Isotopic Heterogeneities in the Granitic Intrusion of Monte Capanne (Elba Island, Italy) and Dating Concepts

The Monte Capanne granodiorite-rhyolite-aplite association exhibitsK-Ar ages which cluster around 7.9 m.y. Rb-Sr mineral ages andU-Pb dating on zircons, however, give 6.2 m.y. which is thepreferred age of emplacement. Evidence of subsequent eventsrelated to either dyke emplacement, or conductive/convectivecooling is provided by ages as young as 5.4 m.y. A significantheterogeneity of the whole-rock ⁸⁷Sr/⁸⁶Sr initial ratios, whichfall in the range 0.711–0.715, is interpreted as beingdue in part to exchanges with the low ⁸⁷Sr⁸⁶Sr country rocksat the deuteric stage via fluid interaction, and in part topossible different magma sources. In the geological future,these heterogeneities should be progressively smeared out bythe radioactive decay of ⁸⁷Rb, but even if the system remainsperfectly closed, a residual bias in excess of 100 m.y. shouldpermanently affect the theoretical isochron ages. It is inferredthat Rb-Sr isochron ages determined on samples of Precambrianage must be regarded with caution: a residual bias may somewhatalter the information obtained even from statistically acceptableisochrons. In this respect, when great accuracy is needed forplutonic emplacement ages (to within few m.y.), U-Pb datingappears to be less sensitive to inherited components and providesmore reliable chronometric information.