The local chronology and regional implications of a Rb‐Sr investigation of granitic rocks from the Corryong District, southeastern Australia

Abstract

A Rb‐Sr analysis of granitic rocks in the Corryong District reveals that the regional aureole granites of that area (Corryong Batholith) are 430 ± 8 m.y. old. This age is some 15 m.y. older than the regional aureole Cooma Granite and if both granites are the ultimate products of the metamorphism, then the locus of this metamorphism progressed eastwards. Unlike other continents (e.g. S. America) the migratory nature of this trend of metamorphism and granite emplacement is not readily explainable in terms of possible Palaeozoic plate‐tectonic mechanisms.

The initial Sr⁸⁷/Sr⁸⁶ ratio for the Corryong Batholith is 0.716 ± 0.001. This value is similar to the value for the Cooma Granite and is compatible with the generation of the extensive Corryong Batholith by partial or complete anatexis of Ordovician‐type metasediments. The Rb‐Sr ages do not allow us to preclude the possibility that the Ordovician‐Silurian Benambran Orogeny was responsible for this anatexis. Indeed they could be interpreted to favour this possibility.

Other high‐level igneous rocks dated include two leucogranites (Pine Mountain Granite, 413 ± 12 m.y.; Mt Mittamatite Granite 427 ± 32 m.y.) and the extrusive Jemba Rhyolite (409 ± 8 m.y.). The initial Sr⁸⁷/Sr⁸⁶ ratio of the latter (0.713 ± 0.002) reflects possible sediment contamination effects, while the value for the Mt Mittamatite Granite (0.704 ± 0.003) is suggestive of sub‐geosynclinal derivation.