Petrology and geochemistry of the Salma dike, Raniganj coalfield (Lower Gondwana), eastern India: linkage with Rajmahal or Deccan volcanic activity?

Abstract

The Gondwana (Early Permian to Early Cretaceous) basins of eastern India have been intruded by ultramafic–ultrapotassic (minette, lamproite and orangeite) and mafic (dolerite) rocks. The Salma dike is the most prominent among mafic intrusives in the Raniganj basin. This dike is tholeiitic in composition; MgO varies from 5.4 to 6.3% and the mg number from 54 to 59. In general, the major and trace element abundances are uniform both along and across the strike. There is geochemical and mineralogical evidence for fractional crystallization. The chondrite normalized REE pattern of the Salma dike (La/Yb n =3.5) is similar to that of Deccan dikes of the Son–Narmada rift zone, western India. 87Sr/ 86Sr varies from 0.70552 to 0.70671 suggesting assimilation of crustal material. Some trace element abundances (e.g. Ti, Zr, Y) of the Salma dike are comparable to Group I Rajmahal basalts. The 40Ar– 39Ar whole rock age of 65 Ma for the Salma dike is less than the ca. 114 Ma age for the Rajmahal basalt, but is similar to the generally accepted age for Deccan volcanic rocks. Despite geographical proximity with the Rajmahal basalt, the Salma dike is believed to be related to late phase of Deccan volcanic activity.