The first find of microdiamond in the Nordøyane ultra-high-pressure (UHP) domain of the Western Gneiss Region (WGR) of the Scandinavian Caledonides reshaped tectonic models for the region. Nevertheless, in spite of much progress regarding the meaning and significance of this find, the history of rock that the diamonds were found in is complex and still largely ambiguous. To investigate this, we report U–Pb zircon ages obtained from the exact crushed sample material in which metamorphic diamond was first found. The grains exhibit complicated internal zoning with distinct detrital cores overgrown by metamorphic rims. The cores yielded a range of ages from the Archaean to the late Neoproterozoic / early Cambrian. This detrital zircon age spectrum is broadly similar to detrital signatures recorded by metasedimentary rocks of the Lower and Middle allochthons elsewhere within the orogen. Thus, our dating results support the previously proposed affinity of the studied gneiss to the Seve–Blåhø Nappe of the Middle Allochthon. Metamorphic rims yielded a well-defined peak at 447 ± 2 Ma and a broad population that ranges between c. 437 and 423 Ma. The data reveal a prolonged metamorphic history of the Fjørtoft gneiss that is far more complex then would be expected for a UHP rock that has seen a single burial and exhumation cycle. The data are consistent with a model involving multiple such cycles, which would provide renewed support for the dunk tectonics model that has been postulated for the region.
In-situ monazite Th-U-total Pb dating and zircon LA-ICP-MS depth-profiling was applied to metasedimentary rocks from the Vaimok Lens in the Seve Nappe Complex (SNC), Scandinavian Caledonides. Results of monazite Th-U-total Pb dating, coupled with major and trace element mapping of monazite, revealed 603 ± 16 Ma Neoproterozoic cores surrounded by rims that formed at 498 ± 10 Ma. Monazite rim formation was facilitated via dissolution-reprecipitation of Neoproterozoic monazite. The monazite rims record garnet growth as they are depleted in Y 2 O 3 with respect to the Neoproterozoic cores. Rims are also characterized by relatively high SrO with respect to the cores. Results of the zircon depth-profiling revealed igneous zircon cores with crystallization ages typical for SNC metasediments. Multiple zircon grains also exhibit rims formed by dissolution-reprecipitation that are defined by enrichment of light rare earth elements, U, Th, P, ± Y, and ± Sr. Rims also have subdued Eu anomalies (Eu/Eu* ≈ 0.6-1.2) with respect to the cores. The age of zircon rim formation was calculated from three metasedimentary rocks: 480 ± 22 Ma; 475 ± 26 Ma; and 479 ± 38 Ma. These results show that both monazite and zircon experienced dissolution-reprecipitation under high-pressure conditions. Caledonian monazite formed coeval with garnet growth during subduction of the Vaimok Lens, whereas zircon rim formation coincided with monazite breakdown to apatite, allanite and clinozoisite during initial exhumation.
Garnet and zircon geochronology combined with trace element partitioning and petrological studies provide tight constraints on evolution of the UHT-(U)HP terrain of the Orlica-Śnieżnik Dome (OSD) in the NE Bohemian massif. Lu-Hf dating of peritectic garnet from two mesocratic granulites constrained the time of its initial growth at 346.9±1.2 and 348.3±2.0 Ma recording peak 2.5 GPa pressure and 950 °C temperature. In situ, U-Pb SHRIMP dating of zircon from the same granulite gave a younger age of 341.9±3.4 Ma. Tiin-zircon thermometry indicates crystallization at 810-860 °C pointing to zircon formation on the retrograde path. Lu partitioning between garnet rim and zircon suggest equilibrium growth and thus U-Pb zircon age constrain the terminal phase of garnet crystallization which lasted about 6 Ma. All Sm-Nd garnet ages obtained for mesocratic and mafic granulites are identical and consistently younger than the corresponding Lu-Hf dates. They are interpreted as reflecting cooling of granulites through the Sm-Nd closure temperature at about 337 Ma. The estimated PTt path documents the ca. 10 Ma evolution cycle of the OSD characterized by two distinct periods: (1) 347->342 Ma period corresponds to nearly isothermal decompression resulting from crustal scale folding and vertical extrusion of granulites, and (2) at >342-337 Ma which corresponds to a fast, nearly isobaric cooling.
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