE modelling testified that 40–45% partial melting of the khondalite in H2O undersaturated condition at around 5‐kbar pressure might have given rise to the parent magma of the garnetiferous granite gneiss (GGG). LA‐ICPMS zircon (U–Pb) dating yielded the crystallization ages of the megacrystic granite gneiss (MGG) and pink granite (PG) as ~1.0 Ga, and EPMA monazite (U–Th–total Pb) dating suggested that the GGG was metamorphosed during 1.0 Ga. The 1.0–0.90 Ga age of high‐grade metamorphism and anatexis in the CGGC may be correlated with that in the Rayner Complex–Eastern Ghats Belt in the Indo‐Antarctic sector, which is considered as the result of Indo‐Antarctic collision during the assembly of Rodinia.