Two-stage regional rare-element pegmatite formation at Tysfjord, Norway: implications for the timing of late Svecofennian and late Caledonian high-temperature events

Abstract

Pegmatite fields within granite plutons are commonly considered to have formed from residual melts of their host. This is not always true as demonstrated by the Tysfjord granite gneiss and its two groups of pegmatites. The Tysfjord granite gneiss, exposed in a tectonic window of the Caledonides of northern Norway, is part of the transscandinavian igneous belt (TIB) that includes several phases of granitic magmatism. In the northern Hamarøy area (Drag-Finnøy), where most rare-element pegmatites occur, Paleoproterozoic and metamorphosed Group 1 allanite–(Ce)–fluorite metapegmatites have similar bulk rock chemical composition as the TIB granite gneiss rocks, indicating that these pegmatites are residual melts. Group 1 metapegmatites, which are up to 400 m in size, are among the largest known intra-plutonic pegmatites with Nb–Y–F (NYF) signature. The formation of these unusually large granite-hosted NYF pegmatites may have been facilitated by the overall high F content of TIB granite gneisses. Undeformed Group 2 amazonite–tourmaline pegmatites yield columbite and zircon U–Pb ages in the range 400–379 Ma. These pegmatites are interpreted to be anatectic melts that formed from the partial melting of Tysfjord granite gneiss. Group 2 pegmatites, including those from Træna Island and the Sjona tectonic window (400 and 414 Ma), formed during late Caledonian ductile shearing and incipient unroofing of the central Scandinavian Caledonides and record progressively younger ages of this event from SW to NE.

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“…The lens-shaped pegmatites are emplaced in the Tysfjord granite gneissetype rocks, which is part of the Trans-Scandinavian Igneous Belt (TIB) and occurs here in a tectonic window in the northern Caledonites [2] . The observed pegmatites identified in the region can be subdivided into two groups: (i) older Paleoproterozoic and metamorphosed pegmatites with up to 400 m in size and (ii) younger, smaller undeformed pegmatites (400-379 Ma) [18] . The first group is considered to be formed from residual melts of the hosting granite gneiss, which subsequently underwent possibly multiple phases with ductile deformation and latest during the Caledonian orogeny, resulting in large-scale shearing of the pegmatite bodies and partial or complete recrystallization of pegmatite minerals [18] .…”

“…The observed pegmatites identified in the region can be subdivided into two groups: (i) older Paleoproterozoic and metamorphosed pegmatites with up to 400 m in size and (ii) younger, smaller undeformed pegmatites (400-379 Ma) [18] . The first group is considered to be formed from residual melts of the hosting granite gneiss, which subsequently underwent possibly multiple phases with ductile deformation and latest during the Caledonian orogeny, resulting in large-scale shearing of the pegmatite bodies and partial or complete recrystallization of pegmatite minerals [18] . However, the second group is considered anatectic due to partial melting of the granite gneiss during late Caledonian events [18] .…”

“…The first group is considered to be formed from residual melts of the hosting granite gneiss, which subsequently underwent possibly multiple phases with ductile deformation and latest during the Caledonian orogeny, resulting in large-scale shearing of the pegmatite bodies and partial or complete recrystallization of pegmatite minerals [18] . However, the second group is considered anatectic due to partial melting of the granite gneiss during late Caledonian events [18] . Locally, the Tysfjord granite gneiss thrusts over the metasedimentary rocks of the Caledonian nappes [18] .…”

“…However, the second group is considered anatectic due to partial melting of the granite gneiss during late Caledonian events [18] . Locally, the Tysfjord granite gneiss thrusts over the metasedimentary rocks of the Caledonian nappes [18] . The geological and tectonic evolution of the larger Tysfjord area is complex and is described in relation to the different types of pegmatites in [18] .…”

“…Locally, the Tysfjord granite gneiss thrusts over the metasedimentary rocks of the Caledonian nappes [18] . The geological and tectonic evolution of the larger Tysfjord area is complex and is described in relation to the different types of pegmatites in [18] . However, the Tysfjord granite gneiss as the general host of the pegmatites was emplaced as part of the TIB and later reworked during different tectonic events with different predominant stress fields and generation and reactivation of faults and thrust accordingly.…”