Abstract

Ophiolite-Chert complexes containing ultramafics, mafic dykes and pillow lavas, overlain by radiolarian cherts and other pelagic sediments are significant features of mountain belts, especially in the Alpine-Himalayan chain (Vine & Hess 1970), and in the Caledonides (Bird et al. 1971). The sedimentology and fauna of the cherts and the similarity of the igneous succession with inferred oceanic igneous structure are consistent with the hypothesis that these complexes are tectonically emplaced fragments of oceanic crust and upper mantle (Vine & Hess 1970). Elsewhere in orogenic belts, mafic rocks of oceanic aspect are frequently found without the full ophiolite succession being present. Although these are broadly similar to oceanic mafic lavas it has not proved possible to establish the genetic affinities of such rocks by their major element chemistry (Vallance 1969).

It has recently been proposed that the genetic affinities of basaltic rocks may be distinguished by the ratios of the three elements, Ti, Zr and Y which have been shown to be insensitive to alteration affecting major elements during metamorphism up to greenschist facies (Cann 1971; Pearce & Cann 1971). Five magma types (number of analyses given in parentheses) have been considered by Pearce & Cann; island arc andesites (12), island arc tholeiites (14), ocean island alkali basalts (8), ocean island tholeiite basalts (13) and mid-ocean ridge basalts (39). Using discriminant analysis they constructed functions in Ti, Zr and Y which distinguished these lava types. Analyses were by X-ray fluorescence on pressed powder pellets. On this basis, samples of supposed oceanic rocks

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