TIMING OF PLUTONISM IN THE PROTEROZOIC ALBANY MOBILE BELT, SOUTHWESTERN AUSTRALIA

Zircon U-Pb ages for two adamellites from the Albany Mobile Belt, in the Albany-Fraser Orogen of southwestern Australia, fall within the range 1174 ± 12 Ma and are interpreted as dating post-tectonic granite emplacement in the belt. Slightly younger K-Ar ages on hornblendes from granulite and amphibolite facies metamorphic rocks from the southern part of the belt are interpreted as cooling ages recording uplift associated with the closing stages of orogeny and granite emplacement. A U-Pb age of 1289 ± 10 Ma for zircons from an enderbitic gneiss adjacent to the 1177 ± 12 Ma old Albany Adamellite is interpreted as dating the emplacement of the granodioritic parent of the gneiss, probably during granulite facies metamorphism. This, together with a previously reported Rb-Sr age of ∼ 1300 Ma for the granulite facies metamorphism in the northeast section of the Albany-Fraser Orogen, suggests that granulite facies conditions were widespread in the orogen at ∼ 1300 Ma. Previous Sm-Nd results suggest that the granitoids were derived from ∼ 2000 Ma old crust. So far no older zircon xenocrysts, which could support this model, have been found in the granitoids. © 1990.