Ultramafic to Mafic Cumulates From a Mesoarchean Accretionary Belt in Southern India

ABSTRACT The Mercara Suture Zone sandwiched between the Western Dharwar Craton and the Coorg Block, recently identified as a Mesoarchean suture, provides significant insight into the tectonics during early Earth history. The remnants of Archean oceanic lithosphere, which occur as ultramafic to mafic cumulates, were investigated for the first time in this study from the accretionary belt within the Mercara suture zone. With a view to gaining insights into the regional crustal evolution through Mesoarchean subduction–accretion‐collision processes, in this study, we present petrology, whole‐rock geochemistry including major, trace, and rare earth elements, and zircon U–Pb and Lu‐Hf geochronology to trace the petrogenesis of the cumulates and associated rocks and the timing of magmatism. These mainly comprise hornblendite, amphibolite and metagabbro, with minor felsic rocks. The ultramafic to mafic cumulates display moderate SiO 2 (44.76–52.08 wt%), low Zr/TiO 2 *0.0001 (0.001–0.007), and low to moderate T FeO/( T FeO + MgO) (0.31–0.61), indicating their generation from magnesian, sub‐alkaline basaltic magma. The chondrite‐normalised and primitive mantle‐normalised trace elements of the ultramafic‐mafic cumulates show transitional features between normal mid‐ocean ridge basalt (N‐MORB) and enriched mid‐ocean ridge basalt (E‐MORB), while MORB‐normalised trace elements display pronounced positive Pb anomaly and flat HREE patterns, suggesting oceanic backarc (OBA) and island arc tholeiite (IAT) affinities. Most of the cumulates display low to moderate V/Ti (0.05–0.08), low Nb/Zr (0.04–0.19) and moderate Zr (13.1–71.4 ppm), consistent with subduction settings. Most zircon grains in these rocks have ca. 3.1 Ga magmatic cores with metamorphic overgrowths at ca. 3.1–3.0 Ga. Zircon Lu–Hf analyses yield ages of ca. 3.3 Ga with εHf(t) values in the range of −0.5 to 1.3, suggesting juvenile late Paleoarchean sources. Our study suggests that the ultramafic to mafic cumulates could be remnants of oceanic lithosphere preserved within the accretionary prism, as an archive of the Mesoarchean convergent margin processes.