Whole-Rock Rb-Sr Age of Norite and Micropegmatite at Sudbury, Ontario

Rb-Sr isotopic analysis of six whole-rock samples from the norite-micropegamtite intrusion at Sudbury, Ontario, indicates the micropegmatite age to be 1,630 ± 55 m.y. for λRb = 1.47 × 10⁻¹¹yr⁻¹. The norite age is less certain and may range from contemporaneous with, to about 100 m.y. older than, the micropegmatite. The age of the hybrid transition zone between the norite and micropegmatite is indistinguishable, within limits of experimental error, from the micropegmatite. The initial ratio Sr⁸⁷/Sr⁸⁶ was found to be comparable with that for basalt. It is concluded that the source of the norite and micropegmatite is subsialic, and that emplacement within the crust did not involve any significant contamination by ancient crustal rocks rich in radiogenic Sr⁸⁷. Comparison of the whole-rock ages with seven mineral ages in the norite and micropegmatite shows that, although the latter are discordant, there is an over-all age pattern of low biotite (1,200-1,575 m.y.) and higher K-feldspar (1,480-1,575 m.y.). This is interpreted for the biotite as evidence of postmagmatic redistribution of radiogenic Sr; the host rocks, as shown by an isochron plot, remaining essentially closed systems. The scatter of the K-feldspar samples may derive in part from loss by redistribution of radiogenic Sr, in part from additions to plagioclase intergrown or included with the K-feldspar.