New U-Pb zircon data and constraints on the age and mode of migmatization in the Aar massif, Central Alps

A genetic link between the Lauterbrunnen migmatites (LM) of the Innertkirch en-lauterbrunnen Crystalline Zone (ILC) in the Aar massif and the nearby 30 3 +/- 4-Ma Gastern granite is indicated by chemical similarities as well as spatial proximity. The new U-Pb zircon ages of 289 +/- 9 Ma (igneous zirco n, nearly concordant) and 303.4 +/- 1.2 Ma (metamorphic zircon, concordant) for the LM in this study indicate that the LM and Gastern granite are also coeval. The relatively mafic composition of the LM sample that yielded the nearly concordant U/Pb ages is interpreted to reflect intrusion of a paren tal magma into the LM, which would have been too mafic to reflect in situ p artial melting of the mid-level crust at geologically reasonable temperatur es. Highly discordant U-Pb zircon ages for other LM samples are interpreted to reflect mixing of the intruded ma,nma and country rocks. The new U/Pb a ge data provide the evidence for a temporal link of the LM to the Gastern g ranite, which is interpreted to belong to thr same Variscan-age magmatic ev ent that formed the LM.