BIOTITE RB-SR AGE EVIDENCE FOR EARLY PALEOZOIC TECTONISM ALONG THE CRATONIC MARGIN IN SOUTHWESTERN AUSTRALIA

Most whole-rock Rb-Sr dates on plutonic rocks of the Yilgarn Craton of Western Australia cluster closely around 2650 Ma. Biotite Rb-Sr dates tend to be 150 to 450 million years younger than the whole-rock dates except at the western margin of the craton where biotite dates are su bstantially younger. West of a line from Mundaring, east of Perth, to the south coast between Denmark and Walpole, biotite Rb-Sr dates decre ase across a transition zone 20-40 km wide to stabilise at the western edge of the craton at about 430 Ma, defining a western domain of rela tively young biotite dates. The domain of young dates continues across the southern margin of the craton into the Mesoproterozoic Albany Mob ile Belt, where rocks emplaced or strongly metamorphosed between 1200 Ma and 1100 Ma have also been reset to give biotite Rb-Sr dates of abo ut 430 Ma, confirming that the relative positions of the Yilgarn Crato n and Albany Mobile Belt were substantially fixed by Early Palaeozoic time. The domain of reset biotite continues to the south coast of West ern Australia and can be correlated with biotite with similar dates on the Antarctic continent. The eastern limit of reset biotite dates on the south coast of Western Australia can be correlated with a similar eastward limit of reset dates in Antarctica thus allowing refinement o f the fit between the two continents. The resetting of biotite dates i n the western biotite domain may be attributable to heating as a conse quence of tectonic loading by thrusting during the Late Neoproterozoic or Early Phanerozoic.