Deciphering the structural and metamorphic history of the Series in the Upper Allochthon of the Scandinavian Caledonides in northern

*stephan.m.hopfl@uit.no 1Department of Geosciences, UiT The University of Norway, Tromsø 9037, Norway 1 1 1,2 1 2Institut des Sciences de la Terre (ISTO), Université d'Orleans, Orleans 45100, France Höpfl Stephan *, Konopásek Jiří , Stünitz Holger , Bergh G., Steffen Department of Geosciences, UiT

Introduction Geological Map and Structural Geology Metamorphism and Age Dating

70°00´ 17°30´ 18°00´ 18°30´ 19°00´ 19°30´ 20°00´ 70°00´ 17°30´ 18°00´ 18°30´ 19°00´ 19°30´ 20°00´ The Balsord Series is located in the central part of – 1 70°00´ 17°30´ 18°00´ 18°30´ 19°00´ 19°30´ 20°00´ N B N B N B County, , and is part of the upper allochthon of the D1 Scandinavian Caledonides. It consists of an Ordovician–Silurian 3 metsedimentary sequence lying on top of the mostly gabbroic B B Basement Magmac Complex (LMC). The unit exhibits an inverted metamorphic 69°30´ 69°30´ gradient, where the metamorphic condions increase from the base to the B D2 B 69°30´ top, from very low grade in the southeast to medium grade in the west and 1 2 1 B

B B northwest. The Balsord Series is sandwiched between two high-grade Reisa Reisa

Tromsø Nappe Tromsø Nappe Nakkedal Nappe Nakkedal Nappe units, the Nakkedal + Tromsø Nappe Complex in the hanging wall and the R R Balsfjord Series Balsfjord Series B 0 5 10 15 20 25 Lyngen Gabbro 0 5 10 15 20 25 Lyngen Gabbro R km R km Reisa Nordmannvik Nappe as the top part of the Reisa Nappe Complex (RNC) in 69°00´ 17°30´ 18°00´ 18°30´ 19°00´ 19°30´ 20°00´ 69°00´ 17°30´ 18°00´ 18°30´ 19°00´ 19°30´ 20°00´

70°00´ 17°30´ 18°00´ 18°30´ 19°00´ 19°30´ 20°00´ 70°00´ 17°30´ 18°00´ 18°30´ 19°00´ 19°30´ 20°00´ Peak-Met. Age 2 3 P-T Modelling (PS) the footwall. R N B N B Balsfjord Series 0 5 10 15 20 25 R km 69°00´ 17°30´ 18°00´ 18°30´ 19°00´ 19°30´ 20°00´ One of the main movaons in this project is to resolve the Caledonian 3 Fig. 4: A map showing the Balsfjord-Series and its inverted heterogeneous metamorphic grade. The lower part of the unit in the SE displays distinctly B B low grade rocks like chloritic schists without biotite (right) and features characteritic metapelites like garnet-mica schists in the upper NW-W part (left). deformaon history in the Balsord Series, ideally leading to a regional This increase in metamorphic grade appears to be tied to tectonic contacts. The higher grade Balsfjord Series is usually found in close proximity to the nappe contact with the Tromsø Nappe, whereas the lower grade parts are situated closer to the Lyngen Gabbro in the E. First results on tectonic model explaining the tectonostragraphic and metamorphic constraining the conditions and timing of the peak metamorphism of the Balsfjord-Series can be observed in Fig.5. The analysed localities where the ? respective samples were taken are marked (yellow star and blue rectangle). 69°30´ 69°30´ 9 B B Ab relaonships between the abovemenoned units. +Qz Grt Pl Ms Bt Rt

Pg Bt Zo Sp

Pl Pg Bt Zo Sp

Chl Ms Pg Bt Zo Sp 8 Pl Ms Pg Bt Zo Rt

Pl Chl Pg Bt Zo Sp Pl Pg Bt Zo Rt 2 8.3 data-point error ellipses are 2s Geological History and Area Pl Ms Pg Bt Rt B B Ab 0.075 Reisa Reisa Pl Pg Bt Rt B Pl Chl Pg Bt Rt 7.6 460

Tromsø Nappe Tromsø Nappe Pl Chl Pg Bt Zo Rt A Pl Ms Pg Bt Rt Ilm Pl Ms Pg St Bt Rt

