GEOLOGY OF SUPRA-SUBDUCTION ZONES - IMPLICATIONS FOR THE ORIGIN OF OPHIOLITES

Abstract

Ophiolites are rock assemblages recognized as relict fragments of oceanic crust and upper mantle. They provide insights into petrologic and seismic layering of in situ oceanic lithosphere at mid-ocean ridges and into deep oceanic crustal layers not easily sampled. Ocean crust also forms at other settings. At convergent oceanic plate margins, aged oceanic lithosphere is subducted into depleted mantle. Above the subduction zone, new oceanic crust is formed in the forearc, volcanic arc, and backarc basins. The magmas, products of supra-subduction zone (SSZ) processes, carry chemical and isotopic signatures reflecting SSZ sources and can be distinguished from mid-ocean ridge magmas. Ophiolite assemblages within belts of accreted terranes (e.g., the North American Cordillera) indicate an oceanic origin for some terranes, but their close geologic association with arc volcanic and volcaniclastic material, granitoids, and silicic extrusives makes a mid-ocean ridge origin suspect. There is much similarity between Cordilleran ophiolites, many other ophiolites that are associated both with tectonically juxtaposed volcanic arc complexes, and the Cenozoic SSZ systems of the Western Pacific Basin. The latter includes immature SSZ systems, such as the Tonga-Kermadec, Vanuatu, Mariana-Izu-Bonin, and also more evolved systems, such as Palau-Kyushu Ridge, Fiji, Luzon, and Mindanao. All geologic units that characterize ophiolites are found in SSZ rock assemblages, as well as distinctive rock types, such as boninite and island arc series rocks. Backarc basin crust is predominantly formed of rocks similar to mid-ocean ridge basalt, but commonly includes variants gradational to island arc tholeiites. Forearc and backarc basins may have thick ponds of arc-derived tuffs and siliciclastics. These may lie on nearly coeval oceanic crust. Parts of backarc basins, starved of clastics, may have pelagic sediments, metalliferous sediments, "umbers" and protoliths for cherty argillites. Mature arcs may be underlain by plagiogranites and tonalites that are plutonic complements of silicic arc volcanism.