The age and source of late Hercynian magmatism in the central Alps: evidence from precise U?Pb ages and initial Hf isotopes

Abstract

This study presents U−Pb ages for zircon, titanite, allanite and epidote, and initial Hf isotopic compositions for zircon of Upper Carboniferous granites, diorites and syenites from the Aar massif, central Alps. The rocks were emplaced during three magmatic pulses after Hercynian collisional tectonics: (A) a shoshonitic-ultrapotassic series at 334±2.5 Ma; (B) scattered diorites and granites at 308–310 Ma; and (C) a high-K cale-alkaline granite batholith at 298±2 Ma. Inheritance of old zircons is negligible among all three groups. The Southern Aar granite, in contrast, is a syn-tectonic, probably ca. 350 Ma old granite that contains large amounts of inherited Precambrian zircons. Alpine metamorphism caused weak lead loss in many analyzed zircon fractions, but left the titanite U−Pb system undisturbed: thorites were almost completely reset by Alpine and recent lead loss. Mineral isochrons defined by titanite, allanite, epidote and apatite yield initial Pb isotopic compositions that are in agreement with the model values of Stacey and Kramers. Initial Hf isotopic compositions range from ɛHf=−8 to +3.5. The data follow a trend of increasing ɛHf with decreasing age. The ɛHf versus element concentration relationships suggest mixing between a mantle and a crustal component. These relationships can be explained in terms of generation of the melts from a subcontinental mantle that had been enriched during subduction events at about 1 Ga and by 300 Ma had developed an isotopic signature distinct from that of MORB-type mantle. Further contamination of the melts occurred during ascent and differentiation in the crust. This late Hercynian magmatism can be related to post-collisional strike-slip tectonics.