Neoarchean arc–juvenile back-arc magmatism in eastern Dharwar Craton, India: Geochemical fingerprints from the basalts of Kadiri greenstone belt

• Tholeiitic to transitional basalts of variable REE contents from Dharwar Craton.
• Melting of MORB-type mantle and metasomatized mantle wedge in a fore arc–arc regime.
• A juvenile back-arc rift system proximal to the intraoceanic arc setting.
• Metasomatism of back arc mantle by slab-derived fluids and sediments.

The Neoarchean Kadiri greenstone belt (KGB) of eastern Dharwar Craton is dominated by metavolcanic rocks including basalts, andesites, dacites and rhyolites that have experienced lower amphibolite grade of metamorphism. The geochemical investigations of the basalts and basaltic andesites of KGB show 47–56 wt.% SiO2 and Mg# ranging from 38 to 64. The rocks display sub-alkaline tholeiitic to transitional affinity. Based on REEs, the KGB basalts are classified as two types. Type I basalts show flat to slightly enriched LREE patterns while type II basalts have relatively enriched LREE patterns. Both types of basalts exhibit negative Nb, Zr, Hf and Ti anomalies along with distinct enrichment in Th and U. Type II basalts are characterized by a greater magnitude of negative Nb anomaly (Nb/Nb* = 0.1–0.32) compared to type I (Nb/Nb* = 0.41–0.82) basalts. Prominent island arc signatures are evident in both the types in terms of their LILE, LREE enrichments and relative HFSE depletion. Ba/Th vs. Nb/Th relationships reflect a frontal arc setting for type I basalts whereas type II basalts correspond to a rear arc setting. Th/Nb, La/Nb and Ce/Yb ratios show a gradual increase from type I to type II basalts reflecting melting of sediments from the subducted slab and their increased addition in type II basalts. The Zr/Hf (31–43), Zr/Sm (11–32) and Nb/Th (1–9) ratios suggest a depleted to enriched mantle source with variable influx of subduction-derived fluids and sediments. The flat to slightly enriched LREE patterns of type I basalts indicate a gradual progression from depleted MORB-type mantle melting in a fore-arc to slab-dehydration–mantle wedge metasomatism and fluid-fluxed melting in an arc regime within spinel peridotite compositional domain. Island arc signatures in type II basalts suggest their generation through melting of a subduction-modified MORB-type mantle within spinel to garnet peridotite stability field in a juvenile back-arc rift system that developed proximal to the intraoceanic arc. Variable input of subduction derived fluids and sediments in type I and type II basalts attests to across-arc geochemical variations. The subduction processes operating within arc–back arc system provided favourable pathways for the migration of gold-bearing fluids leading to economic mineralization in the northern (Hutti) and southern (Kolar) parts of this belt.

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