SmNd geochronology of greenstone belts in the Yilgarn Block, Western Australia

Abstract

SmNd geochronology has been used to date 3 widely spaced metavolcanic (greenstone) belts in the Yilgarn Block. The measured isochron ages are 2.78 ± 0.03 Ga (initial ϵ Nd = 2.5 ± 0.3) for greenstones at Kanowna in the Eastern Goldfields Province, 3.05 ± 0.10 Ga (initial ϵ Nd = 0.9 ± 0.7) for Diemals-Marda in the Southern Cross Province and 2.98 ± 0.12 Ga (initial ϵ Nd = 0.7 ± 1.2) for the Warriedar fold belt in the Murchison Province. These data show that substantial crustal components of age ∼3.0 Ga exist in the Southern Cross and Murchison Provinces, and support published evidence that the Eastern Goldfields Province contains no crustal material ⩾ 2.8 Ga. These ages are all significantly younger than ages commonly observed in the Western Gneiss Terrain and so support the suggestion of a progressive crustal age trend across the Yilgarn Block. The measured age values depend strongly on the inclusion of data for felsic components of the greenstone belts; data for mafic and ultramafic units allow that eruption in the 3 localities could have been contemporaneous at ∼ 2.8 Ga. The age range of metavolcanic rocks within each province is possibly comparable to the maximum possible age difference between study areas (∼ 0.2 Ga), so the documented lithological variations between the greenstone successions of the 3 provinces cannot be attributed to depositional age differences. Leaching experiments on altered Warriedar samples indicate that both alteration and isotopic homogenisation occurred early in the evolution of the system, probably prior to the ∼ 2.6 Ga regional metamorphic event. The initial ϵ Nd values do not conform to previously suggested mantle depletion curves. When considered with published data for greenstone belts they suggest complex mantle heterogeneity, with e Nd diverging over time to both positive and negative values.