Structural and U–Pb isotopic study of late Archaean migmatitic gneisses of the Presvecokarelides, Lylyvaara, eastern Finland

Abstract

ABSTRACTThe migmatitic gneiss complex of Lylyvaara in the eastern part of Finnish Presvecokarelides of the Baltic Shield shows evidence of a polyphase deformational and metamorphic history and of the emplacement of a number of mafic and felsic igneous intrusions at various stages during this history. Sequential structural development has been established on the bases of refolding and cross-cutting relationships. U–Pb zircon and sphene isotopic data combined with structural studies indicate that the first six deformational phases took place in late Archaean (=Presvecokarelian) times. The seventh deformational phase is constrained as being early Proterozoic (=Svecokarelian) from regional considerations.The gneissic foliation in the dominant tonalitic to trondhjemitic palaeosome is parallel to lithological layering (So). Mostly it is composite S1–S2; only in F2 fold hinges can separate S1 and S2 be unequivocally distinguished. There, both of these fabrics, which were formed in amphibolite facies conditions of metamorphism during D1 and D2 have retained their identity despite extensive tectonic overprinting. Further tonalitic or granodioritic material was intruded during D3 or between D2 and D3. Effects of the third deformational phase (D3) ar6e expressed only locally as asymmetrical and polyclinal folds, which deform S1–S2 and F2. These folds now have a northeasterly axial trend and they show considerable variations in the style of their parasitic structures. F4 folds are common. They are dextral and asymmetrical, have NW—NNW-trending axes and show complex interference patterns with F2 and F3 folds. During D4, much aplogranitic neosome material was emplaced in NW—SE-trending movement zones, which correspond to the axial planes of F4 folds. Superimposition of F5 and F6 structures on previously formed patterns add to the structural complexity although they only result in minor modifications. Both are open and upright and locally have associated cleavages or healed fractures (S5, S6). D7 is expressed throughout the migmatitic complex as narrow NW—SE-trending shear zones which reactivate the S4 trend.U–Pb zircon isotopic data indicate that the metamorphism associated with gneiss formation took place 2843 ± 18 Ma ago. U–Pb sphene ages of c. 2660 Ma and 2620 Ma indicate that metamorphic conditions prevailed for a very considerable time. An aplogranitic neosome related to F4 axial planes gave a 2657 ± 32 Ma U–Pb zircon age, while granodiorite and pegmatite dykes related to D6 yielded U–Pb zircon ages of c 2670 Ma and 2640 Ma, respectively.