Anticlockwise metamorphic pressure–temperature paths and nappe stacking in the Reisa Nappe Complex in the Scandinavian Caledonides, northern Norway: evidence for weakening of lower continental crust before and during continental collision

Abstract

Abstract. This study investigates the tectonostratigraphy and metamorphic and tectonic evolution of the Caledonian Reisa Nappe Complex (RNC; from bottom to top: Vaddas, Kåfjord, and Nordmannvik nappes) in northern Troms, Norway. Structural data, phase equilibrium modelling, and U-Pb zircon and titanite geochronology are used to constrain the timing and pressure–temperature (P–T) conditions of deformation and metamorphism during nappe stacking that facilitated crustal thickening during continental collision. Five samples taken from different parts of the RNC reveal an anticlockwise P–T path attributed to the effects of early Silurian heating (D1) followed by thrusting (D2). At ca. 439 Ma during D1 the Nordmannvik Nappe reached the highest metamorphic conditions at ca. 780 ∘C and ∼9–11 kbar inducing kyanite-grade partial melting. At the same time the Kåfjord Nappe was at higher, colder, levels of the crust ca. 600 ∘C, 6–7 kbar and the Vaddas Nappe was intruded by gabbro at > 650 ∘C and ca. 6–9 kbar. The subsequent D2 shearing occurred at increasing pressure and decreasing temperatures ca. 700 ∘C and 9–11 kbar in the partially molten Nordmannvik Nappe, ca. 600 ∘C and 9–10 kbar in the Kåfjord Nappe, and ca. 640 ∘C and 12–13 kbar in the Vaddas Nappe. Multistage titanite growth in the Nordmannvik Nappe records this evolution through D1 and D2 between ca. 440 and 427 Ma, while titanite growth along the lower RNC boundary records D2 shearing at 432±6 Ma. It emerges that early Silurian heating (ca. 440 Ma) probably resulted from large-scale magma underplating and initiated partial melting that weakened the lower crust, which facilitated dismembering of the crust into individual thrust slices (nappe units). This tectonic style contrasts with subduction of mechanically strong continental crust to great depths as seen in, for example, the Western Gneiss Region further south.

Th–U–total Pb monazite geochronology records Ordovician (444 Ma) metamorphism/partial melting and Silurian (419 Ma) thrusting in the Kåfjord Nappe, Norwegian Arctic Caledonides

“…419 Ma are consistent within the errors with the previous whole rock 87 Rb/ 86 Sr ages of Dangla et al (1978). Timing of migmatization event recorded by zircon in the Nordmannvik Nappe i.e., 441 ± 1 Ma and 439 ± 2 Ma (Faber et al 2019) is similar to reported herein. Also, the age constrains on shearing from the Heia locality of 420 ± 4 Ma (Augland et al 2014) support the uniform metamorphic evolution of the Skibotn Nappe Complex.…”

“…The Vaddas Nappe rocks were metamorphosed mostly under low amphibolite facies conditions, reaching kyanite stability field. In Arnøya, the pressure-temperature (P-T) conditions of metamorphism related to shearing were estimated to 11.7-13.0 kbar at 630-640°C (Faber et al 2019).…”

“…Geochronological data for metapelites are limited to whole rock 87 Rb/ 86 Sr ages of 439 ± 5 Ma for non-migmatitic gneisses and 414 ± 3 Ma for migmatitic gneisses (Dangla et al 1978). In Arnøya the P-T conditions for the peak of prograde metamorphism were constrained to 5.8-7.1 kbar at 590-610 °C, while subsequent amphibolite facies shearing reached 9.2-10.1 kbar at 580-605 °C (Faber et al 2019).…”

“…In Arnøya the peak metamorphic conditions were constrained to 9.4-11 kbar at 760-790 °C. The mig matization was dated to 441 ± 2 Ma and 439 ± 2 Ma using SIMS U-Pb dating on zircon from melanosome and leucosome, respectively (Faber et al 2019). 87 Rb/ 86 Sr dating performed on metadiorites in the Heia locality yielded an age of 492 ± 5 Ma (Lindstrøm & Andresen 1992), while the emplacement of the Heia gabbro was dated to 435 ± 1 Ma using U-Pb ID-TIMS technique (Augland et al 2014).…”

“…Information about the pre-migmatitic evolution of the Skibotn Nappe Complex is limited to the late Neoproterezoic magmatic age of the Rappesvare granite (Corfu et al 2007) and the late Cambrian age of a metadiorite in Heia, leaving a certain degree of uncertainty about its exotic origin with respect to the Baltica-derived allochthons (Andresen 1988). Faber et al 2019). An extension required for the formation of mafic intrusions of such age is unlikely to occur in the subducted Baltican margin.…”