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Metamorphic and structural evolution of the Early Proterozoic Puolankajärvi Formation, Finland – I. Structural and textural relations
Author(s) -
LAAJOKI K.,
TUISKU P.
Publication year - 1990
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.1990.tb00478.x
Subject(s) - geology , staurolite , andalusite , geochemistry , sillimanite , schist , kyanite , dalradian , pelite , metamorphic rock , cassiterite , greenschist , metamorphic facies , biotite , quartz , facies , geomorphology , paleontology , materials science , tin , metallurgy , structural basin
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 (D 1 , D 2 , D 3a , D 3b , D 4 and D 5 ). During D 3a ‐D 5 the maximum principal stress (σ 1 ) changed in a clockwise direction from south‐west to north‐east. Between D 2 and D 3 the intermediate principal stress (σ 2 ) changed from horizontal to vertical and the interval between D 2 and D 3 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‐D 1 ), poryphyroblastic biotite (inter‐D 3/4 ) and staurolite (inter‐D 3/4 ) during the pre‐D 4 stage. (2) A syn‐D 4 ‐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 D 4 shear zones. (3) A syn‐ to post‐D 5 generation of andalusite, ilmenohematite and sheet silicates after staurolite and after cordierite occurred near D 5 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.