Rhyolites from the northeastern margin of Singhbhum Craton: petrography and geochemistry

The Singhbhum Craton of eastern India comprises Archean TTG basement, granite-greenstone belt, and Neoarchean–Paleoproterozoic mobile belts, and records a major phase of Mesoarchean potassic to ferro-potassic granitoid magmatism (~3.12–3.05 Ga) following earlier TTG crust formation. Earlier literatures have largely focused on granitoids and associated mafic–ultramafic suites, with rhyolites remaining undocumented. The rhyolites identified in the vicinity of the Bangriposi area are spatially associated with the Mayurbhanj granite–granophyre suite. They are dark grey in colour, predominantly aphanitic and show rare primary flow banding. Some samples exhibit porphyritic textures, with quartz and feldspar phenocrysts (~1–2 cm) set within a fine-grained groundmass. Under the microscope they are extremely fine grained, and some relict feldspar grains are preserved despite alteration, indicating partial recrystallization. The primary mineral assemblage consists of quartz, plagioclase, and microcline, with accessory phases including muscovite, apatite, ilmenite, iron oxides, galena, zircon, and monazite. Zircon grains are fractured and display pitted texture. Quartz phenocrysts are typically anhedral, fractured, and show strong undulose extinction, with some grains forming polycrystalline aggregates indicating recrystallization.  

Whole-rock XRF data show high SiO₂ (71-78 wt%) content with moderate to high total alkalis (Na₂O + K₂O ≈ 6.8–9.1 wt%). K₂O is always greater than Na₂O and their Na₂O/K₂O ratios lie within a restricted range varying between 0.52 and 0.95. The higher K₂O value compared to Na₂O suggests significant input from pre-existing crustal materials. On TAS diagrams, the samples plot in the rhyolite field. Harker variation diagrams show decrease in MgO, Fe₂O₃, CaO, and TiO₂ with increasing SiO₂ signifying magmatic differentiation, primarily through fractional crystallization. The rhyolites are metaluminous to weakly peraluminous, calc-alkaline to alkali-calcic, ferroan, and very low in Mg. Normative QAP compositions plot mainly in the monzogranite field. The data suggest that the rhyolites might have formed from partial melting of existing continental crust or from the fractional crystallization of a parental magma possibly combined with the assimilation of older crustal rocks. A comparative geochemical and petrogenetic evaluation of these rhyolites with the associated granite–granophyre suite will provide critical insights into crustal evolution and tectono-thermal history of the Singhbhum Craton, while further trace-element and isotopic studies are required to fully constrain their source characteristics and petrogenetic history.