Zirconology of the Meeberrie gneiss, Yilgarn Craton, Western Australia: an early Archaean migmatite

Abstract

The Meeberrie gneiss, a major component of the early Archaean Narryer Gneiss Complex of Western Australia, is a polyphase migmatite, comprising monzogranites together with minor tonalitic and trondhjemitic components, ranging in age from 3730 to 3300 Ma, with important resolvable age components at ∼ 3670, 3620 and 3600 Ma. Ion-probe U-Pb studies of zircons from numerous localities show that almost all hand specimens of the gneiss consist of multiple generations of magmatically crystallized zircons, even down to the scale of individual centimetre-sized bands. The earliest age components generally are not found in inherited cores but rather in separate zircon grains, usually indistinguishable from the younger grains in terms of morphology, internal structure and trace-element composition. Only in rare low-strain zones are original cross-cutting relationships between individual magmatic components and pegmatite segregations preserved. For the remainder, late Archaean deformation has blended units and erased pre-existing structures. However, deformation state appears to have little bearing on the zircon isotopic systematics. Geochemical data for such rocks are thus composite, as are Nd isotopic compositions which nonetheless show that the younger generations of material in the gneisses are produced predominantly by reworking of older crust.