No abstract

No abstract

The Neoarchean Sonakhan greenstone belt (SGB) in the north‐eastern Bastar Craton is mainly composed of an association of pillowed and massive tholeiitic basalts emplaced within an intraoceanic environment. Two types of ultrabasic rocks were identified and are designated as TH‐1 and TH‐2 on the basis of geochemical parameters. Both sample suites exhibit depletion of HFSE with reference to LILE and LREE, LREE/HFSE ratios, and negative Nb–Ta–Ti anomalies in the primitive mantle‐normalized multi‐element diagrams. The geochemical characters of TH‐1 and TH‐2 are consistent with Island arc tholeiites (IAT) and boninite‐like rocks of both Archean and Phanerozoic terranes. Mineral chemistry of clinopyroxenes from both volcanic suites indicate an oceanic arc affinity. The chromite chemistry indicate its derivation from a boninite‐like magma in a suprasubduction zone (SSZ) environment. Trace element modelling depicts that source depletion with significant influence of fluids derived from the subducting oceanic slab collectively controlled the trace element inventory of the mantle wedge, which result to the higher abundance of LREE and LILE compared to HREE and HFSE. The occurrence of lower basalt sequence followed by IAT and boninite‐like rocks in the SGB define a magmatic evolution which is comparable to the Phanerozoic ophiolite suites that exhibit subduction initiation prior to forearc rifting and suprasubduction zone magmatism. The subduction initiation in the intraoceanic lithosphere and the initial decompression melting followed by the upwelling of the MORB‐related magmas were attributed to the formation of the lower pillow basalts. Dehydration of the subducting plate and the melt extraction processes in the hydrous lherzolite mantle wedge account for the formation of TH‐1. Subsequent depletion and significant modification of a more refractory harzburgite source by the slab‐derived fluids generated the TH‐2 rocks. The compositionally diverse magmatic imprints with IAT and boninite affinity in the SGB indicate the significance of episodic melt extraction processes, dehydration of the subducting plate, and hydrous fluxing in SSZ tectonic setting.

The Neoarchean Sonakhan Greenstone Belt, located in the northeastern fringes of Bastar Craton, Central India, is dominated by basalts, andesites, dacites, and rhyolites and also contains some basic rocks with very high MgO (up to 33.4 wt%). Chromite mineralization is present in these rocks along with the cumulates of olivine and clinopyoxenes. The rocks are classified as siliceous high‐magnesium basalts (SHMB) exhibiting enriched large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to the high field strength elements. Elevated Th/Yb ratios and negative Nb‐Ta‐Ti anomalies in the primitive mantle normalized multielement diagram indicates a significant role of subduction‐related melts/fluids in their genesis. The chromites in SHMB have high Cr# (0.67–0.75) and moderate Mg# (0.11–0.5) values. Parental melt calculations in the chromites indicate that they are crystallized from an SHMB magma in an island‐arc setting. A plausible model for the genesis of the rocks of Sonakhan Greenstone Belt includes initial subduction of an intraoceanic lithosphere followed by eruption of lava in an oceanic environment. Continued subduction of the slab followed by slab rollback followed by the generation of SHMB parental magma, which was introduced into the basal portions of the lithosphere in which cumulates of olivine and clinopyroxene have been developed and the chromite mineralization occurred in the inter cumulus space. Final emplacement of the magma took place in a forearc suprasubduction‐zone environment with SHMB signature carrying the cumulates, which were located in the lower part of the lithosphere.