Genesis of Epidiorites Associated with Dhalbhum Formation of Proterozoic Singhbhum Basin

Abstract

Epidiorites occurring as conspicuous bands and sill-like bodies interstratified with schistose rocks of Dhalbhum Formation, belonging to Proterozoic Singhbhum Group, are rocks of enigmatic nature characterized by volcanic flows associated with tuffs and other volcaniclastic materials, the genesis of which has so far remained a matter of debate. They are very fine-grained, less schistose with irregular joints, partings and splintery habits. Petrographically, they are characterized by greenschist facies mineralogy represented by albite–epidote–chlorite ± actinolite assemblages similar to the Dalma rocks occupying the mid basinal part of the Singhbhum mobile belt but with higher proportion of saussuritized plagioclase and sericitized K-feldspar often containing oval and elliptical bombs and tephra of glassy matters revealing their tuffaceous character. Their overall compositional homogeneities and other chemical attributes indicate their tholeiitic to calc-alkaline nature. Some of them show very close chemical proximity with the Dalma metavolcanic rocks which are characterized by exceedingly low-K tholeiites comparable to oceanic abyssal tholeiitic basalts falling within the chemical spectrum of enriched MORB ranging in composition from mafic to somewhat ultramafic lavas. The various chemical trends amply suggest that epidiorites are the late differentiates of Dalma parental magma ranging in composition from basalt–basaltic trachy andesite–trachy andesite following the iron suppressed calc-alkaline line of descent. The MORB and Chondrite normalized patterns showing more spiky nature in contrast to relatively smooth patterns of Dalmas are also suggestive of their more evolved nature. Enrichment of LREE (~7xHREE), higher range of total REE (200–434 ppm) as well as (La/Sm)_N (3.45–4.78) and (La/Yb)_N (7.81–11.60) also indicate their fractionated character. A distinctive negative Eu-anomaly marks the REE patterns of the more felsic epidiorites. A narrow range of variation in Ce/Nd ratio in Dalma metavolcanics (1.26–1.55) and epidiorites (1.80–2.49) together with Mg number (58–74) and FeO⁰/MgO (0.85–1.59) suggest their comagmatic character. The enriched MORB composition and bimodal mafic to ultramafic effusion of Dalma rocks are specific of back-arc extensional tectonic regime showing signs of violent eruptive nature both subaerial and subaqueous. Dalmas as well as the epidiorites with 4–5% MgO simulate the common chemical features of the back-arc basalt from East Scotia basin in south Atlantic. The study of epidiorites assumes significance as components of Dalma volcanics are also found within the Dhalbhum succession. The larger implication of the work has suggested that the volcanic dominated Singhbhum Group of rocks resemble the Precambrian greenstone associations related to island-arc assemblages.