Abstract

Chromium-spinels have been widely used as petrogenetic indicators to infer parent melt compositions and the tectonic setting of their formation. This study integrates petrographic, quantitative textural and geochemical analyses of Cr-spinel seams within the Dawros Peridotite, NW Connemara, Ireland to determine the composition of their parental magmas. Calculation of Cr no. (Cr/(Cr + Al)) (0.50–0.77) values and TiO2 (0.18–0.36 wt%) contents of the Cr-spinel seams, coupled with an estimation of the Al2O3 and TiO2 contents (~11.86 wt% and ~0.39 wt%, respectively) of their parental melts, indicates that they probably formed from boninitic melts sourced from a highly depleted mantle. This implies that the Cr-spinel seams formed in a supra-subduction zone undergoing high degrees of partial melting. The Cr-spinel data support tectonic models for the formation of the Dawros Peridotite (and Connemara Metagabbro-Gneiss Complex) during island arc collision, immediately prior to Grampian orogenesis at ~470 Ma. The occurrence of the Dawros chromitite seams at the approximate transition between the lower harzburgite sequence and the upper lherzolite sequence bears marked similarities to the positions of such seams in larger anorogenic layered mafic-ultramafic intrusions, and implies that the Dawros Peridotite behaved as an open-system magma chamber.

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