Stratigraphic and structural synthesis of the New England Orogen

The Spring Well volcanic complex in the Eastern Goldfields Province of Western Australia, is a relatively fresh and well exposed Archaean felsic volcanic centre that is preserved in a synclinal structure at the top of the local greenstone succession. Subaerial acid pyroclastic deposits and subordinate lava flows, intruded by anastomosing intermediate‐acid dykes and sills, comprise the near‐vent facies. In the distal regions of the centre, subaqueous crystal tuff and other tuff units are intercalated with epiclastic sediments.

Geochemical modelling indicates that the acid rocks are unlikely to have been derived by batch partial melting of probable crustal sources. However, differentiation from intermediate parents is compatible with the available geochemical data. The intermediate rocks, in turn, have critical geochemical characteristics comparable with all other studied intermediate calc‐alkaline rocks in the Yilgarn Block. Since it can be demonstrated that many of these rocks have an ultimate mantle source (through differentiation of LIL element enriched mafic primary magmas) it follows that such an origin is applicable in the Spring Well rocks. Therefore, it is concluded that the Spring Well volcanic complex represents a mantle‐derived, calc‐alkaline differentiation series, in which the more silicic members of the suite predominate. Apart from the diagnostic geochemical characteristics of these acid volcanic rocks, their spatial association with intermediate rocks distinguishes them from anatectic acid volcanic rocks that also occur in the greenstone sequences of the Yilgarn Block.