Brittle reactivation of plastic shear zones is frequently observed in geologically old terranes. To better understand such deformation zones, we have studied the >700 Ma long structural history of the Himdalen–Ørje Deformation Zone (HØDZ) in SE Norway by K–Ar and 40Ar–39Ar geochronology, and structural characterization. Several generations of mylonites make up the ductile part of HØDZ, the Ørje Shear Zone. A 40Ar–39Ar white mica plateau age of 908.6 ± 7.0 Ma constrains the timing of extensional reactivation of the Ørje mylonite. The mylonite is extensively reworked during brittle deformation events by the Himdalen Fault. 40Ar–39Ar plateau ages of 375.0 ± 22.7 Ma and 351.7 ± 4.4 Ma from pseudotachylite veins and K–Ar ages of authigenic illite in fault gouge at c. 380 Ma are interpreted to date initial brittle deformation, possibly associated with the Variscan orogeny. Major brittle deformation during the Early–Mid Permian Oslo Rift is documented by a 40Ar–39Ar pseudotachylite plateau age of 294.6 ± 5.2 Ma and a K–Ar fault gouge age of c. 270 Ma. The last datable faulting event is constrained by the finest size fraction in three separate gouges at c. 200 Ma. The study demonstrates that multiple geologically significant K–Ar ages can be constrained from fault gouges within the same fault core by combining careful field sampling, structural characterization, detailed mineralogy and illite crystallinity analysis. We suggest that initial localization of brittle strain along plastic shear zones is controlled by mechanical anisotropy of parallel-oriented, throughgoing phyllosilicate-rich foliation planes within the mylonitic fabric.

A detailed electron microprobe (EMP) and laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) study of zircon from six types of miaskitic and agpaitic alkaline pegmatite from the Larvik Plutonic Complex, Oslo rift valley, Norway, was undertaken to shed light on the pegmatite petrogenesis. Detailed rare earth element (REE) analyses indicate important differences between the zircon from each type of pegmatite. Primary zircon from miaskitic Stavern-, Tvedalen- and Stålaker-type pegmatites has a mean ΣREE = 704 ppm, is depleted in LREE and has a significant positive Ce anomaly (Ce/Ce* = 44–67) and negative Eu anomaly (Eu/Eu* = 0.15–0.18). Secondary Tvedalen-type zircon is REE-enriched (ΣREE = 5035 ppm), with a flatter REE pattern, Ce/Ce* = 0.97 and a Eu anomaly similar to primary Tvedalen-type zircon (Eu/Eu* = 0.21). Secondary zircon from agpaitic Langesundsfjord-type pegmatites display a distinctive flat REE pattern characterized by overall REE enrichment (ΣREE = 967), Ce/Ce* = 1.92, and a minor negative Eu anomaly (Eu/Eu* = 0.37). Zircon from agpaitic Bratthagen-type pegmatites occurs as both altered primary and secondary phases and is strongly enriched in REE relative to other zircon (ΣREE = 4178 and 8388, respectively). Primary Bratthagen-type zircon has a similar REE pattern to miaskitic zircon, with a steeper HREE profile and smaller Ce and Eu anomalies (Eu/Eu* = 0.73; Ce/Ce* = 6.22). Secondary Bratthagen-type zircon is strongly enriched in LREE compared to primary zircon, does not display a positive Ce anomaly and has Eu/Eu* = 0.56. The altered primary and secondary Bratthagen-type zircons have elevated Th/UN ratios, suggesting a different melt source for Bratthagen-type agpaitic pegmatites. Zircon from external pegmatites has trace-element signatures similar to Stavern-, Tvedalen- and Staålaker-type primary zircon with Ce/Ce* = 214 and Nb/Ta and Th/U ratios that are similar to those of secondary Langesundsfjord- and Bratthagen-type zircon. It is suggested that the parental melt of the external pegmatites is the same as the miaskitic pegmatites, but that it has undergone alteration by hydrothermal fluids derived from the host basalt, or by post-magmatic F-rich fluids which mobilize Nb and Th. On the basis of texture, morphology and geochemistry, two populations of zircon can be recognized: (1) primary zircon from miaskitic pegmatites; and (2) secondary zircon from post-magmatic, hydrothermal assemblages. The U–Th–Pb isotope analyses indicate that the secondary and altered zircon are depleted in 238U, and enriched in LREE. Interaction of a post-magmatic hydrothermal fluid with an externally derived meteoric fluid is suggested to have influenced the REE signatures, and in particular the Eu and Ce anomalies of the late-stage zircons.

