EVOLUTION OF OCEANIC CRUST IN SUBDUCTION ZONE ENVIRONMENTS AS REVEALED IN PHANEROZOIC AND PRECAMBRIAN OPHIOLITES

Suprasubduction zone (SSZ) ophiolites represent oceanic crust developed in subduction rollback cycles during the closing stages of basins prior to terminal continental collisions. Collision-induced mantle flow results in subduction rollback & one or more episodes of arc splitting and basin opening, producing a collage of ‘forearc oceanic lithosphere'. Unusual occurrence of fertile peridotites and high-Mg andesites in forearc ophiolites is likely to have resulted from the injection of high-temperature asthenospheric material into the mantle wedge in these rollback cycles. Igneous accretion of SSZ ophiolites involved upper plate extension & advanced melting of previously depleted asthenosphere, showing a progressive evolution from MORB-like to IAT to boninitic, extremely refractory proto-arc assemblages. SSZ ophiolite genesis & emplacement during the assembly of large continental masses overlapped with increased production rates of juvenile crust & rapid continental growth. Cordilleran ophiolites in accretionary-type orogenic belts structurally overlie subduction-accretion complexes & are incorporated into active continental margins via progressive underthrusting of oceanic material or through ridge-trench interactions. These ophiolites are commonly polygenetic, developed on/across a heterogeneous oceanic basement & may include fully developed island arc sequences having IAT to calcalkaline affinities and pyroclastic rocks, as seen in many Neoproterozoic volcanic arc terranes. Prolonged history of subduction with variable polarity and kinematics may generate nested Cordilleran ophiolites with different ages and chemical compositions. Archean greenstone belts may in part represent fragments of arc-forearc-generated oceanic crust & plume-derived, LIP-type oceanic crust, analogous to their Phanerozoic counterparts, suggesting that mantle heterogeneity and modern plate tectonic-like geodynamic processes may have existed prior to 2.5 Ma. Recognition in Archean belts of boninites as the products of primary liquids evolved from 2nd-stage, high-T°C, low-P melting of highly refractory mantle wedge metasomatized by subduction-derived fluids & melts indicates that subduction zone processes that produce modern boninites have been operating since ~3.12 Ga.