THE AGE OF THE ORMISTON POUND GRANITE - IMPLICATIONS FOR MESOPROTEROZOIC EVOLUTION OF THE ARUNTA-INLIER, CENTRAL AUSTRALIA

The Ormiston Pound Granite, southern Arunta Inlier, intruded during the terminal stages (late- to post-D-2) of a major, N-directed overthrust event called the Chewings Orogeny, which deformed supracrustal rocks of the Simpsons and Chewings assemblages. Ion probe zircon U-Pb analysis of the granite indicates an emplacement age of 1603 +/- 10 Ma, more than 500 Ma older than the Rb-Sr isochron age. Timing of emplacement is confirmed by an age of 1575 +/- 20 Ma for zircon mantles from a gneissic xenolith in the granite. Zircon cores from the xenolith yield an age of 1678 +/- 14 Ma, which is taken to represent the protolith age of the surrounding gneisses. The SHRIMP age is similar to, but more precise than the 1586 +/- 69 Ma Rb-Sr isochron age for the Chewings Orogeny, determined by Marjoribanks and Black (1975). Penecontemporaneous similar to 1600 Ma magmatism is evident as pegmatites and associated fluid flow zones in the Anmatjira and Reynolds ranges, northern Arunta Inlier. 1100-1030 Ma Rb-Sr mica ages from the similar to 1600 Ma Ormiston Pound Granite, coeval pegmatites and similar to 1680 Ma host-rock gneisses are similar to a Sm-Nd isochron age for the mafic Stuart Dyke Swarm of the southern Arunta Inlier, and the contemporaneous Alcarra Dyke Swarm in the Musgrave Inlier, some 500 km farther south. The Rb-Sr mica ages reflect isotopic resetting by a thermal perturbation, either associated with regional crustal extension during similar to 1080 Ma dyke intrusion or with the similar to 1150 Ma magmatic event in the southern Arunta (cf. Black and Shaw, 1992). Irrespective, the 1050-900 Ma Ormiston Event, described by Shaw et al. (1984) as granite intrusion associated with high-grade metamorphism, migmatization and folding, needs to be re-defined or abandoned.