Abstract

A genetic link between the Lauterbrunnen migmatites (LM) of the Innertkirchen-Lauterbrunnen Crystalline Zone (ILC) in the Aar massif and the nearby 303 ± 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 zircon, nearly concordant) and 304.4 ± 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 parental magma into the LM, which would have been too mafic to reflect in situ partial melting of the mid-level crust at geologically reasonable temperatures. Highly discordant U-Pb zircon ages for other LM samples are interpreted to reflect mixing of the intruded magma and country rocks. The new U/Pb age data provide the evidence for a temporal link of the LM to the Gastern granite, which is interpreted to belong to the same Variscan-age magmatic event that formed the LM.

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