Diachronous collision in the Seve Nappe Complex: Evidence from Lu–Hf geochronology of eclogites (Norrbotten, North Sweden)

Agentúra na Podporu Výskumu a Vývoja, Grant/Award Number: APVV-18- 0107; Deutsche Forschungsgemeinschaft, Grant/ Award Number: FR700/18-1We thank Christopher Barnes (AGH University of Science and Technology, Kraków) for providing us with some of the studied samples. Kathrin Fassmer thanks Svenja Trapp and Matthias Hauke (University of Bonn) for help during Lu–Hf laboratory work. We would also like to thank M. Smit, F. Corfu and A. Kylander-Clark for their reviews which greatly contributed to improving the manuscript. This research was funded by DFG-Grant FR700/18-1 to N. F. and the Slovak Research and Development Agency project APVV-18- 0107 to M.J, and partially supported by the National Science Centre (Poland) project 2014/14/ST10/00321 to J. Majka. M.Bukała acknowledges The Polish National Agency for Academic Exchange for the scholarship no. PPN/ IWA/2018/1/00046/U/0001. This is contribution no. 64 of the DFG-funded LA-ICP- MS Laboratory at the Institute for Geosciences, University of Bonn, Germany.The collision of Baltica and Laurentia during the Caledonian Orogeny happened at c. 400-420 Ma. However, subduction and collision processes also took place before this main collisional phase and the tectonic history of these is still not fully resolved. The Seve Nappe Complex in Sweden has recorded these earlier phases. The Seve Nappe Complex in Norrbotten (North Swedish Caledonides) comprises four superimposed nappes emplaced by eastward thrusting (from base to top according to the conventional structural interpretation): Lower Seve Nappe, Vaimok, Sarek, and Tsakkok Lenses. Eclogites occur in the Vaimok and Tsakkok Lenses. The Vaimok Lens represents rocks of the Baltican continental margin intruded by Neoproterozoic dolerite dikes which were later eclogitized and boudinaged. By contrast, eclogites of the Tsakkok Lens are former oceanic basalts associated with calcschists, possibly representing the ocean-continent transition between Baltica and Iapetus. Previous age determinations for eclogitization yielded various ages between c. 500 and 480 Ma, in contrast to younger (460-450 Ma) ages of ultra high-P metamorphism in the Seve Nappe Complex further south in Jamtland. Eclogites from the Vaimok (one sample) and Tsakkok (three samples) lenses were dated using Lu-Hf garnet geochronology. Garnet from all samples shows prograde zoning of major element and Lu contents and yielded well-defined isochrons of the following ages: 480.4 +/- 1.2 Ma (Vaimok); 487.7 +/- 4.6 Ma, 486.2 +/- 3.2, 484.6 +/- 4.6 Ma (Tsakkok). The ages from Tsakkok are interpreted to date the burial of the Iapetus-Baltica ocean-continent transition in a west-dipping subduction zone around c. 485 Ma, and the age from the structurally deeper Vaimok Nappe the following subduction of the continental margin. Previously reported ages of 500 Ma and older are not supported by this study. The age difference between eclogites in the Seve Nappe Complex in Jamtland (c. 460-450 Ma) and Norrbotten (c. 488-480 Ma) may reflect the collision of an island arc with an irregularly shaped passive continental margin of Baltica or alternatively the collision of a straight margin with a microcontinent (Sarek Lens) accreted to the upper plate.Agentura na Podporu Vyskumu a Vyvoja APVV-18-0107German Research Foundation (DFG) FR700/18-

Eclogites in peridotite massifs in the Western Gneiss Region, Scandinavian Caledonides: Petrogenesis and comparison with those in the Variscan Moldanubian Zone

