Metamorphic and structural diachroneity in the Finnmarkian nappes of north Norway

Two periods of garnet growth (Gt 1 and Gt 2 ) have been found in the Finnmarkian nappes of north Norway. In the Kolvik Nappe (the lowest nappe) Gt 1 has preserved an S 2 syntectonic spiral inclusion fabric; in the Olderfjord Nappe an earlier S 1 fabric and an interkinematic inter‐D 1 –D 2 fabric have been preserved in Gt 1 whilst only the S 1 fabric has been found in Gt 1 in the Brennsvik Nappe (the highest nappe). In each nappe Gt 2 overgrew a penetrative fabric (S 2 ) wrapped around Gt 1 . In the Kolvik Nappe inclusion fabrics may be continuous from Gt 1 into Gt 2 but in the higher nappes there is a distinct break. Gt 2 may have been partially syntectonic with D 3 in the Brennsvik Nappe. Chemically Gt 1 in the Kolvik Nappe and in parts of the Olderfjord and Brennsvik Nappes has antithetic Fe‐Mn zoning. In all nappes X Ca and X Mg are weakly zoned in Gt 1 ; X Mg increases outwards and is greater in the higher nappes in Gt 1 suggesting higher nucleation temperatures. In the Olderfjord and Brennsvik Nappes Gt 2 is marked by increasing X Ca , probably due to changing garnet‐plagioclase equilibria, although the Fe/Mg ratio remains constant. X Mg is higher in Gt 2 than Gt 1 . Basement rocks within the nappe pile have an early pre‐Finnmarkian growth (Gt 1 ) and a later Finnmarkian growth (Gt H ) correlated with Gt 2 on the basis of chemical zoning patterns. The diachroneity of Gt 1 is ascribed to progressively earlier (compared to the structural development) cessation of overstepping of garnet‐forming reactions before peak metamorphism in the higher nappes, resulting in earlier structural events being preserved.