Crustal Evolution of the Lofoten Vesteralen Area in Norway Through Solution Hf Isotope, Coupled U-Pb and Hf Isotopic Analysis and Garnet Geochronology.

Understanding the evolution of continental crust, including its nature, timing, and rate of crustal growth, gives important insight into mechanisms of mantle differentiation and the nature of Earths evolving crust. The Lu-Hf isotopic system is a reliable tool to constrain the composition, differentiation, and evolution of the crust-mantle system. In this study we present new Lu-Hf data from Archean-Proterozoic rocks exposed in the Lofoten Vesteralen area of Norway. The samples are subdivided into three groups: 1) High-grade metamorphic rocks from the granulite facies zone (GFZ); 2) Medium-grade metamorphic rocks from the amphibolite facies zone (AFZ); and 3) Intrusive rocks from the Anorthosite-Mangerite-Charnockite-Granite suite (AMCG). Solution Hf isotope data obtained for zircons previously dated by Isotope Dilution Thermal Ionization Mass Spectrometry (ID-TIMS) [1,2] are combined with U-Pb and Hf isotopic data from zircon analyzed by laser ablation split-stream (LASS). Garnet Lu-Hf and Sm-Nd ages from the GFZ are also reported. LASS data from the GFZ yield two populations: Neoarchean (2608 to 2618 Ma) with Hf(i) from -1.6 to -3.0 and Paleoproterozoic (1802 and 1796 Ma) with Hf(i) from -13.8 to -15.7. Rocks within the AFZ yield Neoarchean ages between 2690 and 2615 Ma with Hf(i) from +0.3 to -2.0. Zircon U-Pb ages from the AMCG suite vary between 1870 and 1750 Ma with Hf(i) from -2.7 to -13.8. In general, there is a good agreement between solution and in-situ data, but some solution analysis from GFZ samples have mixed compositions due to analysis of zircon with both components. This underscores the importance of using in-situ analysis of complex multi-component grains. Rocks within the GFZ yield garnet Lu-Hf and Sm-Nd ages of 1879 and 1791 Ma, respectively. This Paleoproterozoic metamorphism is not recorded by rocks within the AFZ, supporting the hypothesis of a tectonic boundary between those terranes [2]. The Hf isotope composition of the Paleoproterozoic magmatism suggests incorporation of both recycled ancient crust and juvenile material. Chondritic to sub-chondritic Hf isotope compositions from Neoarchean rocks suggest no significant presence of pre-Neoarchean crust within the source regions of these rocks. [1] Corfu, F (2004) J Pet, 45, 17991819. [2] Corfu, F (2007) Chem Geol, 241, 108128.