The petrogenesis of modern and ophiolitic lavas reconsidered: Ti-V and Nb-Th

Tectonic discrimination diagrams are a key tool for understanding ancient volcanic rock origins. In this contribution we compile over 15,000 whole rock compositions to re-evaluate the Ti-V discrimination diagram and compare it to another commonly used tool, the Nb/Yb-Th/Yb diagram. We have reformulated the Ti-V diagram into a log-log plot to distinguish samples more clearly at lower concentrations. The compilation shows that MORB are dominated by Ti/V = 20-43, whereas juvenile arc tholeiites and boninites are characterized by Ti/V < 20 generally, although there is minor overlap at the boundary (Ti/V = 20-22). Plume-related volcanic rocks (ocean island basalts, oceanic plateaux, and continental flood basalts) generally have Ti/V > 43, although there may be significant overlap with MORB-like ratios for ridgecentered OIB and for some oceanic plateaux. About 56% of alkaline OIB have Ti/V > 70. Back-arc basins are dominantly MORB-like. Melt models show that MORB and juvenile arc volcanics most likely formed under different fO(2) conditions, but are permissive of similar fO(2) if the arc rocks form by much higher melt fractions. The Nb/Yb vs. Th/Yb plot clearly distinguishes most oceanic basalts (MORB, plateaux, OIB) from subduction-related volcanic rocks (boninite, juvenile arc tholeiite, calc-alkaline) and from flood basalts. We propose here a new two-proxy diagram of Ti/V vs. Th/Nb, which incorporates the advantages of both. (C) 2021 China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V.

Automated summary

Tectonic discrimination diagrams are important tools for understanding the origins of ancient volcanic rocks. This study re-evaluates the Ti-V and Nb/Yb-Th/Yb diagrams using a compilation of over 15,000 whole rock compositions. A new two-proxy diagram, Ti/V vs. Th/Nb, is proposed to combine the advantages of both diagrams.