Comparative RbSr and UPb zircon geochronology of late- to post-tectonic plutons in the Winnipeg River belt, Northwestern Ontario, Canada

RbSr and UPb zircon geochronological studies of the unmetamorphosed undeformed late- to post-tectonic Archean plutons of the Winnipeg River belt within the northwestern Superior province of the Canadian shield reveal complex isotope systems within rocks that have a simple geologic history. The UPb zircon results suggest that these plutons were emplaced over the short time interval of 2705-2660 Ma. Both inherited zircons and zircons with complex Pb loss histories are found but careful selection of uniform zircon populations in conjunction with techniques designed to concentrate less discordant zircons has minimized this complexity.The RbSr whole-rock isochron dates are slightly (∼ 50 Ma) to significantly (∼ 200 Ma) younger than the crystallization dates inferred from the UPb zircon results. The redistribution of radiogenic 87Sr accompanying deuteric alteration during the subsolidus cooling of these plutons is the most probable cause of this discordance in ages. Rocks with high Sr and Ca and low (< 1.0) 87Rb/86Sr (granodiorites, diorites) are more likely to remain closed systems because newly-formed Ca-rich deuteric minerals can accomodate Sr liberated from high-Rb/Sr mineral phases. Rocks having low Ca and Sr concentrations and high (> 1.0) 87Rb/86Sr are more likely to be open systems because there are fewer Ca-rich deuteric minerals that can accept Sr partitioned into a fluid phase during deuteric alteration. The RbSr isochron dates for such rocks may have no significance in spite of being relatively precise and satisfying statistical requirements for an isochron.