Geochemical discrimination between ocean-floor and island-arc tholeiites—application to some ophiolites

Abstract

Two discriminative diagrams are proposed to separate island-arc tholeiites (1AT) and ocean-floor tholeiites (OFT). The first diagram, Ti/Cr vs. Ni, has been drawn using 84 island-arc (IAT) and 178 ocean-floor (OFT) samples with silica contents between 40 and 56%. About 97% of OFT and 93% of IAT samples fall, respectively, on opposite sides of the empirical boundary. In the second diagram, where the Ba/Y is less than 4.4 for the OFT and more than 3.9 for the IAT, the overlap between the two groups is about 6%.Owing to alteration effects, only the discrimination diagram Ti/Cr vs. Ni has been applied to ophiolitic basalts from the Mediterranean belts, Newfoundland, Central and North America, and Mongolia. The effusive and hypabyssal formations plotting either in one group or in the other lead to the suggestion that they have been formed in several possible geotectonical environments. It appears that ophiolites generated in a mid-oceanic ridge are scarce in opposition to those formed in an island-arc setting. In this latter case, ophiolite associations may correspond to the juxtaposition of either island-arc – marginal basin or island-arc – offshore oceanic crust formations.

Petrology and geochemistry of orthoamphibolites from the Variscan metamorphic sequences of the South Tisia in Croatia - an overview with geodynamic implications

“…On the Ti/Cr vs. Ni diagram (Beccaluva et al 1979), most of the points fall along the line separating oceanic tholeiites and island arc tholeiites; the Ti-Zr-Y diagram (Pearce and Cann 1973) demonstrates that most analyzed rocks plot in field B of ocean-floor basalts, and on the Nb-Zr-Y diagram (Meschede 1986), these amphibolites are gathered close to the triple point of the fields of volcanic arc basalts, within-plate tholeiites, and E-and N-MORBs. Consequently, with such unequivocal geochemical discrimination data, any geotectonic consideration becomes dubious.…”

Tectonic significance of mafic volcanic rocks in a Mesozoic sequence of the Menderes Massif, West Turkey

“…11a). The plot of Ti/Cr vs Ni (ppm) (Beccaluva et al 1979) also shows an oceanic affinity for the metabasalts (Fig. 11b).…”

GEOCHEMISTRY OF METAMORPHIC ROCKS OF THE KURAI BLOCK (Gorny Altai)

“…На диаграммах Ni-Ti/Cr [Beccaluva et al, 1979] и Cr-Ti [Pearce, 1975], используемых для разде-ления океанических и островодужных толеитовых базальтов (рис. 11, а, б), точки составов изученных амфиболитов образуют тренд в поле базальтов океанического дна, а на диаграммах Nb-La [Gill, 1981] и Zr-Nb-Y [Meschede, 1986] (см.…”

“…а -диаграмма Ni-Ti/Cr [Beccaluva et al, 1979]. OFT -толеитовые базальты океанического дна; IAT -толеитовые базальты островных дуг.…”