Coexisting arc and MORB signatures in the Sonakhan greenstone belt, India: late Neoarchean – early Proterozoic subduction rollback and back-arc formation

Abstract

Differentiation of rock suites related to mid-ocean ridge and subduction zone in Archean greenstone belts is important in tracing back tectonic processes related to evolution of these belts. The late Neoarchean – early Paleoproterozoic Sonakhan greenstone belt (SGB) lying between Mesoarchean gneisses of the Bastar craton and the Mesoproterozoic Chattisgarh Supergroup in central India was earlier interpreted to have arc-like affinity. New data from the SGB is presented to reinterpret the Archean tectonic setting. NNW-SSE trending SGB is constituted of three domains. The Baghmara domain in the west is dominantly a mafic metavolcanic rock succession (BGMV group), with repeated cycles of massive to pillowed basalts, pillow breccia and thin chert-BIF-shale and greywacke interlayers, representing an oceanic back-arc system. The Bilari domain in the east, with mixed mafic and felsic metavolcanic rocks (BLMV group) and minor clastic metasediments, presents an ancient magmatic arc. Overlapping these, a polymictic conglomerate-sandstone (greywacke) intercalation of the Arjuni Formation occurs in the central part of steep fold-fault belt of the SGB. Basic to intermediate intrusives (SMI group) and syn- to late-tectonic granitoids occur in all three domains.

The BGMV group samples are low-K tholeiites and characterized by modern MORB like major element composition and near-flat REE patterns, reminiscent of some basalts of back-arc spreading centres, such as Parece Vela off Mariana arc. These features together with plots in Sm/Yb versus La/Sm diagram suggest derivation of their parental magmas from primitive spinel lherzolite mantle source with an N-MORB affinity that subsequently fractionated under low-pressure conditions. The BLMV and SMI samples with calc-alkaline major element composition are characterized by E-MORB type REE profiles, with enriched LREE and fractionated HREE patterns, and enrichment in trace elements more incompatible than Ti, relative to N-MORB. In addition, plots in Sm/Yb versus La/Sm diagram indicate derivation of parental magmas from partial melting of enriched garnet lherzolite mantle source at different depths, less and more deep for the BLMV and SMI groups, respectively. The BLMV magmas evolved via crystal fractionation under high water pressure conditions. The intermediate to acidic intrusives of the SGB are calc-alkaline and metaluminous, similar to I-type granites.

Although in Th/Yb versus Nb/Yb diagram all the SGB mafic rocks plot above MORB array, restriction of the BGMV samples near N-MORB – PM field and distribution of the BLMV and SMI samples along AFC curve above the MORB array confirm juxtaposition of two contrasting suites, with oceanic back-arc and arc affinities, in the SGB. The Arjuni Formation apparently represents an accretionary wedge lodged in between the Baghmara and Bilari domains. Based on geological and geochemical characteristics, we suggest influence of subduction rollback and oceanic back-arc spreading in the tectonic evolution of the Sonakhan greenstone belt, which may have been common in other late Neoarchean – early Proterozoic greenstone belts.