P-T history and zircon geochronology of a felsic gneiss hosting ultrahigh pressure metamorphic rocks from the Tromso Nappe, Caledonian orogenic belt

第8回極域科学シンポジウム/個別セッション：[OG] 極域地圏12月6日（水）国立極地研究所 3階セミナー室The Eighth Symposium on Polar Science/Ordinary sessions: [OG] Polar GeosciencesWed. 6 Dec./3F Seminar room, National Institute of Polar Researc

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

The northern extent of the Scandinavian Caledonides includes the Skibotn Nappe Complex of still debatedstructural position. This paper is focused on part of this complex and presents new U–Th–total Pb monazite dating resultsfor the migmatitic gneiss of the Kåfjord Nappe. The rocks show mineral assemblage of garnet + plagioclase + biotite +white mica + kyanite + rutile ± K-feldspar ± sillimanite. Thermodynamic modelling suggests that garnet was stable at P–Tconditions of ca. 680–720 °C and 8–10 kbars in the stability field of kyanite and the rocks underwent partial meltingduring exhumation following a clockwise P–T path. This episode is dated to 444 ± 12 Ma using chemical Th–U–total Pbdating of the Y-depleted monazite core. Second episode highlighted by growth of secondary white mica resulted fromsubsequent overprint in amphibolite and greenschist facies. Fluid assisted growth of the Y-enriched monazite rim at419 ± 8 Ma marks the timing of the nappe emplacement. Age of migmatization and thrusting in the Kåfjord Nappe issimilar to the Kalak Nappe Complex, and other units of the Middle Allochthon to the south. Nevertheless, the obtainedresults do not allow for unambiguous definition of the tectonostratigraphic position of the Skibotn Nappe Complex

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 degrees 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

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

High- and ultrahigh-pressure rocks occur in the Austroalpine Nappes in a similar to 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

Timing of eclogite-facies metamorphism of mafic and ultramafic rocks from the Pohorje Mountains (Eastern Alps, Slovenia) based on Lu-Hf garnet geochronometry

The metamorphic series of the Pohorje Mountains represents a part of the Eastern Alpine realm that was subjected to ultrahigh-pressure conditions during the Cretaceous Eo-Alpine orogenic cycle. The Slovenska Bistrica Ultramafic Complex located in the south-eastern Pohorje Mountains is an 8 km wide serpentinite body that contains lenses of garnet-bearing ultramafites and eclogites. It is embedded in and part of a mixed continental unit of metapelitic gneisses, orthogneisses, and eclogites. We present Lu-Hf garnet chronometry coupled with geochemical and petrological data from three samples: one garnet lherzolite, one eclogite from within the ultramafic complex, and one eclogite from the surrounding mixed unit. All obtained ages are identical within error, i.e. 96.6 +/- 1.2 Ma and 94.8 +/- 5.1 Ma, respectively, for the two eclogites and 91.6 +/- 4.1 Ma for the garnet lherzolite. Garnet of all samples shows homogeneous concentrations of major bivalent elements due to high temperature re-equilibration. It does, however, preserve growth-related zoning with respect to Lu in all three samples implying that Lu-Hf ages still record garnet growth. The coincidence of ages suggests that the ultramafic complex and the surrounding continental mixed unit share the same subduction history, i.e. the complex was part of the subducting plate during and after the garnet growth stages. (C) 2016 Elsevier B.V. All rights reserved

Late Cretaceous eclogite in the Eastern Rhodopes (Bulgaria): evidence for subduction under the Sredna Gora magmatic arc

The Rhodopes in Bulgaria and Greece represent a nappe stack of high-grade units with polymetamorphic history. Constraining the time of metamorphism in individual subunits is essential for unraveling the controversial framework of subduction, exhumation and nappe stacking. Here we present new evidence for Late Cretaceous high-pressure metamorphism in the Eastern Rhodopes. In eclogite from the Byala Reka-Kechros Dome (Kazak eclogite), garnet growth is dated at 81.6 +/- 3.5 Ma by Lu-Hf chronometry, indicating that prograde eclogite-facies metamorphism occurred during the Late Cretaceous. Petrological data and modeling suggest peak-pressure conditions of 1.2-1.6 GPa, 570-620 A degrees C. We propose that metamorphism took place in a subduction zone dipping towards north under the Sredna Gora section of the Apuseni-Banat-Timok-Sredna Gora continental magmatic arc. Eclogite-facies metamorphism coincides with the main phase of granitoid intrusions in the Sredna Gora Zone. The site of magmatic activity in this area shifted southward during the Late Cretaceous and arrived in the Eastern Rhodopes at similar to 69 Ma, as shown by granite intrusions of that age only 4 km north of the locality of the dated eclogite sample. This proximity may be explained by south-directed rollback of the subduction zone, although also post-69 Ma tectonic displacement has to be considered. Together with published age data from other parts of the Rhodopes, the new data confirm that multiple subduction events took place between similar to 200 and similar to 40 Ma along this section of the southern European plate boundary

Devonian subduction and syncollisional exhumation of continental crust in Lofoten, Norway

When continents collide, continental crust of the lower plate may be subducted to mantle depth and return to the surface to form eclogite facies metamorphic terranes, as typified by the Western Gneiss Complex of the Scandinavian Caledonides. Proterozoic basement of the Lofoten Islands, located northeast and along strike of the Western Gneiss Complex, contains Caledonian eclogite, although Caledonian deformation is only minor. Previous dating suggested that Lofoten eclogites formed at ca. 480 Ma, i.e., similar to 50 Ma before the collision between the major continents Baltica and Laurentia, and that the Lofoten basement may not originate from Baltica but rather represents a stranded microcontinent. Newly discovered kyanite eclogites from the Lofoten Islands record deep subduction of continental crust during the main (Scandian) stage of Baltica-Laurentia collision ca. 400 Ma. Thermobarometry and thermodynamic modeling yield metamorphic conditions of 2.5-2.8 GPa and similar to 650 degrees C. Lu-Hf geochronology yields 399 +/- 10 Ma, corresponding to the time of garnet growth during subduction. Our results demonstrate that the Lofoten basement belonged to Baltica, was subducted to similar to 90 km depth during the collision with Laurentia, and was exhumed at an intermediate to high rate (>6 mm/yr) while thrusting of a Caledonian allochthon (Leknes Group) was still ongoing. This supports the challenging conclusions that (1) subducted continental crust may stay rigid down to a depth of similar to 90 km, and (2) it may be exhumed during ongoing collision and crustal shortening