Eclogite lenses and boudins are volumetrically minor, but petrologically important, features of peridotite massifs worldwide. In the Western Gneiss Region of the Scandinavian Caledonides, eclogites in the Almklovdalen and Raubergvik peridotites originated as basaltic to picrobasaltic dikes, comprising both olivine–normative and nepheline–normative types, with a wide variation in Mg–number from 34 to 65. Positive anomalies for Pb and Sr and negative anomalies for Zr and Hf reflect a subduction signature in the basic melts, and rare–earth element modelling requires 20% to 70% fractional crystallization, combined with 20% to 70% assimilation of peridotite. Clinopyroxenes in eclogites have a wide variation in εNd(0) from +68 to −26, which is comparable to that for associated garnet peridotites and pyroxenites, +55 to −38, and a range in 87Sr/86Sr from 0.7021 to 0.7099, which is much larger than that in peridotites and pyroxenites, 0.7014 to 0.7033. Plagioclase and amphibole inclusions in eclogite garnet provide evidence for prograde metamorphism, which attained a maximum temperature of ~775 °C and pressure of ~25 kb. Such conditions are allofacial with those of associated garnet peridotites and pyroxenites, which equilibrated at ~825 °C and ~37 kb. Eclogites yield mixed Sm-Nd isochron ages, as do the peridotites and pyroxenites, but ages in eclogites are 1000 Ma. Three eclogites yield Ordovician U-Pb ages for rutile at 440 ± 12, 445 ± 51, and 480 ± 29 Ma, which are coeval with the Taconic Orogeny and are consistent with a Laurentian provenance for the host peridotites. Eclogites in both Norwegian and Czech peridotites originated from melts passing through a mantle wedge above a subduction zone, and both suites exhibit subduction geochemical signatures, although they differ dramatically in petrogenesis. Eclogites in Norwegian peridotites initially crystallized as relatively low–pressure, plagioclase–bearing basaltic or gabbroic dikes and subsequently recrystallized to high–pressure eclogite, whereas most eclogites in Variscan Moldanubian peridotites crystallized directly from magmas at high pressure to produce eclogite facies assemblages

Variscan ultra‐high‐pressure eclogite in the Upper Allochthon of the Rhodope Metamorphic Complex (Bulgaria)

The Rhodope Metamorphic Complex (RMC) in Bulgaria has been established as a Mesozoic ultra‐high‐pressure metamorphic province by findings of microdiamond in gneisses. Additionally, Variscan ultra‐high‐pressure metamorphism has been proposed for the Ograzhden/Vertiskos Unit in the Upper Allochthon of the RMC, based on findings of coesite, graphite pseudomorphs after diamond and indirect age constraints. We confirm ultra‐high‐pressure metamorphism of eclogites in this unit using thermobarometry, phase‐equilibrium modelling and the Variscan age of metamorphism using Lu–Hf garnet–whole‐rock dating. In Belica (southern Rila Mountains), kyanite‐ and phengite‐bearing eclogite enclosed in high‐grade gneisses records P‐T conditions of 3.0–3.5 GPa and 700–750°C. Lu–Hf dating of eclogite samples from Belica and Gega (Ograzhden Mountain), where coesite was found, yielded ages of 334.1 ± 1.8 and 334.0 ± 2.2 Ma, respectively, interpreted as the age of garnet growth during post‐collisional subduction of continental crust after closure of the Rheic Ocean.Slovak Research and Development Agency http://dx.doi.org/10.13039/501100005357University of BonnVEGA http://dx.doi.org/10.13039/50110000610

Constraining the process of intracontinental subduction in the Austroalpine Nappes: Implications from petrology and Lu‐Hf geochronology of eclogites

AbstractHigh‐ and ultrahigh‐pressure rocks occur in the Austroalpine Nappes in a ~400 km long belt from the Texel Complex in the west to the Sieggraben Unit in the east. Garnet growth during pressure increase was dated using Lu‐Hf chronometry. The results range between c. 100 and 90 Ma, indicating a short‐lived period of subduction. Combined with already published data, our estimates of metamorphic conditions indicate a field gradient with increasing pressure and temperature from the northwest to the southeast, where the rocks experienced ultrahigh‐pressure metamorphism. The P‐T conditions of the eclogites generally lie on the ‘warm’ side of the global range of subduction‐zone metamorphic conditions. The oldest Cretaceous eclogites (c. 100 Ma) are found in the Saualpe‐Koralpe area derived from widespread gabbros formed during Permian to Triassic rifting. In the Texel Complex garnets showing two growth phases yielded a Variscan‐Eoalpine mixed age indicating re‐subduction of Variscan eclogite‐bearing continental crust during the Eoalpine orogeny. Jurassic blueschist‐facies metamorphism at Meliata in the Western Carpathians and Cretaceous eclogite‐facies metamorphism in the Austroalpine are separated by a time gap of c. 50 Ma and therefore do not represent a transition from oceanic to continental subduction but rather separate events. Thus, we propose that subduction initiation was intracontinental at the site of a Permian rift.German Science Foundation (DFG)Slovak Research and Development Agency http://dx.doi.org/10.13039/50110000535