Based on field observations, there are two generations of pegmatites emplaced in the Paleoproterozoic Tysfjord granite gneiss (Husdal 2008). These Paleoproterozoic rocks were metamorphosed and deformed under amphibolite-facies conditions during the Caledonian orogeny about 430 Ma ago (Björklund 1989 Northrup 1997). The Paleoproterozoic granites of the Tysfjord–Hamarøy area in north Norway host a suite of pegmatites that traditionally have been interpreted as a typical example of pegmatites being fractionated derivatives of their granitic host. The absence of an exposed coeval granite, however, does not exclude the possibility that there is a hidden granite intrusion at depth. These studies imply that not all chemically evolved pegmatites form from residual melts of chemically evolved plutons. 2017 Barros and Menuge 2016 Ilickovic et al. Romer and Smeds 1994, 1996 Fuchsloch et al. There is, however, an increasing number of case studies yielding contrasting ages for pegmatites and spatially associated granites and, thus, showing that there are rare-element pegmatites without an associated source granite (e.g. 2018) and that show the same chemical character and age as the associated granite (e.g. Janeczek 2007 Thomas and Davidson 2016 Falster et al. Such a genetic relation certainly applies for numerous pegmatite fields, in particular for pegmatite fields that are hosted within the parental pluton (e.g. Based on this spatial relation, pegmatites are commonly interpreted to have formed from residual melts derived from crystallizing granitic plutons (e.g. Granites in orogenic and anorogenic settings are often spatially associated with pegmatites that may be enriched in incompatible elements. 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. These pegmatites are interpreted to be anatectic melts that formed from the partial melting of Tysfjord granite gneiss. Undeformed Group 2 amazonite–tourmaline pegmatites yield columbite and zircon U–Pb ages in the range 400–379 Ma. The formation of these unusually large granite-hosted NYF pegmatites may have been facilitated by the overall high F content of TIB granite gneisses. 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. 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. 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. This is not always true as demonstrated by the Tysfjord granite gneiss and its two groups of pegmatites. Pegmatite fields within granite plutons are commonly considered to have formed from residual melts of their host.
0 kommentar(er)
