Sr-Nd isotope and chemical evidence that the Ballantrae ‘ophiolite’, SW Scotland, is polygenetic

Abstract

Summary Sr-Nd isotope and major- and trace-element analyses, including REE, are presented for a suite of basaltic lavas from the Ballantrae complex, SW Scotland. The chemistry of the lavas is discussed with reference to ‘spider’ diagrams, using normalization of incompatible element abundances to those estimated in the primordial earth, rather than conventional element-element plots. These diagrams facilitate assessment of element mobility during alteration, and some samples are shown to have close to original igneous concentrations of the mobile elements Rb, K, Sr etc. Clinopyroxene-phyric basalts from Games Loup and Mains Hill have chemical affinities with modern intra-oceanic arc volcanics, and their acid-leached augite phenocrysts have more radiogenic Sr than the mantle array, defined using clinopyroxene separates from Ordovician oceanic basalts from the Southern Uplands. Basalts from Pinbain, Bennane Head and Downan Point are in contrast typical ocean-island tholeiites in both chemistry and Nd-Sr isotopes. At least four independent volcanic sequences can be recognized from the chemical data: these do not form a continuous 5 km thick sequence but rather represent tectonically juxtaposed units generated in very different environments. The ‘ophiolite’ can not therefore be regarded as a single entity, produced in a single tectonic environment. Sm-Nd mineral ages have been determined for samples from Games Loup, Mains Hill and Downan Point in an attempt to provide a chronology for the development of the different units, but ages obtained are indistinguishable from the zircon age reported by Bluck et al. (1980) for a Ballantrae trondhjemite. Evaluation of the significance of the complex to Caledonian plate models will only be possible with more precise age determinations on the different members of the complex.