GEOLOGY OF SUPRA-SUBDUCTION ZONE SYSTEMS - THEIR RELEVANCE TO THE ORIGIN OF OPHIOLITES

Ophiolites, interpreted as relict fragments of oceanic crust and upper mantle, provide insights and constraints for oceanic petrologic layering inferred from seismic velocities, and for the evolution of lithosphere formed at mid-ocean ridges (MOR). Thus “ophiolite=seafloor=ophiolite.” Ophiolite assemblages within accreted terranes (e.g., North American Cordillera) support an oceanic origin for many terranes. But many ophiolites have crustal sections thinner or thicker than seafloor. Silicic plutons and extrusives are common. Both mafic and silicic rocks may have isotopic and trace element chemistry (particularly high field strength elements) different from MOR. These ophiolites are fragments of oceanic lithosphere but probably NOT from deep sea MOR unless we postulate curious crustal tectonic-petrologic schemes to explain their accretion. From where do they come? Oceanic lithosphere is formed not only at MOR but off axis on leaky transforms and seamounts as well as in supra-subduction zone (SSZ) settings of convergent margins. Modern SSZ systems offer insights to the origin of ophiolites as well as the volcanic arc-derived material commonly found with them. In particular, occurrences of nearly coeval volcanic/plutonic/arc clastic rocks, granitoids, silicic tuffs, “cherty argillite”, and boninites with ophiolite series rocks require a geologic setting like SSZ but unlike MOR. The geology of the Tonga-Kermadec, Vanuatu, Mariana-Izu-Bonin SSZ systems (immature intra-oceanic arcs) and Luzon, Fiji, Palau-Kyushu Ridge (more evolved systems), resembles many Cordilleran ophiolites and their tectonically juxtaposed arc complexes. SSZ forearc and volcanic arc magmas include boninite, arc tholeiite and calc-alkaline series. Mature backarc basins are dominated by MORB but may include transitions to island arc basalt and have seamounts of ocean island and arc series basalt. Forearc and backarc basins may have thick accumulations of arc-derived tuffs and siliciclastics. Parts of backarc basins, starved of clastics, have metalliferous deposits, “umbers”, sedimentary protoliths for cherty argillite, and pelagic sediments. Mature island arcs may have intrusive granitoids (e.g., tonalite, plagiogranite) complementary to silicic tuffs and pyroclastics.