Evidence for a Caledonian amphibolite to eclogite facies pressure gradient in the Middle Allochthon Lindås Nappe, SW-Norway

Abstract

The Proterozoic anorthosite–mangerite–charnockite complex dominating the Lindås Nappe in the Scandinavian Caledonides was locally eclogitized in the southwestern part of the nappe during the Caledonian orogeny, whereas only amphibolite facies assemblages are recorded in the rest of the nappe. Sveconorwegian granulites of anorthositic to jotunitic composition in the northernmost eclogite-free exposures of the nappe exhibit large garnet phenoblasts (ca. 900°C) that are fractured and partly replaced by a Caledonian symplectitic amphibolite facies assemblage (ca. 515°C). Metamorphic zircon attributed to this garnet breakdown is dated by ID-TIMS U–Pb at 430 ± 3 Ma, suggesting that the amphibolite facies overprint was coeval with the formation of eclogite 30 km further south, probably implying that the section across the nappe represents a Caledonian pressure gradient. The rocks also preserve a complex Sveconorwegian history including an age of 969 ± 6 Ma, which we interpret as dating magmatic emplacement of jotunitic–anorthositic portions of the complex, 936 ± 12 Ma reflecting the granulite facies metamorphism, and 908 ± 16 Ma, representing a late generation of zircon best explained as having formed by metasomatic processes. Caledonian shearing severely deformed zircon grains in an amphibolite facies shear zone, resetting their U–Pb systems, and forming new ones, hereby also demonstrating a case of resetting and recrystallization of low-U zircon. Our data, gained from diverse lithologies, illustrate several processes involved in making and resetting zircon as well as indicate the contemporaneous evolution and similar origin of the Lindås Nappe and the Jotun Nappe Complex.