GNEISS DOMES AND EXTENSIONAL DEFORMATION IN THE HIGHLY MINERALIZED ARCHEAN-EASTERN-GOLDFIELDS PROVINCE, WESTERN-AUSTRALIA

The recognition of large areas of gneiss within granitic complexes of the northeastern part of the Eastern Goldfields Province of Western Australia, based on analysis of regional aeromagnetic data, has resulted in a re-evaluation of the structural data for that region. Five magnetic domains have been established in the granitic complexes, which are marginal to the main greenstone belts. At least two of these domains have a distinctive domal form, and also have internal dome-shaped regions. The Ballard and Laverton domains have extensive areas of layered gneissic and migmatitic rocks which flank the internal domes. Boundary relationships at the Laverton and Ballard domains show that the domes are concordant sheets, and the metamorphic conditions of the boundary rocks imply that the sheets were intruded at a middle crustal level. Strain conditions in the boundary zones indicate that emplacement of the gneiss domains to the crustal level of the surrounding lower-grade greenstones was initially along relatively gently-dipping shear zones which were active during uplift. Structural relationships around both the domal granitic complexes and the later granite batholiths, however, show that regional shortening deformation events were not caused directly by the emplacement of any of the granitic rocks. The tectonic hypothesis presented for the Eastern Goldfields is similar to that proposed for the formation of Cordilleran metamorphic core complexes. The uplift event responsible for the crustal extension required to allow the emplacement of the gneiss, may have been driven by the same thermal input responsible for the generation of the voluminous granites intrusive into both the gneiss complexes and the surrounding greenstone belts.