Sm-Nd, Pb-Pb and Rb-Sr geochronology of the manfred complex, Mount Narryer, Western Australia

Abstract

The Sm-Nd, Pb-Pb and Rb-Sr systems in Manfred Complex samples ranging from metaleucogabbro to metapyroxenite indicate formation of the complex ∼ 3700 Ma ago and multiple metamorphic disturbances over the succeeding ∼ 2000 Ma. The isotopic responses to metamorphism were largely determined by the accessory mineral contents of the rocks and by country rock influences. Whole-rock data give a five-point Sm-Nd isochron date of 3680±70 Ma, and the six samples with least radiogenic Pb give a Pb-Pb isochron date of 3689±146 Ma. Whole-rock Rb-Sr data reflect the widespread Yilgarn ‘event’ at ∼ 2700 Ma, but some samples show additional Rb-Sr disturbance < 2000 Ma ago. Data for various mineral fractions show substantial internal re-equilibration of Sm-Nd during granulite facies metamorphism at ∼ 3320 Ma, and further disturbance at least as recently as ∼ 3100 Ma. About half of the samples examined show evidence of partial Pb isotopic re-equilibration or U gain at ∼ 2900 Ma, possibly a time of retrograde amphibolite facies metamorphism. A high apparent source μ = 10.2 ± 0.4 from the Pb-Pb whole-rock isochron could indicate assimilation of significant quantities of older (? > 4000 Ma) crustal material. The measured initial ϵ Nd = −0.2 ± 0.7 is consistent with this, but is not definitive. The various dates correspond well to those determined by Kinny et al. (Kinny, P.D., Williams, I.S., Froude, D.O., Ireland, T.R. and Compston, W., 1988. Precambrian Res., 38: 325–341) and Wijbrans (Wijbrans, J.R., 1985, Ph.D. thesis. Australian National University), and complement the geological framework presented by Myers (Myers, J.S., 1988. Precambrian Res., 38: 309–323).