Nakkedal Nappe Nakkedal Nappe Pl Chl Pg Bt Zo Sp Pl Ms St Bt Rt R R Chl Ms Bt Zo Sp 0.073 Balsfjord Series Balsfjord Series 7 0 5 10 15 20 25 Lyngen Gabbro 0 5 10 15 20 25 Lyngen Gabbro 9 U R km R km 2 6.9 Pl Chl Ms Pg Bt Rt 10 238 Pressure (P) [kbar] 69°00´ 17°30´ 18°00´ 18°30´ 19°00´ 19°30´ 20°00´ 69°00´ 17°30´ 18°00´ 18°30´ 19°00´ 19°30´ 20°00´ 4 Pl Chl Ms Pg Bt Rt Ilm Ab Pl Chl Ms Pg Bt Ilm 11 3 Pb/ 0.071 440 Fig. 2: Top Left: Geological Map modified from Augland et al. (2014) of the investigated area with idealized models of the two deformational events (D1 1 206 12 Pl Ms St Bt Rt Ilm and D2) at the locations where they are they are most prominent. The area is subdivided into three zones that are compared by their structural properties 13 16 6.2 15

(Schmidt Net). Red dots are flat lying Stretching Lineations (SL), blue Great Circles symbolize Fold Axial Planes (FAP) and blue dots represent Fold Axes Ab Pl Chl Ms Bt Ilm 14 (FA). 0.069 Concordia Age = 443 ± 2 Ma (2s, decay-const. errs included) Pl Chl Ms Bt Rt Ilm Pl Ms St Bt Ilm Pl Chl Ms Bt Sp Rt Pl Chl Ms Bt Rt 1: In Zone 1 in the NW the SL show an average NW-SE direction, albeit some spread. Note, that this is parallel to the orientation of the FA that are linked Pl Bt Sil Ilm 6 17 MSWD (of concordance) = 3.3, Pl Chl Ms Bt Sp Pl Chl Ms St Bt Ilm to flat lying W-dipping FAP (D1). Going further SE in the area we see the SL are more concentrated in the NW and there no longer any N-S 5 Pl St Bt Sil Ilm Probability (of concordance) = 0.067 5.5 420 measurements. Looking at the folding, a new population emerges (D2) with NE-SW directed FA and steeply dipping FAP. Finally, at the very south it is 450 490 530 570 610 650 0.067 evident the SL have reoriented even more and are now organised almost E-W with some even being WSW-ENE. Analysis of the folding here needs more Temperature (T) [C°] 0.49 0.51 0.53 0.55 0.57 0.59 0.61 207Pb/235U data but there are first signs of both D1 and D2 being active here as well. 1 Pl Chl Ms Pg Bt Sp Ab 9 Pl Ms Pg St Bt Rt Ilm 2 Pl Chl Ms Pg Bt Zo Sp 10 Pl Ms Pg Bt Ilm 2: Zone 2 lies further to the E of Zone 1. D2 is very dominant in the southern part but there is no sign of D1. Moving further north D1 picks up again and 3 Pl Chl Ms Pg Bt Sp 11 Pl Ms Pg St Bt Ilm 4 Pl Chl Ms Pg Bt Sp Rt 12 Pl Chl Ms Pg St Bt Ilm coexists with D2. SL were difficult to verify in this zone. 5 Pl Chl Ms Bt Zo Sp Ab 13 Pl Ms Bt Sil Rt Ilm 6 Pl Chl Ms Bt Sp Ab Rt 14 Pl Ms St Bt Sil Rt 3: Starting in the W of Zone 3, the SL have the same orientation as the SL in the NW of Zone 1. Measured FA indicate the existence of both D1 and D2, 7 Pl Chl Ms Pg Bt Zo Rt 15 Pl Ms Bt Sil Ilm 8 Pl Pg Bt Zo Sp Rt 16 Pl Bt Sil Rt Ilm albeit D1 being more dominant. Looking at the E side of the zone across the fjord we can see SL in a NE-SW orientation. Judging by their orientation this 17 Pl Ms St Bt Sil Ilm may actually be FA or crenulations. More fieldwork needs to give clarification on this matter. The FAP are typical for D2 and are exceptionally steep. Fig.5: Left: P-T Pseudosection calculated in Perple_X (Connolly, 2005) in the NCKFMMASHT system modelling a metapelitic sample (garnet- micaschist) of the Balsfjord-Series. The location of the sample is marked on Fig. 4 (blue rectangle). A and B represent garnet growth P-T conditions a D1 b D2 where A is the garnet core and B is the garnet rim. The P-T conditions for the core and rim are quite similar and lie at around 550 C° and 7.5 kbar. Fig. 1: Left: Schematic Reconstruction modified Right: U-Pb dating of the peak metamorphism in a garnet-micaschist with monazites that are assumed to have grown during peak metamorphism. from Ziegler (1985) of the ancient collision between The location of the sample is in the E of the Balsfjord-Series (yellow star in Fig. 4). Laurentia and Baltica approximately 420 Ma ago resulting in the formation of the Caledonian mountain range. Right: Map of Scandinavia with rectangle marking the working area of the project Interpretations and Conclusions around the city of Tromsø The two folding events (D1 and D2) are interpreted to be genetically related but slightly diachronous. The earlier folding phase D1 was likely generated during nappe thrusting and peak

