Metamorphic history of the Precambrian lower cratonic crust from U-Pb dating of granulite xenoliths (Anabar province, Siberia)

Garnet granulite xenoliths from the Nurbinskaya diatreme in the central part of the Archean Anabar province in Siberia are fragments of the local lower crust that experienced multiple metamorphic events in the Paleoproterozoic and reheating events in the Mesoproterozoic and later. This study addresses the timing of metamorphic transformations, and constrains the cooling rate and the time of stabilization of the lower crust. The observed metamorphic mineral assemblage of garnet, clinopyroxene, plagioclase, amphibole, rutile and ilmenite was formed at ~800°C, 1.1-1.2 GPa under water-undersaturated conditions at ~1.88 Ga. However, the mineral assemblage is not well equilibrated and retains evidence of earlier and subsequent metamorphic stages. Late titanite formed in response to hydrous fluid influx according to phase equilibria modeling. U-Pb dating shows two events of titanite formation at 1850 ±5 Ma and at 1788±2 Ma. After deformation, which led to the porphyroclastic rock textures, the granulites underwent near-isobaric cooling. The cooling rate was higher than ~6°C/Myr, to retain the garnet compositional zoning. Rutile ages are discordant, with 207Pb/206Pb dates ranging from 1.43 to 1.53 Ga. However, rutile may have responded to earlier thermal pulses, and was also reset later, so it does not record the stabilization of the crust. Crustal stabilization after Paleoproterozoic orogenic events may have occurred shortly after titanite formation

1.86 Ga key paleomagnetic pole from the Murmansk craton intrusions – Eastern Murman Sill Province, NE Fennoscandia: Multidisciplinary approach and paleotectonic applications

© 2019 Elsevier B.V. We present the first 1.86 Ga paleomagnetic key pole of Fennoscandia obtained for the dolerite sills of the Murmansk craton – Eastern Murman Sill Province, that outcrop in the northern part of the Kola Peninsula along the Barents Sea coast for a distance of 200 km (Slat = 68.5°; Slong = 37.9°; N = 16 sites; Plat = 54.7°; Plong = 234.7°; dp/dm = 4.3°/6.3° Qv = 5). The age of the sills and their characteristic remanent magnetization (ChRM) was determined by four independent geochronometers: U-Pb – 1860 ± 4 and 1863 ± 7 Ma (ID-TIMS, baddeleyite), Sm-Nd – 1889 ± 57 Ma, Rb-Sr – 1850 Ma, Ar/Ar – 1865 ± 8 and 1857 ± 20 Ma (biotite). The primary nature of the ChRM is confirmed by the results of petrographic, geochemical, paleo- and rock magnetic studies, as well as by thermochronological data. The similarity of the petrographic and geochemical characteristics of sills from different localities indicates that these dolerite sills were formed during a single magmatic event and their cooling down to 580 °C occurred at depths of about 10 ± 2 km and lasted ∼2800 years or even faster. Paleogeographic reconstruction of Fennoscandia on the basis of the obtained paleomagnetic pole is in general agreement with the previously suggested configuration of core of the Nuna/Columbia supercontinent (Evans and Mitchell, 2011; Meert and Santosh, 2017). A new reliable Thellier-Coe paleointensity determination for this time reveals a rather low mean VDM = 1.8 (±0.1) × 10 22 Am 2 that supports the Proterozoic dipole low hypothesis (Biggin et al., 2009)

1.86 Ga key paleomagnetic pole from the Murmansk craton intrusions – Eastern Murman Sill Province, NE Fennoscandia: Multidisciplinary approach and paleotectonic applications

© 2019 Elsevier B.V. We present the first 1.86 Ga paleomagnetic key pole of Fennoscandia obtained for the dolerite sills of the Murmansk craton – Eastern Murman Sill Province, that outcrop in the northern part of the Kola Peninsula along the Barents Sea coast for a distance of 200 km (Slat = 68.5°; Slong = 37.9°; N = 16 sites; Plat = 54.7°; Plong = 234.7°; dp/dm = 4.3°/6.3° Qv = 5). The age of the sills and their characteristic remanent magnetization (ChRM) was determined by four independent geochronometers: U-Pb – 1860 ± 4 and 1863 ± 7 Ma (ID-TIMS, baddeleyite), Sm-Nd – 1889 ± 57 Ma, Rb-Sr – 1850 Ma, Ar/Ar – 1865 ± 8 and 1857 ± 20 Ma (biotite). The primary nature of the ChRM is confirmed by the results of petrographic, geochemical, paleo- and rock magnetic studies, as well as by thermochronological data. The similarity of the petrographic and geochemical characteristics of sills from different localities indicates that these dolerite sills were formed during a single magmatic event and their cooling down to 580 °C occurred at depths of about 10 ± 2 km and lasted ∼2800 years or even faster. Paleogeographic reconstruction of Fennoscandia on the basis of the obtained paleomagnetic pole is in general agreement with the previously suggested configuration of core of the Nuna/Columbia supercontinent (Evans and Mitchell, 2011; Meert and Santosh, 2017). A new reliable Thellier-Coe paleointensity determination for this time reveals a rather low mean VDM = 1.8 (±0.1) × 10 22 Am 2 that supports the Proterozoic dipole low hypothesis (Biggin et al., 2009)