CHARACTER AND REGIONAL IMPLICATIONS OF THE SHEARED ARCHEAN GRANITE-GREENSTONE CONTACT NEAR LEONORA, WESTERN-AUSTRALIA

The boundary between greenstone belt rocks and deformed domal bodies o f granite and gneissic granite in the area west and northwest of Leono ra is marked by a wide zone of high strain within which the dominant f abric elements are parallel to the boundary. The high-strain zone is a rcuate; the granitic body is domal in form. Consistent sense-of-shear movement indicators show that the greenstones moved down relative to t he granite. Rocks from a continuous core of 1.6 km, drilled 4 km south east of Leonora, exhibit mesoscale recumbent folding and ductile fault ing. The upper 900 m of core is characterised by recumbent flexural sl ip folds with half wavelengths up to 660 m separated by discrete shear zones. Below about 1200 m, rock type and structural character change. At this depth greenschist-facies rocks are juxtaposed against amphibo lite-facies rocks across a major ductile shear. Thermodynamic data giv e estimates of 370-degrees-C/2.1 kbar for greenschist and 610-degrees- C/5.6 kbar for amphibolite facies, with high fluid pressures dominated by water in low-grade rocks and CO2 in high-grade ones. Several kilom etres of section have therefore been lost across this zone. The amphib olite hosts broad and narrow shear zones within which movement sense i s consistently normal. The structural and metamorphic data suggest tha t emplacement of the domal granitic and gneissic rocks was temporally linked with normal ductile shearing. The regional structural setting a nd juxtaposition of high- and low-grade rocks across the shear zone wi th abrupt changes in temperature and fluid composition suggests that t he domes were emplaced as uprising hot, deep-seated rock in a regional extensional tectonic environment; comparable to the tectonic setting of metamorphic core complexes.