EARLY PALEOZOIC BIOTITE RB-SR DATES IN THE YILGARN CRATON NEAR HARVEY, WESTERN-AUSTRALIA

Biotite from granite and gneiss in the southwestern part of the Yilgar n Craton near Perth has been dated by the Rb-Sr technique at about 430 -500 Ma in a belt at the western edge of the craton. The belt widens s outhward from 30km near Perth to 55km east of Harvey. The eastern boun dary of the belt passes along the east margin of the Saddleback greens tone belt. A transition zone 15-40 km wide separates the belt of young dates from an eastern chronological plateau where biotite dates, main ly 2300-2600 Ma, are marginally younger than regional Rb-Sr whole rock dates which average about 2550 Ma. In contrast to biotite dates, whol e rock dates greater than 2500 Ma persist to the western edge of the c raton. The 430-500 Ma dates probably represent resetting during uplift in the Early Palaeozoic.