Geology and geochronology of the Saddleback Greenstone Belt in the Archaean Yilgarn Block, southwestern Australia

Abstract

The Saddleback Group is a volcanogenic greenstone sequence developed in the Western Gneiss Terrain of the Archaean Yilgarn Block. It consists of three major units; the Hotham Formation composed mainly of metasediments, the Wells Formation consisting of metamorphosed felsic volcanics and the Marradong Formation which is made up of metabasalts. The sequence has been metamorphosed to greenschist facies and intruded by adamellite. The sequence is poorly exposed, due to an extensive capping of bauxitic laterite which is also host to a major lateritic gold deposit (the Boddington deposit). The Saddleback Greenstone Belt contrasts with the rest of the Western Gneiss Terrain, which is composed mainly of linear, high grade, polyphase metamorphic belts enclosed by complex migmatite and intruded by late‐stage granitoids. Zircon U‐Pb results indicate that felsic volcanism in the Saddleback Greenstone belt extended from ca. 2670 to 2650 Ma ago. This was immediately followed by greenschist facies metamorphism, deformation and granitoid intrusion at ca. 2640 Ma ago. The broad similarity between the age of Saddleback Group magmatism and late greenstone events in the Eastern Goldfields Province argues against models involving eastward tilting of the Yilgarn Block after final greenstone belt formation.