The mildy peralkaline Eikeren-Skrim granite belongs to the Permian magmatic province of the Oslo rift, south-east Norway. Euhedral quartz crystals from the abundant miarolitic cavities contain primary inclusions of partly crytallized silicate melts and coexisting primary, aqueous fluid inclusions. Micro-thermometric measurements give maximum estimates for the granite solidus of 685–705°C. Quenched silicate melt inclusions are not peralkaline, have normative Or/Ab weight ratios of 1.15–1.44 (compared to 0.49–0.80 in whole-rock samples) and F and Cl contents of 0.1 and 0.21–0.65 wt. %, respectively. Coexisting magmatic fluid inclusions are highly enriched in Na, Cl, S and to some extent K. These chemical characteristics are the results of late-magmatic melt-mineral-fluid interaction in the miarolitic cavities.

Amphibole in the Larvik alkaline plutonic complex in the Oslo rift, Norway, has Ti-rich compositions from edenite through pargasite to kaersutite, and has a large H+ deficiency (0.7–1.1 atoms per formula unit: a.p.f.u.) with a large oxy component in the amphibole OH– site (O2– = 2 – (OH + F + Cl) = 0.2–0.9 a.p.f.u.), similar to the mantle-derived kaersutites. Their compositions reveal a characteristically low Fe3+/(Fe3++Fe2+) ratio (<0.23) and a high F concentration (0.3–0.9 a.p.f.u.). Correlation with the Fe3+ ratio caused by Fe2+ + OH– = Fe3+ + O2– + 1/2H2 substitution is negligible, which is supported by H and O isotope compositions. A possible substitution, [6]Al3+ + OH– = [6]Ti4+ + O2– may be operative for Larvik kaersutites when the O2–/Ti is 1.0. A relatively larger O2–/Ti ratio (1.2—2.0) suggests an another kaersutite substitution mechanism, [6]R2+ + 2OH– = [6]Ti4+ + 2O2–, where [6]R2+ = Fe2+ + Mg + Mn. These effects might result in the limited O2–/Ti ratio value from 1.0 to 2.0.

A negative correlation between Ti and F, suggesting F incorporation into kaersutite may diminish the O2–/Ti ratio, not only due to the occupation of this non-oxy species in the O3 site, but also due to F—Ti avoidance. Composition-dependent H and O isotope variations (δD = –106 to –71% and δ18O = 4.6–5.2%) suggest equilibrium in the closed-system magma with differentiation. The mineral chemistry of Larvik oxy-kaersutitic amphibole could reflect the crystallization in a closed-system magma during rifting with passive crustal thinning at the Oslo palaeorift.

The Permian magmatic province of the Oslo rift, south-east Norway, includes large volumes of felsic and silicic rocks. Based on their geochemical character, these rocks may be divided into two main groups. The Larvik larvikites (monzonites) are highly enriched in large ion lithophile elements (LILE) (e.g. 10–32 ppm Th. 8–15 ppm Ta), and have an initial 87Sr/86Sr of 0.70391 ± 5. The syenites and granites have moderate to high concentrations of LILE (e.g. 7–88 ppm Th, 4–25 ppm Ta), and initial 87Sr/86Sr ratios between 0.705 and 0.707. The Larvik larvikites and extrusive equivalents (rhomb porphyry lavas) have similar initial Sr isotope ratios to uncontaminated basalts and gabbros in the rift, and are believed to have a mantle origin. The higher initial 87Sr/86Sr ratios in the silicic than in the felsic rocks reflect a crustal component representing the intermediate or low crust. After intrusion into the upper crust, the major and traceelement concentrations of the silicic magmas were modified through fractional crystallization dominated by removal of alkali feldspar, and transport of elements with a fluid phase. The silicic magmas appear not to have interacted significantly with the side rock at this stage.