Palaeomagnetic and geochemical characteristics of the Rajmahal Traps, eastern India

The Rajmahal Traps of Early Cretaceous age (117 ± 1 Ma) are medium/fine-grained rocks of quartz-tholeiitic composition. Plagioclase phenocrysts in the basalts show compositional zoning from An₅₃ to An₆₈, whereas groundmass plagioclase compositions range from An₅₀ to An₅₈. Clinopyroxene phenocrysts range in composition from En_46–48Wo_35–39Fs_14–19, with groundmass clinopyroxene compositions in the range En_32–46Wo_33–37Fs_18–34. Titanomagnetite occurs as small specks, prismatic grains and elongate needles. Secondary minerals include smectite-chlorite replacing groundmass plagioclase and augite, suggesting that the basalts have undergone hydrothermal alteration subsequent to their emplacement. Major, trace and rare-earth element data suggest two distinct catergories of basalts; Group I, characterised by high MgO, K₂O, Sr, Rb, Cr, Ba and Nb; and Group II, characterised by high Fe₂O₃, TiO₂ and rare-earth element contents. Both basalt groups appear to have been derived by melting of the same mantle source. Rajmahal basalts collected from three sites show very good grouping in their natural remanent magnetic vectors, with normal and reverce polarities. Detailed AF and thermal demagnetisation treatment of these rocks reveals characteristic components at all three sites. Sites A and B exhibit normal magnetisation, and site C exhibits both normal and reverse magnetisations. These results are critically evaluated in relation to models for widespread basaltic magmatism during the break-up of eastern Gondwana.