LA-ICP-MS ТРЕКОВОЕ ДАТИРОВАНИЕ АПАТИТА ИЗ ИНТРУЗИВНЫХ ТЕЛ СИБИРСКОЙ ТРАППОВОЙ ПРОВИНЦИИ: МЕТОД, ПЕРВЫЕ РЕЗУЛЬТАТЫ И ИХ ИНТЕРПРЕТАЦИЯ

In this paper we present the results on apatite fission-track dating, which was first performed at the Schmidt Institute of Physics of the Earth, Russian Academy of Sciences, using the laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS, Dobretsov Geological Institute, Siberian Branch of the Russian Academy of Sciences) for two intrusions of the Siberian Permian-Triassic large igneous province: Magan alkaline-ultramafic pluton (two samples) and Kontay intrusion (one sample). The obtained AFT ages are 217.6±18.6 and 238.8±35.8 Ma (95% confidence) for the Magan intrusion and 150.0±23.0 Ma for the Kontay intrusion and mark the time since they have been cooled below 120 °C. The distributions of track lengths in apatite grains from the studied samples indicate their rapid cooling to near-surface temperatures. We provide a detailed description of the method used, and also demonstrate that the results of fissiontrack analysis performed on the "sample-to-sample" principle by the classical external detector method (EDM) and the LA-ICP-MS method in the modification of the zeta calibration coincide within the age uncertainty.В статье представлены результаты трекового анализа апатита, впервые выполненного в Институте физики Земли им. О.Ю. Шмидта РАН с использованием метода масс-спектроскопии с индуктивно связанной плазмой и лазерной абляцией (LA-ICP-MS, Геологический институт им. Н.Л. Добрецова Сибирского отделения РАН) для двух интрузивных тел Сибирской пермско-триасовой трапповой провинции: щелочно-ультраосновного плутона Маган (две пробы) и Контайской интрузии (одна проба). Полученный трековый возраст составляет 217.6±18.6 и 238.8±35.8 млн лет – для интрузии Маган и 150.0±23.0 млн лет (2σ) для Контайской интрузии и маркирует время, прошедшее с момента их остывания ниже 120 °С. Распределение длин треков в зернах апатита из исследованных образцов свидетельствует об их быстром остывании до приповерхностных температур. Приводится подробное описание используемой методики, а также показано, что результаты трекового анализа, выполненного по принципу «образец-в-образец» классическим методом внешнего детектора и методом LA-ICP-MS в модификации зета-калибровки, совпадают в пределах погрешности метода

Geomagnetic Secular Variations at the Permian-Triassic Boundary and Pulsed Magmatism During Eruption of the Siberian Traps

©2019. American Geophysical Union. All Rights Reserved. The tempo of Large Igneous Province emplacement is crucial to determining the environmental consequences of magmatism on the Earth. Based on detailed flow-by-flow paleomagnetic data from the most representative Permian-Triassic Siberian Traps lava stratigraphy of the northern Siberian platform, we present new constraints on the rate and duration of the volcanic activity in the Norilsk and Maymecha-Kotuy regions. Our data indicate that volcanic activity there occurred during a limited number of short volcanic pulses, each consisting of multiple individual eruptions, and that the total duration of discrete eruption pulses did not exceed ~10,000 years (hiatuses are not included). Our study confirms the occurrence of a thick interval in the lower part of the Norilsk lava sections, which contains a record of geomagnetic reversal and excursion. Based on combined evidence from paleomagnetic secular variation and typical timescales for such reversals, we conclude that the ~1-km-thick lava stratigraphy, corresponding to ~20,000 km 3 of basalt, of the Kharaelakh, Norilsk, and Imangda troughs was formed during a brief, but voluminous, eruptive period of several thousand years or less. Our data further suggest that the ore-bearing Norilsk-type intrusions are coeval or nearly coeval with the boundary between the Morongovsky and Mokulaevsky formations. We calculated a new Siberian Permian-Triassic paleomagnetic pole Norilsk-Maymecha-Kotuy (NMK): PLat = 52.9°, PLong = 147.1°, A95 = 4.3°, K = 23.2, and N = 49 lava flows. It is shown that geomagnetic field variations circa 252 Ma were similar to those observed in the latest Cenozoic

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

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) × 1022 Am2 that supports the Proterozoic dipole low hypothesis (Biggin et al., 2009)