Detrital zircon record of the Mesoproterozoic to Lower Cambrian sequences of NW Russia: implications for the paleogeography of the Baltic interior

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb detrital zircon isotope data from Mesoproterozoic to Lower Cambrian strata of the St Petersburg region are used to characterize the paleogeographic and tectonic evolution of the Baltica continent. We dated fifteen samples and divided them into three groups based on their distribution of detrital zircons. The first group (comprising Lower Mesoproterozoic rocks) is dominated by Early Mesoproterozoic and Late Paleoproterozoic zircons, mostly derived from weathering of proximal source region including rapakivi granites exposed across the neighboring Baltic Shield. The second group includes Upper Ediacaran samples (Redkino and Kotlin Regional Stages), with major zircon populations ranging in age between 1970–1850 and 1600–1550 Ma, respectively, correlating with magmatic and metamorphic events within the Svecofennian Orogeny and rapakivi granite igneous activity in the interior of Fennoscandia. The third group of samples, collected from both the uppermost Ediacaran and lowermost Cambrian deposits (Kotlin, Lontova and Dominopol Regional Stages), contains older Paleo-Mesoproterozoic zircons as well as Late Neoproterozoic-earliest Cambrian zircons, indicating a Timanian source area and exhibiting a age spectra similar to spectra for coeval rocks of the Scandinavian Caledonides. Therefore, we conclude that reworking and transport of continental detritus from the Timanian Orogen began during Late Ediacaran, earlier than previously supposed, with transport of Timanian detritus not only to the marginal part of Baltica (known from the Scandinavian Caledonides), but also to the distal interior of Baltica.

本数据集采用采自圣彼得堡地区中元古代至下寒武统地层的激光剥蚀电感耦合等离子体质谱法（Laser ablation inductively coupled plasma mass spectrometry，LA-ICP-MS）铀-铅（U-Pb）碎屑锆石同位素数据，用以表征波罗的古大陆（Baltica）的古地理与构造演化历程。我们对15件样品开展了定年分析，并依据碎屑锆石的分布特征将其划分为3组。第一组（对应下中元古代岩石）以早中元古代和古元古代晚期锆石为主，其物质来源主要为邻近波罗的地盾（Baltic Shield）出露的拉帕基维花岗岩（rapakivi granite）等近源区的风化产物。第二组为埃迪卡拉纪晚期样品（对应雷德金诺阶与科特林区域阶），其主要锆石群体的年龄区间分别为1970~1850 Ma与1600~1550 Ma，这与斯维科芬造山带（Svecofennian Orogeny）内的岩浆与变质事件，以及芬诺斯坎迪亚（Fennoscandia）内部的拉帕基维花岗岩岩浆活动具有相关性。第三组样品采自埃迪卡拉纪最顶部与寒武纪最底部地层（对应科特林、伦托瓦与多米诺波尔区域阶），其既包含古元古代-中元古代早期的老锆石，也含有新元古代晚期至寒武纪最早期的锆石，这指示其物质来源为蒂曼造山带（Timanian Orogen），且年龄谱特征与斯堪的纳维亚加里东造山带（Scandinavian Caledonides）的同期岩石高度相似。据此我们得出结论：来自蒂曼造山带的大陆碎屑物的再改造与搬运作用始于埃迪卡拉纪晚期，早于此前的学术认知；且蒂曼碎屑物不仅被搬运至波罗的古大陆的边缘地带（该结论已在斯堪的纳维亚加里东造山带的研究中得到证实），还被搬运至波罗的古大陆的远端内部区域。