Geology of western Ullsfjord, North Norway, with emphasis on the development of an inverted metamorphic gradient at the top of the Lyngen Nappe Complex

Abstract

The Ullsfjord area of northern Troms comprises rocks from two allochthons: (1) a fragmented ophiolite complex and non-conformably overlying fossiliferous (Upper Ordovician-Lower Silurian) metasedimentary rocks (Balsfjord Group) of the Lyngen Nappe Complex and (2) exotically derived metasedimentary and meta-igneous rocks of the overlying Tromso Nappe Complex. Tromso Nappe Complex rocks are exposed as isolated klippen on the highest peaks in Ullsfjord. A synmetamorphic, inverted metamorphic gradient exists within the prograde assemblages of the Balsfjord Group, such that chlorite-zone assemblages occur at the base of the sequence and sillimanite-zone assemblages occur immediately beneath the thrust contact with the overlying Tromso Nappe Complex. The entire Barrovian sequence of mineral zones, with the exception of kyanite, is present in the footwall. Complexity of rock fabrics increases sympathetically with metamorphic grade progressing structurally upwards. Simple bedding-cleavage relations at the base of...