TiV plots and the petrogenesis of modern and ophiolitic lavas

Abstract

Plots of Ti vs. V for many modern volcanic rock association 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 f{hook}O2 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 suittes 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" Karmøy volcanics have Ti/V trends similar to MORB; the "upper" Karmøy 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. © 1982.