Precambrian Research, 26 (1984) 333--361 333 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands Sm-- Nd GEOCHRONOLOGY OF GREENSTONE BELTS IN THE YILGARN BLOCK, WESTERN AUSTRALIA I.R. FLETCHER and K.J.R. ROSMAN School of Physics and Geosciences, Western Australian Institute of Technology, Kent Street, Bentley, 6102, Western Australia (Australia) I.R. WILLIAMS, A.H. HICKMAN and J.L. BAXTER* Geological Survey of Western Australia, 66 Adelaide Terrace, Perth, 6000, Western Australia (Australia) (Received September 22, 1983; revision accepted May 21, 1984) ABSTRACT Fletcher, I.R., Rosman, K.J.R., Williams, I.R., Hickman, A.H. and Baxter, J.L., 1984. Sm--Nd geochronology of greenstone belts in the Yilgarn Block, Western Australia. Precambrian Res., 26: 333--361. Sm--Nd geochronology has been used to date 3 widely spaced metavolcanic (green- stone) belts in the Yilgarn Block. The measured isochron ages are 2.78 -+ 0.03 Ga (initial end = 2.5 + 0.3) for greenstones at Kanowna in the Eastern Goldfields Province, 3.05 -+ 0.10 Ga (initial end = 0.9 -+ 0 . 7 ) for Diemals--Marda in the Southern Cross Province and 2.98 -+ 0.12 Ga (initial end = 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 sig- nificantly 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 compo- nents 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 metavol- canic rocks within each province is possibly comparable to the maximum possible age dif- ference between study areas (- 0.2 Ga), so the documented lithological variations be- tween 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 end values do not conform to previously suggested mantle depletion curves. When considered with published data for greenstone belts they suggest complex mantle heterogeneity, with end diverging over time to both positive and negative values. *Present address: School of Physics and Geoseiences, Western Australian Institute of Technology, Kent Street, Bentley, 6102, Western Australia, Australia. 0301-9268/84/$03.00 © 1984 Elsevier Science Publishers B.V.