LARGE-SCALE CRUSTAL STRUCTURE OF THE NORTHWESTERN YILGARN-CRATON, WESTERN-AUSTRALIA - EVIDENCE FROM ND ISOTOPIC DATA AND ZIRCON GEOCHRONOLOGY

U-Pb zircon geochronology and Nd isotope geochemistry have been used in the northwestern Yilgarn Craton region of Western Australia to map gneiss units of different ages and to Provide a model for late Archean crustal evolution, despite the inherent difficulties of minimal exposure and variations in the appearance of units brought about by heterogeneous strain. The 3300-3730 Ma gneisses and intercalated metasedimentary units crop out in a less-than-or-equal-to 100 lan wide tract, bounded on both sides by areas containing 2920-3000 Ma gneisses. Between 2750 and 2620 Ma, several generations of granitoids were emplaced throughout the region. Regardless of whether the granitoids intruded the 3300-3730 Ma or 2920-3000 Ma gneisses, modeling of their initial Nd isotopic compositions shows that they were most likely formed by partial melting of predominantly 2920-3000 Ma gneisses; only a minor input from early Archean sources is indicated for granitoids cutting the 3300-3730 Ma gneisses and intercalated metasediments. Some of the granitoids that intrude the 3300-3730 Ma gneisses contain 2920 Ma inherited zircons, supporting the Nd evidence that their source is dominated by 2920-3000 Ma gneisses. The 3300-3730 Ma gneisses are interpreted as belonging to an allochthonous terrain emplaced over a younger terrain comprising the 2920-3000 Ma gneisses. Subsequently, partial melting concentrated in the underlying 2920-3000 Ma terrain gave rise to the late Archean granitoids.