FOREARC-ARC-BACKARC SYSTEMS OF THE CENOZOIC WESTERN PACIFIC: THEIR RELEVANCE TO CORDILLERAN ARC-OPHIOLITE TERRANES

The western Pacific basin (WPAC) preserves ~50 Ma history of oceanic plate convergence and episodic development of subduction systems. Ocean lithosphere has been recycled to the mantle via trenches and new crust has formed in volcanic arcs, backarc basins, and forearcs. Supra-subduction zone (SSZ) mantle was depleted of basaltic melt by previous melting processes. Subduction dehydration of altered oceanic lithosphere and sediments re-enriches it with water and low ionic potential elements mobile in aqueous fluids. SSZ magmas are depleted in high field strength elements (HFSE, e.g. Ti, Zr, Ta, Nb) because their source is depleted, however they are variably enriched in fluid-mobile elements (e.g., Rb, Sr, Ba) - the subduction component. Thus, SSZ magmas have distinctive chemistry useful in recognizing them in other settings, e.g., accreted terranes. Basaltic crust, nearly identical to N-MORB, commonly has minor HFSE depletion that sets it apart from "true" mid-ocean ridges basalt. Many SSZ volcanic arcs evolve to classic "calc-alkaline" (CA) andesite, but early stages of many, e.g., Tonga, have low-K island arc tholeiite (IAT) basalts, basaltic andesites, and, rarely, picrite. Dacites have low K, high Ca, relative to CA dacites. Arcs may evolve to CA series with variable K-enrichment. Mature island arcs have mid-level crust of granitoids complementary to silicic tuffs and pyroclastics. Boninite is a distinctive SSZ magma type having high MgO (8 - 18%), high Cr, Ni, low Al, 53-58% SiO2, very low HFSE, and varied fluid-mobile elements. It forms early in subduction history and may be inter-layered with IAT. Their origin is restricted to forearcs; they have never been found on mid-ocean ridges. Boninite in arc-ophiolite terranes probably is unequivocal evidence for an SSZ origin. There is no doubt that ophiolites are relict fragments of oceanic crust and upper mantle; their geology gives insights and constraints for understanding oceanic petrologic layering inferred from seismic data, and for evolution of lithosphere formed at mid-ocean ridges. However, many, perhaps all, ophiolites formed in an SSZ setting. Cordilleran arc-ophiolite terranes appear to have many geologic features and rock associations like those of WPAC SSZ systems.