A U-Pb zircon, Archaean age for granitoid rocks in the Kunes Nappe, Laksefjord Nappe Complex, Finnmark, North Norway

Abstract

Nappes in the middle parts of the Caledonian tectonostratigraphy in Finnmark, northern Norway, contain slices of granitoid basement that hitherto have not been reliably dated. Zircons extracted from two, high-K, calc-alkaline granites from the Storfjord Basement Complex of the Kunes Nappe, Laksefjord Nappe Complex, are strongly discordant and display some scatter, but provide evidence indicating that the granites formed in the Archaean (c. 2850 Ma; 2600 Ma minimum age). Mature arc, alkali-rich granites of this type and similar age are known from some of the nearby Archaean terranes in this northernmost part of the Fennoscandian Shield, southeast of the Caledonian front. Most of these NW-SE-trending terranes or cratonic blocks can be followed northwestwards, on geophysical evidence, for some 350 km beneath the Caledonian nappes and far into the southwestern Barents Sea. The granites and related plutonic rocks of the Kunes Nappe are considered to derive from a former highland region (termed the Finnmark Ridge) of Fennoscandian (Baltican) Archaean to Palaeoproterozoic rocks, now largely concealed beneath the SE-transported Caledonian allochthons of northwestern Finnmark and the nearby continental shelf.