ZIRCONOLOGY OF THE MEEBERRIE GNEISS, YILGARN CRATON, WESTERN-AUSTRALIA - AN EARLY ARCHEAN MIGMATITE

The Meeberrie gneiss, a major component of the early Archaean Narryer Gneiss Complex of Western Australia, is a polyphase migmatite, compris ing monzogranites together with minor tonalitic and trondhjemitic comp onents, ranging in age from 3730 to 3300 Ma, with important resolvable age components at similar to 3670, 3620 and 3600 Ma. Ion-probe U-Pb s tudies of zircons from numerous localities show that almost all hand s pecimens 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 i nherited cores but rather in separate zircon grains, usually indisting uishable from the younger grains in terms of morphology, internal stru cture and trace-element composition. Only in rare low-strain zones are original cross-cutting relationships between individual magmatic comp onents and pegmatite segregations preserved. For the remainder, late A rchaean deformation has blended units and erased pre-existing structur es. However, deformation state appears to have little bearing on the z ircon isotopic systematics. Geochemical data for such rocks are thus c omposite, as are Nd isotopic compositions which nonetheless show that the younger generations of material in the gneisses are produced predo minantly by reworking of older crust.