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

Abstract

The Monte Capanne granodiorite-rhyolite-aplite association exhibits K-Ar ages which cluster around 7.9 m.y. Rb-Sr mineral ages and U-Pb dating on zircons, however, give 6.2 m.y. which is the preferred age of emplacement. Evidence of subsequent events related to either dyke emplacement, or conductive/convective cooling is provided by ages as young as 5.4 m.y. A significant heterogeneity of the whole-rock ⁸⁷Sr/⁸⁶Sr initial ratios, which fall in the range 0.711–0.715, is interpreted as being due in part to exchanges with the low ⁸⁷Sr⁸⁶Sr country rocks at the deuteric stage via fluid interaction, and in part to possible different magma sources. In the geological future, these heterogeneities should be progressively smeared out by the radioactive decay of ⁸⁷Rb, but even if the system remains perfectly closed, a residual bias in excess of 100 m.y. should permanently affect the theoretical isochron ages. It is inferred that Rb-Sr isochron ages determined on samples of Precambrian age must be regarded with caution: a residual bias may somewhat alter the information obtained even from statistically acceptable isochrons. In this respect, when great accuracy is needed for plutonic emplacement ages (to within few m.y.), U-Pb dating appears to be less sensitive to inherited components and provides more reliable chronometric information.