Abstract
A geochronological, isotopic, and geochemical study of the Suordakh event of mafic magmatic intrusions on the southeast Siberian margin was undertaken. U-Pb baddeleyite dating of a mafic sill intruding lower Cambrian rocks, yielded a 458 ± 13 Ma emplacement age. The chemical composition and stratigraphic setting of this dated sill differed from that previously attributed to the Suordakh event, implying that additional intrusions, previously mapped as Devonian, potentially belonged to the Suordakh event. No correlation between L.O.I. and concentration of highly mobile major and trace elements was documented, showing small or no influence of hydrothermal alteration on the chemical composition of the intrusions. A new tectonic reconstruction located an island arc and active margin relatively close to the study area. However, all samples had chemical compositions close to that of OIB and did not display Ta-Nb and Ti-negative anomalies, nor other features typical for subduction-related magmatism. The major and trace element distribution was most characteristic of within-plate basalts with the mantle source composition being transitional from spinel to garnet lherzolite. Combining four U-Pb baddeleyite dates of mafic sills and dykes from southeast Siberia, the age of the Suordakh event was estimated at 454 ± 10 Ma. The area of the Suordakh event was at least 35,000–40,000 km2 (an estimate including sills previously interpreted as Devonian), and could be increased with additional dating in Southeastern Siberia. Similar ages for within-plate intrusions were reported from South Korea, West Mongolia, South Argentina, North Iran and Northwest Canada, and these ca. 450 Ma ages were collectively close in timing with the latest Ordovician (Hirnantian) mass extinction. More high-precision dating is necessary to fully test a link between the Suordakh event (and the other age-correlative events) and the end-Ordovician mass extinction.
Highlights
The southeastern margin of the Siberian Craton is commonly known as a passive margin with well-recognized early Neoproterozoic and Middle-Late Devonian rifting events associated with mafic magmatism, normal faulting, and deposition of clastic rocks and evaporites [1,2,3,4]
U-Pb baddeleyite dating of a small mafic sill cutting an Ediacaran (Vendian) sedimentary succession for the first time, identified a Late Ordovician mafic magmatic event commonly known as the Suordakh [7,8]
Recent tectonic reconstructions [11] located an island arc of similar age nearby, leading to uncertainty regarding the tectonic setting of the Suordakh event—extension or subduction related
Summary
The southeastern margin of the Siberian Craton is commonly known as a passive margin with well-recognized early Neoproterozoic (ca. 1000–950 Ma) and Middle-Late Devonian rifting events associated with mafic magmatism, normal faulting, and deposition of clastic rocks and evaporites [1,2,3,4]. 1000–950 Ma) and Middle-Late Devonian rifting events associated with mafic magmatism, normal faulting, and deposition of clastic rocks and evaporites [1,2,3,4]. In contrast to these events, most of the Ordovician and Silurian are reported as a period of tectonic quiescence, with deposition of thick units of carbonates with numerous fossils and reef-like build-ups [3,4,5,6]. U-Pb baddeleyite dating of a small mafic sill cutting an Ediacaran (Vendian) sedimentary succession for the first time, identified a Late Ordovician mafic magmatic event commonly known as the Suordakh [7,8]. Recent tectonic reconstructions [11] located an island arc of similar age nearby, leading to uncertainty regarding the tectonic setting of the Suordakh event—extension or subduction related.
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