Transition from vertical to horizontal tectonic regime during the Paleoarchean-Mesoarchean: Evidence from mafic-ultramafic rocks of the Holenarsipur Greenstone Belt, Western Dharwar Craton, Southern India

The Dharwar Craton (DC) in southern India is one of the classic Archean terrains, preserving an evolutionary history from the Paleoarchean to Mesoarchean. The present study attempts to understand the origin and evolution of mafic (amphibolites) and ultramafic (harzburgite) rocks from the Holenarsipur Greenstone Belt (HGB), to decipher the evolution of the cratonic core of the Western Dharwar Craton (WDC). Relict olivine and orthopyroxene pseudomorphs and Neodymium isotopic compositions reveal that harzburgites from the HGB represent a preserved section of the early Archean mantle, re-fertilized by fluid/rock interaction during serpentinization. The ultramafic and mafic rocks from the southern and northern parts of the belt exhibit coherent whole-rock geochemical and Nd isotopic compositions. Ultramafic rocks were derived from a depleted mantle source (epsilon Nd-(t=3300)= +0.65 to +8.70) while, the amphibolites were generated by partial melting of an enriched mantle source (epsilon Nd-(t=3300) = -4.16 to +0.92). The ultramafic rocks with fore-arc affinity may signify the prevalence of a subduction setting during the early stages of the first accretionary event in the WDC. The compositionally uniform mafic rocks from the HGB with oceanic plateau affinity affirm a plume environment. Our study establishes the prevalence and transition from vertical to horizontal tectonics during the PaleoarcheanMesoarchean evolution of the WDC.