Rb–Sr whole-rock age of the Sudbury lopolith and basin sediments

Abstract

The concordant norite, granophyre ("micro-pegmatite"), rhyodacite tuff, slate, and graywacke ("sandstone") that form the rimmed elliptical basin at Sudbury, Ontario, are unique in the Canadian Shield and exhibit several features characteristic of lopoliths. A previous Rb–Sr whole-rock isochron for the norite-granophyre intrusive pair gave 1704 ± 19 m.y., with initial 87Sr/86Sr of 0.7052 ± 0.0003. New work presented here concerns the sediments and tuff. An isochron for the slate–graywacke sedimentary sequence in the center of the basin gives 1720 ± 30 m.y., with initial 87Sr/86Sr of 0.7062 ± 0.0018. A definitive age for the underlying tuff has not been obtained owing to scatter and small Rb/Sr range; however, its concordance and chemical relationships with both sediments and norite–granophyre point to an equivalent age. The combined isotopic data for tuff and sediments give 1684 ± 43 m.y., with initial 87Sr/86Sr of 0.7092 ± 0.0015, in reasonable agreement with the norite–granophyre age. The decay constant λRb = 1.39 × 10−11 yr−1 has been used throughout.The investigation thus far supports consanguinity of the entire complex, with geologically rapid development about 1700 m.y. ago. It imposes no barrier due to age differences on either conventional models of lopolith generation or on generation initiated by meteoritic impact. It raises a problem concerning initial 87Sr/86Sr if impact is assumed. It rules out proposals to correlate the tuff–sediment sequence either with Keweenawan rocks (about 1000 m.y.) or with older volcanic sequences (2000 ± m.y.) outside the loplith.