Structures, intrusive contacts and metamorphic mineral assemblages
reveal a polyphase tectonothermal evolution for the Caledonian basement
rocks of the Bockfjorden area, NW Spitsbergen. The oldest deformation
D-1 is documented by isoclinal folds F-1 in biotite schist. F-1 folds
are crosscut by an intrusion I-1 of tonalitic to monzogranitic
composition. Both, F-1 folds and I-1 are overprinted by D-2 structures,
that are considered to be related to the Silurian Caledonian
continental collision. D-1 and I-1 are thus regarded as the record of a
complex pre-Silurian evolution of the basement rocks. The Caledonian
continental collision that is represented by the isoclinal folding F-2
and by the main schistosity S-2 was simple shear dominated and
associated to S-N thrusting in the studied area. During D-2 the I-1
granitoid rocks were transformed to orthogneiss. The related
metamorphism M-2 reached upper amphibolite facies conditions. During or
after late stage (2), deformation the orthogneiss provides evidence for
incipient partial melting. The resulting migmatites and S-2 were
subsequently crosscut by the granodiorite and monzogranite of the
intrusive event I-2, that display partly a peraluminous composition,
pointing to partial melting of crustal rocks as the magma source.
Subsequently, cordierite formed in all Al-rich rock types including I-1
and I-2 rocks. Cordierite growth was caused by a high T-low P
metamorphism at conditions of 630-700 degrees C and 0.3-0.4 GPa, the
geotherm of which was about 50 degrees/km. The transition from
migmatite formation to granite emplacement followed by a high T-low P
metamorphism indicates near isothermal decompression under upper
amphibolite facies conditions. This evolution reflects a continuous
exhumation of mid-crustal rocks that is considered as the consequence
of crustal extension during the collapse of Caledonian thickened crust.