A Petrochemical Appraisal of Dalma Volcanism and its Bearing on the Geodynamic Evolution of the Proterozoic Singhbhum Mobile Belt: Eastern Indian Shield

The Dalma metavolcanic rocks occupying the median tract of the Singhbhum mobile belt of eastern Indian peninsular shield, despite being extensively studied, still continue to pose serious questions about their precise chemical nature, stratigraphic position and tectonic regime. The present study is an attempt to address the underlined issues through a renewed petrochemical treatment of these metabasites in the type area north of Jamshedpur in Jharkhand state where they occur as metabasalts, agglomerates and tuffs which constitute low-grade greenschist facies rocks typically characterized by tremolite – actinolite as well as albite - epidote – chlorite ± actinolite associations. The chemical attributes suggest tholeiitic protolith for them. The exceedingly low K (0.09-0.24 wt%) and FeO /MgO (1.20-1.59) < 1.7 are comparable to the oceanic or abyssal tholeiites similar to the MORB. Presence of ol in the norms indicates their Mg-rich basaltic nature. The Mg number (58-65) characteristically conform to the mafic to somewhat ultramafic character and also the tholeiitic nature which is further substantiated using various chemical parameters as well as on the basis of abundance of trace elements such as V, Cr, Ni, Co, Cu, Rb, Sr. The extremely low Rb (av. 2.7 ppm), Sr (av.137 ppm) and Th/U (~ 4) values also indicate their affinity with MORB low-K tholeiites. The major and trace element data are consistent with fractional crystallization model of differentiation within a limited range. The MORB and Chondrite normalized trace element patterns show similar trends marked by parallelism, negative slope and presence of characteristic Nb-Ta-Ti anomaly typical of subduction related island arc set-up. A relatively smooth pattern showing close proximity to MORB line suggests their back-arc extensional tectonic regime further supported by tectonomagmatic diagrams as well as their close similarity in composition to the East Scotia back-arc tholeiites.