Chromitite petrogenesis in the mantle section of the Ballantrae Ophiolite Complex (Scotland)

Podiform chromitites from the Ballantrae Ophiolite Complex (BOC), NW Scotland, are examined to investigate their petrogenesis and elucidate the nature of melt percolation in the supra-subduction zone oceanic mantle more generally. The mantle portion of the BOC comprises two petrologically distinct serpentinite belts, whose differences have previously been attributed to different degrees of upper mantle melt extraction. Chromitite occurs in each of the northern and southern serpentinite belts, at Pinbain Bridge and Poundland Burn, respectively. Field relationships suggest that chromitites were formed by melt-rock reaction in channel-like conduits in the upper mantle. Chromitite Cr-spinel compositions from the two localities show marked differences to one another, with the Pinbain Bridge chromitite Cr-spinels being characterised by relatively high Cr# [Cr/(Cr+Al); 0.62-0.65] and lower abundances of certain trace elements (e.g., Ti, Ga, V), whereas the Poundland Burn chromitite Cr-spinels exhibit relatively low Cr# (0.44-0.46) and higher concentrations of these trace elements. The contrasting Cr-spinel compositions are used to estimate parental magma compositions for the chromitites; the Pinbain Bridge chromitites crystallised from magmas resembling arc tholeiites whereas MORB-like magmas were involved in formation of the Poundland Burn chromitites. While it is possible that this dichotomy points to early derivation of the BOC at a MORB spreading centre, with subsequent processing in a supra-subduction zone, we suggest that the differences reflect melt extraction from different parts of an evolving subduction zone, such that the MORB-like magmas were generated in a back-arc setting. This interpretation finds support in the Ti/Fe3# versus Ga/Fe3# systematics of peridotite-hosted accessory Cr-spinel that we present here, as well as previously published trace element data and geochronological constraints on the basalt lava sequences associated with the BOC, which collectively favour formation of the Poundland Burn chromitites in supra-subduction zone mantle.