Evolution of the Archaean granite-greenstone terranes of the Eastern Goldfields, Western Australia: SHRIMP UPb zircon constraints

Abstract

Thirty-five high-precision SHRIMP UPb zircon dates (with an average 95% confidence error of ±6 Ma) have been obtained from the granite-greenstone terranes in the southern part of the Eastern Goldfields of Western Australia. These dates establish the timing of major deformation, metamorphism and granite intrusion events, and test inferred stratigraphic relationships within the greenstones. These new data reveal that felsic volcanic rocks were erupted throughout the southern part of the Eastern Goldfields between c. 2713 and c. 2672 Ma. Analytically indistinguishable dates of c. 2705 Ma were obtained for felsic volcanic rocks interbedded with komatiitic lavas at three sites. Dates on felsic volcanic rocks within tectonostratigraphic packages that include komatiites are also consistent with an inferred eruption age for these komatiites of c. 2705 Ma. Komatiitic lavas in the southern part of the Eastern Goldfields were probably emplaced during a single voluminous eruption that resulted in the flooding of a vast area of the late Archaean landscape, filling the topographic lows around older felsic volcanic vents. The stratigraphic complexity within tectonostratigraphic domains is attributed to the existence of an undulating topography during the eruption of the mafic and ultramafic lavas, and to the rapid and localized deposition of felsic volcanic rocks close to a number of isolated volcanic centres prior to, and after, the komatiite lava eruption event. Between c. 2685 and c. 2675 Ma, early (pre-D2) intrusive equivalents of the felsic volcanic rocks were emplaced as thick granite sheets into the base of the greenstone sequences. Regional east-west compression between c. 2675 and c. 2657 Ma reactivated early extensional structures and resulted in the stacking of the greenstones. This was accompanied by further intrusion of granite within developing antiforms. Burial of greenstone slices by stacking, and the effect of hydrothermal fluids emanating from both the granites and buried greenstones, resulted in regional metamorphism to greenschist facies prior to c. 2640 Ma. Monzogranite and granodiorite emplaced near the western, southern and eastern margins of the greenstones were deformed and metamorphosed to upper amphibolite facies, and converted into gneisses during D2 and D3 compression events. Movement along large-scale transcurrent D3 shear zones had ceased by c. 2635 Ma. Remnants of a c. 2958 to c. 2930 Ma greenstone sequence preserved near Norseman and Ravensthorpe in the southeastern Yilgarn Craton, and xenocryst zircons ranging in age from c. 2910-2730 Ma in the c. 2713-2672 Ma felsic volcanic rocks, indicate that the exposed greenstone sequences were deposited on continental basement. The Eastern Goldfields granite-greenstone terranes were formed by the closure of a series of narrow back-arc rift basins that developed along a continental margin and above an active subduction zone located to the east of the rift basins.