Isotopic ages of zircon from granites and pegmatites

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Abstract
Isotopic lead age determinations have been made on 13 zircons obtained from rocks 185 to 1400 million years old. Concordant or nearly concordant ages are found for all of the samples which contain no detectable common lead, and discordant ages are found for most, if not all, of the samples which contain common lead. A comparison is made between the concordant isotopic age patterns given by three zircons from the Grenville subprovince in Ontario and the discordant patterns given by three zircons from the Cordilleran region of western United States. This indicates that the discordant ages can be related to the recent orogenies which occurred in the Cordilleran region. The Grenville is a stable shield area. There is no relation between the agreement of the isotopic ages of zircon and crystal size, amount of radiation damage or optical appearance—that is, zoning, cloudiness, or inclusions.When a discordant age result is compared with the potassium‐argon and rubidium‐strontium ages of associated mica the Pb207‐Pb206 age is found to be the closest to the mica age. Isotopic ages are compared with the simpler alpha‐lead and chemical lead ages, which do not require isotopic analysis of lead. The non‐isotopic ages are approximately correct for zircons which have concordant isotopic ages but are in error when discordant isotopic ages are found. No explanation is offered as to why the mica ages are apparently unaffected by the process or processes which altered the zircon ages. An understanding of this phenomenon would doubtless provide valuable information concerning the post‐crystallization history of the samples.

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