A sheeted dyke complex within the Coolac Ophiolite,southeastern New South Wales

A unit composed of sheeted dykes and an associated unit of sodic felsic rocks have been found within the Coolac Ophiolite, about 5 km east of Coolac township, southeastern N.S.W. The dyke complex consists of small multiple dykes intruding gabbro screens. About half the dykes are basaltic in composition and half are ande‐sitic. Felsic differentiates occur as minor intrusions within the dyke complex, and also constitute the sodic felsic unit. It is suggested that the dyke complex and associated felsic rocks be included in the Honeysuckle Beds, and that the absence of a mappable dyke unit further south within the Honeysuckle Beds is caused by tectonic dismembering of the ophiolite. The dyke complex bears considerable resemblance to the sheeted dyke members of other ophiolite sequences. However, the scale of development is roughly 10 times smaller than at Troodos or Newfoundland, and one‐way chilling is not well developed. The mechanism of intrusion in the Coolac dyke complex therefore is probably not symmetrical spreading about a well defined axis, but a diffuse injection of small multiple dykes within a relatively broad extensional zone. The environment in which such a mechanism might develop, and which is suggested by geochemical evidence, including the high proportion of intermediate and felsic rocks, is that of a small basin adjacent to or within an island‐arc system.