Multistage metamorphic evolution and nature of the amphibolite-granulite facies transition in Lofoten-Vesterålen, Norway, revealed by U-Pb in accessory minerals

The nature of the amphibolite- to granulite-facies transition zone exposed in the Lofoten-Vesterålen islands of northern Norway, and the relationships between high-grade metamorphism and the genesis of an anorthosite, mangerite, charnockite and granite suite (AMCG) are studied with the help of ID-TIMS U-Pb dating of zircon, monazite and titanite in polyorogenic gneisses. The western part of the region is dominated by granulite facies gneisses intruded by a 1800-1790 Ma AMCG suite and characterized by a strong magnetic and gravity anomaly. To the east this zone borders a crustal section with amphibolite facies gneisses, migmatites and granites. The transition has been considered to reflect the amphibolite to granulite facies metamorphic gradient of a Palaeoproterozoic orogenic event. The results of this study show that the region underwent multiple high-grade events, one in the late Archean at ca. 2640 Ma, one in the period slightly before (< 10 m.y.) the main pulse of AMCG magmatism at 1800-1790 Ma, and another during the main AMCG event, with less distinct evidence pointing to an earlier episode sometime between 1880-1810 Ma. The major Archean and Proterozoic events are recorded on both sides of the metamorphic transition, thus supporting an affinity of these crustal segments. In detail adjacent parts across the metamorphic transition give distinct results. For example at Sigerfjord, a 1800 Ma granulite is matched with Archean migmatites, the latter showing little evidence of having undergone amphibolite facies metamorphism at 1800 Ma. Paired with differences in the timing of Caledonian events these relationships suggest that the transition may be a tectonic boundary. The successive metamorphic events left a very unequal record in various rocks that can be explained by the role played by compositional factors, presence of fluids, specific mineral reactions and as an integration of these factors the ability of each rock to make, modify and preserve zircon, monazite or titanite.