Plots of Ti vs. V for many modern volcanic rock associations are diagnostic of tectonic setting and can be used to determine possible tectonic settings of ophiolites as well. The basis of this plot is the variation in the crystal/liquid partition coefficients for vanadium, which range with increasing oxygen fugacity from > 1 to << 1. Since the partition coefficients for Ti are almost always << 1, the depletion of V relative to Ti is a function of the fo2 of the magma and its source, the degree of partial melting, and subsequent fractional crystallization. Volcanic rocks from modern island arcs have Ti/V ratios of ~20, except for calc-alkaline volcanics which show the effects of magnetite fractionation. MORB and continental flood basalts have Ti/V ratios of about 20-50 and alkaline rocks have Ti/V generally >50. Back-arc basin basalts may have either arc-like or MORB-like Ti/V ratios, and sample suites from single back-arc basins may have Ti/V ratios ranging from 10 to 50. This range in Ti/V ratios in samples from a restricted geographical area may be diagnostic of the back-arc setting. The Ti/V plot is applied here to published data on ophiolites from a variety of postulated settings and in general supports the conclusions of previous investigators. Ophiolites from the western Mediterranean (Corsica, northern Apennines) and the "lower" Karmoy volcanics have Ti/V trends similar to MORB; the "upper" Karmo'y volcanics have alkaline Ti/V ratios. Lavas and tonalites in the Papuan ultramafic belt, the high-Mg andesites of Cape Vogel, and the upper pillow lavas at Troodos all have Ti/V ratios < 20, consistent with formation in an island arc setting. More specific evaluation of the tectonic setting of these and other ophiolites requires application of detailed geologic and petrologic data as well as geochemistry. The Ti/V discrimination diagram, however, is a potentially powerful adjunct to these techniques.

[1] Abstract: Suprasubduction zone (SSZ) ophiolites display a consistent sequence of events during their formation and evolution that suggests that they form in response to processes that are common to all such ophiolites. This sequence includes the following: (1) birth, which entails the formation of the ophiolite above a nascent or reconfigured subduction zone; this stage is typically characterized by the eruption of arc tholeiite lavas and the formation of layered gabbros and sheeted dike complex; (2) youth, during which is continued melting of refractory asthenosphere (depleted during birth) occurs in response to fluid flux from the subducting slab, with extensional deformation of the older plutonic suite, eruption of refractory lavas, and the intrusion of wehrlite-pyroxenite; (3) maturity, with the onset of semistable arc volcanism, typically calc-alkaline, as the subduction zone matures and stabilizes, and the intrusion of quartz diorite and eruption of silicic lavas; and (4) death, which is the sudden demise of active spreading and ophiolite-related volcanism, which in many cases is linked to collision with an active spreading center and the onset of shallow underthrusting of the buoyant spreading axis; expressed as dikes and lavas with oceanic basalt compositions that crosscut or overlie rocks of the older suites; (5) resurrection, with emplacement by obduction onto a passive margin or accretionary uplift with continued subduction. The early stages (1± 3) may be diachronous, and each stage may overlap in both time and space. The existence of this consistent progression implies that ophiolite formation is not a stochastic event but is a natural consequence of the SSZ tectonic setting.

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International Ocean Discovery Program (IODP) Expedition 352 recovered a high-fidelity record of volcanism related to subduction initiation in the Bonin fore-arc. Two sites (U1440 and U1441) located in deep water nearer to the trench recovered basalts and related rocks; two sites (U1439 and U1442) located in shallower water further from the trench recovered boninites and related rocks. Drilling in both areas ended in dolerites inferred to be sheeted intrusive rocks. The basalts apparently erupted immediately after subduction initiation and have compositions similar to those of the most depleted basalts generated by rapid sea-floor spreading at mid-ocean ridges, with little or no slab input. Subsequent melting to generate boninites involved more depleted mantle and hotter and deeper subducted components as subduction progressed and volcanism migrated away from the trench. This volcanic sequence is akin to that recorded by many ophiolites, supporting a direct link between subduction initiation, fore-arc spreading, and ophiolite genesis