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The petrochronological records of eclogites in the Scandinavian Caledonides are investigated using EPMA and LA-ICPMS of zircon and apatite for U–Pb geochronology, combined with major and trace element characteristics. Metamorphic zircon from two eclogites from the Lofoten-Vesterålen Complex (Lofoten Archipelago region) collectively yielded a Concordia age 427.8 ± 5.7 Ma and an upper intercept U–Pb age 425 ± 30 Ma. Apatites from the same eclogites provided U–Pb lower intercepts at 322 ± 28 Ma and 354 ± 33 Ma, with the latter also yielding a younger age of 227 ± 24 Ma. Two eclogites from the Lower Seve Nappe (Northern Jämtland) demonstrate different zircon and apatite age records. Metamorphic zircon provided Concordia ages of 467.2 ± 5.9 Ma and 444.5 ± 5.5 Ma, which resolve the age of prograde metamorphism and zircon growth during retrogression, respectively. The lower intercept U–Pb ages of apatites from the same eclogites are 436 ± 18 and 415 ± 25 Ma, respectively. In combination with their geochemical characteristics, they suggest two separate stages of exhumation of eclogite bodies in the Lower Seve Nappe. Zircons from an eclogite from the Blåhø Nappe (Nordøyane Archipelago) yielded a continuum of concordant U–Pb dates from ca. 435 to 395 Ma, which suggests several cycles of HT metamorphism within short intervals. Distinctive trace element characteristics of apatites from the Blåhø Nappe eclogite suggest formation coeval with zircon and garnet during HT metamorphism, but Pb diffusion behaved as an open system until cooling during exhumation of the nappe at 390 ± 12 Ma (lower intercept U–Pb age of apatite). To summarize, this study presents the high potential of coupled zircon and apatite petrochronology of eclogites in resolving their metamorphic evolution, particularly with respect to using trace element characteristics of apatites to constrain the records of their growth, alterations and the meaning of their U–Pb age record. [ABSTRACT FROM AUTHOR]

Detailed X‐ray compositional mapping and microtomography have revealed the complex zoning and growth history of garnet in a kyanite‐bearing eclogite. The garnet occurs as clusters of coalesced grains with cores revealing slightly higher Ca and lower Mg than the rims forming the coalescence zones between the grains. Core regions of the garnet host inclusions of omphacite with the highest jadeite, and phengite with the highest Si, similar to values in the cores of omphacite and phengite located in the matrix. Therefore, the core compositions of garnet, omphacite, and phengite have been chosen for the peak pressure estimate. Coupled conventional thermobarometry, average P–T, and phase equilibrium modelling in the NCKFMMnASHT system yields P–T conditions of 26–30 kbar at 800–930°C. Although coesite is not preserved, these P–T conditions partially overlap the coesite stability field, suggesting near ultra‐high–pressure (UHP) conditions during the formation of this eclogite. Therefore, the peak pressure assemblage is suggested to have been garnet–omphacite–kyanite–phengite–coesite/quartz–rutile. Additional lines of evidence for the possible UHP origin of the Międzygórze eclogite are the presence of rod‐shaped inclusions of quartz parallel to the c‐axis in omphacite as well as relatively high values of Ca‐Tschermak and Ca‐Eskola components. Late zoisite, rare diopside–plagioclase symplectites rimming omphacite, and minor phlogopite–plagioclase symplectites replacing phengite formed during retrogression together with later amphibole. These retrograde assemblages lack minerals typical of granulite facies, which suggests simultaneous decompression and cooling during exhumation before the crustal‐scale folding that was responsible for final exhumation of the eclogite. [ABSTRACT FROM AUTHOR]

