PETROGENESIS OF BONINITES IN THE ORDOVICIAN-BALLANTRAE COMPLEX OPHIOLITE, SOUTHWESTERN SCOTLAND

Primitive lava and hyaloclastite with unusual, highly refractory compo sitions, form part oi the Early Ordovician Balcreuchan Group within th e ophiolitic Ballantrae Complex, southwestern Scotland. They are ident ified as likely high-Ca boninites on the basis of new XRF and INAA res ults and are the first unambiguous boninites to be discovered in the B ritish Isles. The boninites are interbedded with low-Ti tholeiitic lav as with which they share some distinctive geochemical characteristics suggestive of a close petrogenetic relationship. The low-Ti tholeiite lavas have been interpreted as island-are tholeittes but they also res emble back-are basin basalts. The newly discovered boninites confirm a n intra-oseanic environment of eruption; their distinctive features in clude relatively high SiO2, MgO, Cr and Ni but low Al2O3 and HFSE abun dances, U-shaped REE patterns, low Ti/Zr and high Zr/Hf ratios. Bulk g eochemical trends are indicative of low-temperature, seawater-dominate d alteration of the lavas but these alteration conditions apparently h ad little effect on the distribution of critical diagnostic elements s uch as Zr, Ti, Sc, Ta and the mid-heavy rare earths. We suggest that t he Ballantrae boninites and low-Ti tholeiites represent different batc h melts derived from a common, depleted mantle source region variably modified compositionally (i.e., made ''streaky'') by fluids and/or mel ts during slab interaction (subduction metasomatism) A contribution fr om slab-derived pelagic sediments and/or a carbonatite melt is necessa ry to account for the fractionated, non-chondritic Zr/Hf ratios in the boninites. In view of the close compositional similarity of the Balla ntrae lavas to Cenozoic boninite suites, we believe that these interpr etations may have wider application to the petrogenesis of boninites i n general.