Metamorphism within the Çokkul synform: evidence for detachment faulting within the metamorphic infrastructure of the Norwegian Caledonides (67°30'N)

Within the Çokkul synform, Caledonian metamorphic rocks of the Middle Köli Nappe Complex (MKNC) are in low-angle fault contact with the basement mylonites derived from the Precambrian Tysfjord granite-gneiss. In the synform, the MKNC is composed of four fault-bounded nappes each of which has a distinct tectonic stratigraphy composed of amphibolite-facies metamorphosed pelitic and psammitic schists with minor lensoidal bodies of mafic and ultramafic rocks. Pelitic rocks from the three structurally lowest nappes contain the low-variance AFM mineral assemblages gar + bio + staur and staur + ky + bio with mu + qtz + ilm, whereas staur and ky are absent from the highest nappe, the Kallakvare nappe. AFM mineral assemblages in the three lowest nappes indicate peak metamorphic temperatures of 610–660°C and peak pressures in excess of 600 MPa. Mineral assemblages from the Kallakvare nappe are not as diagnostic of metamorphic grade. However, rocks from that nappe contain coexisting plagioclases from both sides of the peristerite gap, suggesting lower-grade peak P–T conditions than those of the structurally lower nappes. In addition, biotite from the lower nappes is more Ti-rich than biotite from the Kallakvare nappe. However, gar–bio–mu–plag and gar–bio–ky–plag–qtz thermobarometry suggests that all four nappes equilibrated at approximately 525 ± 25°C and 700 ± 100 MPa. Gibbs method thermodynamic modelling of garnet zoning profiles suggests that the lower three nappes followed clockwise P–T paths that involved heating and compression to a metamorphic peak of approximately 575–625°C, 800 MPa followed by cooling and decompression to 525°C, 700 MPa. P–T paths calculated for the Kallakvare nappe show decompression and minor heating to a peak T of 500–525°C. In the lower nappes, staur and ky grew during the heating phase not seen by the highest nappe. The outer parts of the paths from all four nappes are approximately parallel, possibly recording the emplacement of the Kallakvare nappe onto the already stacked lower three nappes at some time following the metamorphic peak. These P–T paths suggest that the sole fault of the Kallakvare nappe is a normal fault. Garnet zonation thus appears to record a previously unrecognized phase of uplift and tectonic thinning of the MKNC. This event appears to be restricted to the MKNC and to have occurred prior to the emplacement of the MKNC onto the Tysfjord granite-gneiss basement of Baltoscandia under greenschist-facies conditions. It may have been responsible for the uplift and cooling of the MKNC from 25–30 km amphibolite-facies conditions prior to its emplacement onto Baltoscandia under 15–20 km greenschist-facies conditions. The deformation zone associated with this normal fault is relatively narrow, generally less than 1 m thick. If this is typical of other detachment faults in the metamorphic infrastructure of the Scandinavian Caledonides, they may be relatively common, but not often recognized due to the detailed study needed to document them.