Even though the high pressure – low temperature (HP-LT) metamorphism on Svalbard has been documented already in 1960' in its type locality on Oscar II Land, still little is known about the metamorphic history of these HP-LT lithologies. The Vestgötabreen Complex is divided into two units based on relative differences in lithological assemblage and metamorphic grade. The Lower Unit consists mainly of phyllites with lenses of dolomite, quartzite, metabasite and serpentinite, whereas the Upper Unit comprises blocks of blueschist, eclogite, and lenses of marble surrounded by garnet-mica schist (Ohta et al. 1986).The suite of HP-LT lithologies from the Lower Unit was collected during the summer 2018 from three areas: 1) an unnamed peak, further called here as "Anticline" mountain, 2) Skipperryggen, and 3) Motalafjella. The collected rocks are composed of sodic amphibole, white mica, epidote, chlorite and minor rutile. Mica, defining the S1 foliation, is phengitic in composition and characterized by high Si content ranging from 3.39 to 3.48 apfu. Sodic amphibole is classified as glaucophane with Na content increasing from the core (1.76 apfu) to the rim (1.86 apfu). The rim of glaucophane is partly replaced by late actinolite. Two generations of chlorite have been identified. Chlorite-I forms flakes inside glaucophane and chlorite-II overgrowths Na-amphibole and mica. Rutile is partly replaced by titanite. The studied samples represent the HP-LT rocks which were formed under lower blueschist facies conditions (M1) and overprinted by greenschist facies assemblages (M2). The latter is demonstrated by growth of chlorite-II, actinolite and titanite.Our preliminary observations confirm that the Lower Unit was metamorphosed along a very low temperature gradient as previously proposed by Agard et al. (2005). The main future focus of this study is to reconstruct detailed pressure-temperature evolutionary path of this unique unit, characterize geochemically protoliths of especially metabasites and calculate subduction-exhumation rates. This work is financially supported by NCN research project no. 2015/17/B/ST10/03114 and AGH-UST statutory funds 11.11.140.158.References:Agard, P., Labrousse, L., Elvevold, S. and Lepvrier, C., 2005. Discovery of Palaeozoic Fe–Mg carpholite (Motalafjella, Svalbard Caledonides): a milestone for subduction zone gradients. Geology, 33, 761–764.Ohta, Y., Hirajima, T. and Hiroi, Y., 1986. Caledonian high-pressure metamorphism in central western Spitsbergen. Geol. Soc. Am. Mem., 164, 205–216. [ABSTRACT FROM AUTHOR]

The Tsäkkok Lens belongs to the Seve Nape Complex which constitutes the upper part of the Middle Allochthon of the Scandinavian Caledonides and represents the outermost parts of the rifted passive margin of Baltica. The Tsäkkok eclogites, hosted by mica schists and marbles, preserve structures of pillow basalt with a tholeiitic E-MORB affinity (Kullerud et al. 1990), which indicates an oceanic or ocean-continent transition environment. Eclogites record pervasive brittle fracturing at high-pressure conditions, from the single-grain to the outcrop scale. X-ray mapping of garnet revealed a network of healed micro-fractures and vermicular micro-channels propagating outward from inclusions of pseudomorphs after lawsonite. Additionally, garnet exhibits larger dilational fractures that are filled with omphacite (<48 Jd mol.%) ± phengite (3.47 Si apfu) whose crystallization was facilitated by fluids. Eclogite lenses are also cut by a network of two types of veins, presumably formed at high-pressure conditions: (i) <2 mm thick garnetite veins, (ii) ~1 cm thick garnetite veins containing abundant remnants of interstitial glaucophane ± omphacite (among others). The composition of garnet rims, omphacite and phengite infilling the fractures in garnet were utilized for P-T estimates based on the garnet-clinopyroxene Fe2+-Mg exchange thermometer and the garnet-phengite-omphacite net-transfer reaction barometer (Ravna & Terry, 2004) and yielded a pressure of 22.2 ± 3.2 kbar and a temperature of 590 ± 60°C. Zr-in-rutile geothermometer (Tomkins et al. 2007) has also been used. The calculated temperature for both, rutile inclusions in garnet and matrix rutile show a broad range (possibly related to recrystallization along a prograde path) varying from 480°C to 680°C. The maximum pressure of metamorphism might be higher than estimated here, but further studies are required to pinpoint the peak P-T conditions.To summarize, the Tsäkkok eclogites record burial in a cold subduction zone and exhibit a wide range of lithological subtypes representing a transition from blueschist to eclogite facies rocks. They bear a record of pervasive dehydration reactions and subsequent hydrofracturing that happened along the prograde part of the clockwise pressure - temperature path.Research funded by NCN project no. 2014/14/E/ST10/00321 and AGH research grant no. 15.11.140.237.References: Kullerud, K., Stephens, M.B., Zachrisson, E. (1990) Contr. to Mineralogy and Petrology, 105, 1-10.Ravna E.J., Terry M.P. (2004) Journal of Metamorphic Geology 22: 579-592.Tomkins H.S., Powell R., Ellis J.D. (2007) Journal of Metamorphic Geology 25: 703-713. [ABSTRACT FROM AUTHOR]