CONSTRAINING THE ALPINE HISTORY OF THE HIGH GRADE GRUF COMPLEX, CENTRAL ALPS, BY CORRELATING DEFORMATION, U-PB ZIRCON GEOCHRONOLOGY AND COMPOSITION OF FELSIC DIKES AND LEUCOSOMES

Correlation of crystallization ages of leucosomes and dikes with deformation phases, metamorphic events and regional scale intrusions can aid reconstruction of complex histories in high-grade metamorphic terranes. The Gruf Complex consists dominantly of migmatitic gneisses and scarce charnockites and is best known for its ultrahigh temperature (UHT) sapphirine granulites. Interpretation of the UHT metamorphism as an Alpine event (Liati & Gebauer, 2003, SMPM; Oalmann et al., this session) has been challenged by Galli et al. (2011, CMP) who interpret it as the result of Permian rifting.

Distinct types of leucosomes and dikes in the Gruf complex are: 1) foliation-defining, commonly folded, bt-bearing leucosomes; 2) medium-grained hbl- and/or bt-bearing granite pods crosscutting earlier leucosomes; 3) bt-bearing pegmatites, boudinaged or crosscutting the main foliation; 4) pegmatitic ms-, grt-, beryll-bearing dikes, crosscutting all other rock types. Changing melt composition during and after regional metamorphism and associated deformation are consistent with the regional change from Bergell tonalite-granodiorite to Novate leucogranite.

U-Pb LA-ICP-MS analyses yield ages of 250–300 Ma for oscillatory-zoned zircons in all samples, interpreted to be inherited magmatic material. A leucosome within a brecciated metaperidotite enclave contains equant, sector-zoned 32.2±0.2 Ma zircons. Most zircons from dike samples have rims with 28–30 Ma ages. An undeformed muscovite-bearing pegmatite crystallized at 25.6±0.3 Ma. Apparent zircon saturation temperatures (T_Zrs) yield a wide range of 680–890°C uncorrelated with age, attributed to undissolved inherited zircon. Another ms-bearing pegmatite lacks zircon domains <28 Ma, due to insufficient Zr in the melt. We interpret its 730°C T_Zrs as formation of magma by partial melting of material similar to the deformed dikes at this T or below.

Conclusions: partial melt crystalliation is coeval with Bergell intrusion and main deformation at 30–32 Ma. Deformation continued until at least 28 Ma. Dike emplacement continued until c. 24 Ma coeval with Novate leucogranite intrusion and granulite cooling below rutile T_c at c. 500°C. 32 Ma zircons in all dikes indicate remelting of older leucosomes and dikes as the mechanism producing the more fractionated pegmatites.