Plagiogranites in the Coolac ophiolite suite, New South Wales, Australia

Abstract

SummaryPlagiogranites occur mainly as intrusive bodies and tectonic inclusions within mafic and ultramafic members of the Coolac ophiolite suite. Quartz diorites and trondhjemites appear to represent localized late-stage felsic differentiates of subalkaline tholeiitic magma, with their formation dominantly controlled by fractionation of hornblende and plagioclase. Albitites are interpreted as altered equivalents of quartz diorites and trondhjemites. Chemical data, while emphasizing the similarity of these rocks to ‘oceanic plagiogranite', also indicate that variation in the latter may be broader than previously defined. The Coolac rocks show less marked Fe-enrichment, higher Ni and Cr, and lower Y and Zr than many ophiolitic plagiogranites. These data are consistent with weak calc-alkaline tendencies demonstrated in the Coolac ophiolite suite.