CHARACTERIZATION AND PETROGENESIS OF INTERMEDIATE TO SILICIC ROCKS IN OPHIOLITES: A GLOBAL SYNTHESIS

Intermediate to silicic rocks exist in most ophiolites as intrusive bodies at different stratigraphic levels and/or lavas and pyroclastic deposits at their upper, volcanic parts. We present a global synthesis of the occurrence of such leucocratic, intrusive and volcanic rocks from 150 Phanerozoic to Archean ophiolites, and evaluate models for their genesis during the development of oceanic crust in different tectonic environments. Evolved rocks in subduction-unrelated, Rift/Continental Margin ophiolites are predominantly basaltic andesite and andesite, whereas MOR type (mid-ocean ridge) ophiolites exhibit nearly equal proportions of basaltic andesite/andesite and rhyodacite and Plume/MOR type ophiolites are characterized by rhyolites. Intermediate to silicic volcanic uints in the Backarc sub-group of subduction-related ophiolites are characterized by similar amounts of basaltic andesite/andesite and rhyodacite, whereas in the Backarc to Forearc, Forearc, and Volcanic Arc sub-groups they are mainly basaltic andesite/andesite. Intermediate to silicic rocks in Rift/Continental Margin and Plume/MOR type ophiolites are generally LREE-enriched, whereas those in the MOR type vary from LREE-depleted to LREE-enriched. The Backarc and Backarc to Forearc types are similar to the MOR type; silicic rocks of the Forearc and Volcanic Arc types are generally LREE-enriched. The main process in the formation of the majority of the intermediate to silicic rocks in both subduction-unrelated and subduction-related ophiolites is partial melting of basaltic and/or gabbroic rocks beneath the spreading centers, whereas a minor volume in subduction-related ophiolites are adakites that were produced from partial melting of a subducting slab. Silcic to intermediate rocks in Plume/MOR type ophiolites are generated by fractional crystallization of basaltic melt. The incompatible, non-conservative elements, such as Ba and Th, are weakly to strongly enriched in subduction-related ophiolites as a result of shallow to deep enrichment associated with subduction zone processes. The field occurrence and the geochemical character of leucocratic rocks in ophiolites show considerable variations, providing additional constraints on the petrogenesis of ophiolites in different tectonic settings.