Granite genesis and migmatization in the western Aar Massif, Switzerland

摘要

U-Pb zircon geochronology, whole-rock Sr-Nd isotope composition and geochemical data obtained from the Lotschberg base tunnel traverse samples offer new information on ages and origins of Variscan granitoids and the Innertkirchen-Lauterbrunnen crystalline from the western Aar Massif, a major pre-Alpine basement complex. The granitoids intruded during two separate time intervals, in the early and the late Carboniferous. The Baltschieder granodiorite is the only example of the early Carboniferous mag-matic pulse in the western Aar Massif. This primitive, lower-crust derived pluton intruded at ca. 336 Ma. The Gastern granite, the Bietschhorn granite and the Lauterbrunnen migmatites postdate the Baltschieder granodiorite and Variscan peak metamorphism. Both the Lauterbrunnen migmatites and the Gastern granite are the product of mid-crustal anatexis of the "Altkristallin", a pre-Variscan metamorphic complex in the Aar Massif. Crustal melting has been induced by decompression during regional uplift and zirconium saturation suggests a temperature of -830 deg C for this event. The Bietschhorn (or Central Aar) granite is a product of a coeval, lower-crustal melting event triggered by magmatic underplating and appears to be unrelated to the Gastern granite.