Metamorphic and structural evolution of the Early Proterozoic Puolankajärvi Formation, Finland – I. Structural and textural relations

The amphibolite facies Puolankajärvi Formation (PjF) occupies the western margin of the Early Proterozoic Kainuu Schist Belt (KSB) of northern Finland. The lower and middle parts of the PjF consist of turbiditic psammites and pelites and tempestitic semipelites. This report concentrates on the pelitic lithologies which include quartz–two-mica–plagioclase schists with variable amounts of garnet, staurolite, andalusite and biotite porphyroblasts as well as sillimanite and cordierite segregations. The KSB forms a major north–south-trending synclinorium between two Archaean blocks. It contains both autochthonous and allochthonous units and is cut by faults and shear zones. The PjF lies on the western side of the KSB and is probably allochthonous. The formation has undergone six major deformation phases (D₁, D₂, D_3a, D_3b, D₄ and D₅). During D_3a-D₅ the maximum principal stress (σ₁) changed in a clockwise direction from south-west to north-east. Between D₂ and D₃ the intermediate principal stress (σ₂) changed from horizontal to vertical and the interval between D₂ and D₃ marks a transition from thrust to strike-slip tectonics. Relict structures in the porphyroblasts indicate the following mineral growth–deformation evolution in the PjF. (1) Throughout the PjF there was a successive crystallization of garnet (syn-D₁), poryphyroblastic biotite (inter-D_3/4) and staurolite (inter-D_3/4) during the pre-D₄ stage. (2) A syn-D₄-inter-D_4/5 crystallization of kyanite, sillimanite (fibrolite), porphyroblastic tourmaline, magnetite, rutile, cordierite and muscovite–biotite–plagioclase pseudomorphs after staurolite was most localized at and near D₄ shear zones. (3) A syn- to post-D₅ generation of andalusite, ilmenohematite and sheet silicates after staurolite and after cordierite occurred near D₅ faults. The evolution outlined here permits the relative dating of the PjF parageneses, which is used in the second part of the study (Tuisku & Laajoki, 1990), and, together with the knowledge of the pressure–temperature conditions during various growth events, makes it possible to compile pressure–temperature–deformation paths for the PjF.