GEOLOGICAL INVESTIGATIONS OF THE REPPARFJORD TECTONIC WINDOW, NORTHERN NORWAY: Backtracking two billion years of geological history by structural analysis, K–Ar and Re–Os geochronology and carbonate chemostratigraphy

Abstract

This thesis presents the results of a multidisciplinary geological study of the Repparfjord Tectonic Window (RTW) in northern Norway. The study was carried out to refine the understanding of the RTW’s long geological evolution and was motivated by the urgent need for a modern, well-constrained geological model for future ore mineral potential assessment in northern Norway. The structural, tectonic and depositional development of the RTW is investigated and presented in five self-standing manuscripts. These contribute also to an improved understanding of several fundamental geological processes in the field of fault-rock dating, fault mechanics, Re–Os pyrite–chalcopyrite systematics and Early Paleoproterozoic C isotopic fluctuations and geodynamics. The RTW is exposed within the Caledonian Kalak Nappe Complex along the northwestern passive margin of the Eurasian plate and comprises a c. 8000 m thick Early Paleoproterozoic sedimentary–volcanic succession, which represents the northwestern-most termination of the Fennoscandian Shield. This succession is intruded by two Paleoproterozoic Svecofennian magmatic suites and is, in turn, overlain by a Neoproterozoic sedimentary cover. In contrast to the intraplate rift-setting inferred for most parts of the Fennoscandian Shield in the Early Paleoproterozoic, the Repparfjord sedimentary–volcanic succession is interpreted to have formed in a continental back–arc domain. The revised stratigraphy of the RTW suggests that arkosic sandstones and volcaniclastic conglomerates and siltstones of the Saltvann Group represent the lowermost exposed stratigraphic level. The group’s lithofacies association is interpreted to reflect deposition in a rapidly subsiding half-graben basin. C isotope ratios in carbonate rocks constrain the depositional age of this group to a relatively short time window of no longer than 75 Myr, and possibly as short as c. 10 Myr, around the termination of the global Lomagundi–Jatuli isotopic event at c. 2060 Ma. The present-day geometry of the northwestern RTW is that of a km-scale upright anticline, the Ulverygg Anticline. Metabasaltic rocks of the Nussir Group rest on the northwestern limb and the chronologically correlative metabasaltic- to rhyolitic volcanic rocks of the Holmvann Group on the southeastern limb. Carbonate chemostratigraphy constraints a post-2060 Ma depositional age of the Porsa Group, the uppermost exposed group in the Repparfjord succession. Cu-rich carbonate veins were emplaced (in metabasalts) and structurally reactivated during episodes of oscillating frictional–viscous deformation that caused repeated brecciation along the vein margins and mylonitization of the core. Re–Os pyrite–chalcopyrite geochronology constrains the initial emplacement of the veins at c. 2069 Ma. K–Ar fault gouge dating indicates reactivation of the veins during the Silurian Caledonian Orogeny. Structural analysis shows that fracturing and vein formation took place in a dextral transpressive deformation corridor during a phase of overall NW–SE shortening, which represents one of the earliest phases of Paleoproterozoic contraction ever documented in Fennoscandia. The study demonstrates that strain and ingress of oxidizing fluids has a significant, yet localized, effect on the isotopic integrity of the Re–Os pyrite–chalcopyrite system. The isotopic disturbance of the Re–Os chronometer in calc-ultramylonitic intervals is interpreted as independent evidence for considerable fluid flow during grainsize sensitive viscous deformation. The northwestern-most part of the RTW is characterized by a series of discrete thrusts that repeat the stratigraphy and form the Porsa Imbricate Stack (PIS). The tectonic repetition of metabasalts, dolostones and slates within the PIS was largely controlled by the geometry of inherited Paleoproterozoic kmscale folds and by localized strain weakening steered by dolomite decarbonation and metabasalt carbonation reactions. K–Ar fault gouge dating of brittle– ductile and brittle faults constrains the timing of PIS development to the main episode of nappe emplacement during the Caledonian orogeny (c. 445–400 Ma), although pre-Caledonian (c. 530 Ma; Finnmarkian?) deformation is also indicated. Extension and fault reactivation occurred in the Carboniferous (c. 330–300 Ma) and Early Cretaceous (c. 120 Ma). The study has demonstrated that the common inclined K–Ar "age–grain-size" spectra obtained from the dating of fault gouges reflect primarily mixing between authigenic and protolithic mineral phases inherited from either the wall rock or from an earlier faulting event.