Age of granite emplacement in the Norseman region of Western Australia

Ion probe U‐Th‐Pb dating of zircons from the Late Archaean granites of the Norseman region of the southeastern Yilgarn shows the existence of two distinct magmatic episodes. Large regional tonalite and granodiorite plutons were emplaced between 2685 and 2690 Ma, whereas large regional granite, and small tonalite and leucogranite plutons that intrude the greenstones have ages of 2660–2665 Ma. A small body of granite that intrudes the western edge of the greenstones has an inferred emplacement of 2672 ± 7Ma, and contains inherited zircon that is ～2800 Ma. The monzogranite core from a second pluton in a similar structural position also contains ～2800 Ma zircon; this age is similar to published Sm‐Nd and Rb‐Sr whole rock ages for banded gneisses associated with other members of this suite of domal plutons and is interpreted as representing the age of a significant component within the source region for these distinctive rocks.

Available geochemical and isotopic data are interpreted as indicating derivation of both the older granodiorite and younger granite suites through anatexis of pre‐existing crust of broadly andesitic composition, whereas both the domal granites and the small, late tonalite plutons could have been derived by anatexis of heterogeneous material similar to that represented by the banded gneisses.

If regional metamorphism was related to the emplacement of large volumes of felsic magma within the upper crust, as suggested by Binns et al. (1976), then the Norseman area has probably undergone two periods of regional metamorphism of comparable intensity at approximately 2660 and 2685 Ma.