Rubidium-strontium geochronology of the Encounter Bay granites and adjacent metamorphic rocks, South Australia

Abstract

Rubidium-strontium and strontium isotope data for eight whole-rock samples of granite varieties from the Encounter Bay area, South Australia, yield an isochron age of 487 ± 37 m.y. Two specimens of albitised granite, formed as a result of late­ stage metasomatic alteration of original megacrystic granite, conform to this isochron. These data support a genetic relation between granites and late-stage metasomatic alteration as suspected from field, petrographical and geochemical studies. Eight samples from contiguous Kanmantoo Group metasedimentary rocks have an isochron age of 487 ± 60 m.y. Thus this metamorphic event is coincident with emplacement of the Encounter Bay Granite. The initial Sr87/Sr86 ratio for the Encounter Bay Granite (0.719) is significantly higher than initial ratios for the Palmer (0.709) and Anabama (0.705) granites from the same region and can be attributed to either remobilisation or incorporation of strontium from older crustal material in the intrusion. The apparent initial Sr87/Sr86 ratio for the Kanmantoo Group metasedimentary rocks (0.722) can not be distinguished from that for the Encounter Bay Granite within the analytical uncertainties. Compatability of ages and high initial Sr87/Sr86 ratios suggest that the granites formed by remobilisation of associated crustal rock.