0 10 20 30 40 metamorphism of the Balsfjord Series (443 Ma). The subsequent open folding D2 is explained as cm Results a result of continued shearing and shortening of the weaker metapelitic Balsfjord Series against the more rigid gabbroic part of the Lyngen Gabbro in the E during late stages of the Caledonian The Balsord Series features two main discernible folding phases. The nappe thrusting. Dating of the peak metamorphism of the overlying Tromsø-Nakkedal complex has yielded ages of earlier phase (D1) displays ght to isoclinal folds with flat lying axial surfaces around 450 Ma (Fassmer et al. 2020, Augland et al. 2014) and 439 Ma for the underlying Reisa Nappe complex (Faber et al. 2019). parallel to the penetrave foliaon. Observed fold axes are parallel with the 0 1 2 0 5 10 15 Both of these units show high metamorphic grades with the lower-grade Balsfjord-Series being cm cm sandwiched in between. The recent dating of its metamorphism (443 Ma) reveals a tectonic stretching lineaon. These folds are best preserved in the northwestern, Fig.3: Phield photos displaying the two c D1+D2 deformational folding phases. continuity between these units and that the final exhumation of the Tromsø-Nakkedal Complex a: Three examples of D1 folds showing took place during prograde metamorphism of the underlying Balsfjord Series. upper part of the unit and are syn-metamorphic in certain areas, as they high strain and flat FAP b: Typical D2 folds with inclined-steep FAP and lower strain; Note the bottom fold original bedding (transposed foliaon). photo where the FAP is subvertical. References c: An outcrop where both D1 and D2 can be observed. The right photo is a A later folding phase (D2) is represented by mainly open folds with inclined Augland, L. E., Andresen, A., Gasser, D., & Steltenpohl, M. G. (2014). Early Ordovician to Silurian evolution of exotic terranes in the Scandinavian C aledonides of the Ofoten–Troms area–terrane close-up of the D1 fold tip that is seen characterization and correlation based on new U–Pb zircon ages and Lu–Hf isotopic data. Geological Society, London, Special Publications, 390(1), 655-678. on the left. D1 (left) exhibits a Connolly, J. A. (2005). Computation of phase equilibria by linear programming: a tool for geodynamic modeling and its application to subduction zone decarbonation. Earth and Planetary Science to steep axial surfaces. Their fold axes are gently plunging with a characteristic flat lying FAP and is Letters, 236(1-2), 524-541. Faber, C., Stünitz, H., Gasser, D., Jeřábek, P., Kraus, K., Corfu, F., ... & Konopásek, J. (2019). Anticlockwise metamorphic pressure–temperature paths and nappe stacking in the Reisa Nappe overprinted by a steeply dipping axial Complex in the Scandinavian Caledonides, northern Norway: evidence for weakening of lower continental crust before and during continental collision. Solid Earth, 10(1), 117-148. 0 2 4 6 cleavage parallel to the FAP of D2 folds Fassmer, K., Martinet, I., Miladinova, I., Sprung, P., Froitzheim, N., Fonseca, R. O. C., ... & Kullerud, K. (2020). Lu–Hf geochronology of ultra-high-pressure eclogites from the Tromsø-Nappe, predominant NE–SW orientaon. cm (right). Scandinavian Caledonides: evidence for rapid subduction and exhumation. International Journal of Earth Sciences, 109(5), 1727-1742. Ziegler, P. A. (1985). Late Caledonian framework of western and central . The Caledonide Orogen-Scandinavia and Related Areas, 3-18.