DISTINCT BASEMENT TERRANES IDENTIFIED FROM GRANITE GEOCHEMISTRY IN LATE ARCHEAN GRANITE-GREENSTONES, YILGARN-CRATON, WESTERN-AUSTRALIA

Hidden basement terranes have been identified by a geochemical study o f granites that intrude deformed greenstones in the Eastern Goldfields of the Yilgarn Craton. The granites are calc-alkaline rocks with a hi gh average silica content (72.5 wt%), lie mainly in the compositional range of granodiorite to monzogranite, and are derived from melting of pre-existing sialic crust. The greenstones comprise several tectono-s tratigraphic terranes that were essentially assembled before intrusion of the granites. The distribution of granite types generally shows no spatial relationship to these terranes or to terrane boundary faults. The Emu Fault, which separates the Kurnalpi Terrane to the east, from the Kalgoorlie and Gindalbie terranes to the west, provides an except ion. Granites to the east of this fault have lower TiO2, FeO2 and MgO, and generally higher Na2O, compared to granites to the west. These co ntrasting compositions result from differences within the source regio ns and are interpreted to reflect two distinct basement terranes. The basement terrane beneath the Kurnalpi region had a higher proportion o f tonalite than was present beneath the Kalgoorlie and Gindalbie terra nes. We suggest that a regional tonalite-trondhjemite-granodiorite bas ement was accreted prior to complete accumulation of the greenstones a nd was later recycled, to form the granites. The Emu Fault is inferred to reflect a suture in the underlying basement of the greenstones. (C ) 1997 Elsevier Science B.V.