Forearc and back-arc mantle characteristics of the mafic-ultramafic rocks of Simlipal complex, Singhbhum Craton, India

The Simlipal complex of Singhbhum Craton is enigmatic in terms of its shape, stratigraphy, age and geodynamic setting. The circular-shaped Simlipal complex has predominant mafic volcanic rocks and quartzites associated with komatiites, komatiitic basalts, picrites, boninites, andesites and other ultramafic rocks. The mafic rocks are subalkaline tholeiitic basalts to basaltic andesites with moderate SiO2 (45.94–54.26 wt%), MgO (6.09–12.21 wt%), Fe2O3 (10.02–16.44 wt%), CaO (8.33–12.12 wt%), Mg # (32–58), low TiO2 (0.4–1.02 wt%), P2O5 (0.08–0.34 wt%), Ni (112–241 ppm) and Cr (104–506 ppm). The chondrite normalized rare earth element (REE) patterns are slightly fractionated [(La/Yb)N = 2.97–5.44] with slight negative to positive Eu anomalies (Eu/Eu* = 0.84–1.46). Their primitive mantle normalized multielement patterns exhibit negative Th, Nb, Ta and positive to negative Ti anomalies. The ultramafic rocks characterized as harzburgites, have high MgO (34.77–38.11 wt%), Ni (1974–2453 ppm), Cr (3875–4610 ppm) and Mg# (80–85), low SiO2 (40.61–44.18 wt%), TiO2 (0.08–0.14 wt%), Al2O3 (2.24–3.79 wt%), Fe2O3 (7.24–9.77 wt%) and P2O5 (∼0.05 wt%). Their chondrite normalized REE patterns exhibit fractionated LREE [(La/Sm)N = 2.07–2.37], depleted MREE, almost flat HREE [(Gd/Yb)N = 0.99–1.14] and a pronounced trough at Nb-Ta; ZrHf and Ti on the primitive mantle normalized multielement patterns. The basalt gave a zircon UPb age of 2843 ± 16 Ma (MSWD = 0.72) which is the crystallization age. HFSE relationship of the basalts indicates back-arc nature and their melting in the spinel-lherzolite field. The peridotites are arc cumulates, exhibiting boninitic characteristics and forearc affinity. Their PGE systematics indicate retention of sulphides in the residue during the partial melting and fractionation of the mantle. These peridotites and basalts were generated in forearc and back-arc environments during the Neoarchean time, subsequently juxtaposed with plume-derived volcanic sequences and the passive margin sediments as the arc progressed towards the continent. The plume-arc accretion model (PAA) is a viable tectonic setting with the presence of komatiites, komatitic basalts, picrites, tholeiitic basalts and peridotites. Further, the metasediments from the passive continental margin accreted with PAA products in the Simlipal complex. The above findings also support the existence of Archean smaller plate configuration and the proximity of the arc to the continental margin. After the formation of the volcano-sedimentary sequences, the meteoritic impact appears to have obliterated the accretionary imprints in the Simlipal complex.