Age relationships in supracrustal sequences of the northern part of the Murchison Terrane, Archaean Yilgarn Craton, Western Australia: a combined field and zircon U-Pb study

Mapping carried out in the northern Murchison Terrane of the Archaean Yilga rn Craton, Western Australia, shows that correlation of units between isola ted greenstone belts is very difficult and an informal stratigraphic subdiv ision is proposed where the greenstone sequences have been divided into a n umber of assemblages. The assemblages may not necessarily be time equivalen t throughout the region. The lower units (Assemblages 13) consist of ultram afic. mafic and intermediate volcanic rocks deposited without significant b reaks in volcanism. Felsic volcanic packages (Assemblage 4) are conformable with underlying units, but are spatially restricted. Discordant units of g raphitic sedimentary rocks are developed along major crustal structures (As semblage 5). SHRIMP and conventional U-Pb study of zircons reveal that fels ic volcanic rocks of Assemblage 4 in the Dalgaranga Greenstone Belt were em placed at 2747 +/- 5 Ma, whereas those in the adjacent Meekatharra - Mt Mag net Greenstone Belt range in age from 2762 +/- 6 to 2716 +/- 4 Ma. The age of emplacement of a differentiated gabbro sill in the Dalgaranga Greenstone Belt at 2719 +/- 6 Ma places a maximum age on major folding in the belt. T he presence of 2.9-3.0 Ga inherited zircons in some of the felsic volcanic rocks indicates contamination with, or reworking of, underlying 3 Ga sialic crust. This distinguishes the Murchison Terrane from the central parts of the Eastern Goldfields terranes to the south, where there is no evidence fo r a 3 Ga imprint in zircons from volcanic or granitic rocks, and also from the Narryer Gneiss Terrane to the north and west, which is composed of olde r gneisses and granitoids. The ca 2.76-2.71 Ga felsic volcanism in the Murc hison Terrane is significantly older than 2.71-2.67 Ga felsic volcanism in the Eastern Goldfields lending support to models advocating assemblage of t he craton by terrane accretion.