16:00 〜 16:15
[BCG06-21] Paleozoic subduction tectonics of the Mongol-Okhotsk Ocean: Detrital zircon ages and basalt geochemistry of the Gorkhi Formation, central Mongolia
キーワード：中央アジア造山帯、付加体、海洋プレート層序、砕屑性ジルコン、U-Pb年代
The Mongol-Okhotsk Belt, one of the tectonic units in the Central Asian Orogenic Belt, is distributed from central to northeastern Mongolia and has grown in the Paleozoic-Mesozoic paleo-oceanic realm surrounded by the Siberia and North China cratons. The Carboniferous accretionary complex called the Gorkhi Formation in the Mongol-Okhotsk Belt consists mainly of sandstone, mudstone, and chert with a small amount of basalt, and constitutes an ocean plate stratigraphy (OPS) (Kurihara et al., 2009; Savinskiy et al., 2022). The tectonic history and the paleo-oceanic environment of the Paleozoic Mongol-Okhotsk Ocean are not fully understood.
In this study, we investigated the outcrops of the Gorkhi Fm in the Sergelen area, reconstructed OPSs in each section, and conducted the U-Pb dating of 8 sandstones and the geochemical analysis of a basalt. The detrital zircons from sandstone samples of the Gorkhi Fm are dominant in Late Devonian-Early Carboniferous grains with few pre-Devonian ones. The U-Pb age histograms show the youngest cluster weighted mean ages from 327.0±3.5 Ma to 314.5±2.4 Ma. The results suggest that the depositional ages of the sandstones are younger than these ages, and the accretional ages of the unit at the trench are briefly in the Early Pennsylvanian. The Mongol-Okhotsk Ocean continued to subduct to the Siberian side until 314 Ma in this area. As the basalt of the Gorkhi Fm shows the hornblende K-Ar age of 412.7±8.6 Ma (Nadmid et al., 2022) and the chert preserves Late Silurian conodonts and Early-Middle-Late Devonian radiolarians (Kurihara et al., 2009), the OPS of the Gorkhi Fm records ca. 100 million years of history in the Mongol-Okhotsk Ocean. The basalt's major and trace elemental compositions show the oceanic island alkaline basalt origin, consistent with the previous study in the same area (Tsukada et al., 2013). The intraplate seamount volcanism occurred in the pelagic realm in the Mongol-Okhotsk Ocean, and the subduction of the seamounts at the trench may have caused the synclinal deformation of a central unit of the Gorkhi Fm and the collapse of the accretionary complex to form the characteristic conglomerate beds in it.
In this study, we investigated the outcrops of the Gorkhi Fm in the Sergelen area, reconstructed OPSs in each section, and conducted the U-Pb dating of 8 sandstones and the geochemical analysis of a basalt. The detrital zircons from sandstone samples of the Gorkhi Fm are dominant in Late Devonian-Early Carboniferous grains with few pre-Devonian ones. The U-Pb age histograms show the youngest cluster weighted mean ages from 327.0±3.5 Ma to 314.5±2.4 Ma. The results suggest that the depositional ages of the sandstones are younger than these ages, and the accretional ages of the unit at the trench are briefly in the Early Pennsylvanian. The Mongol-Okhotsk Ocean continued to subduct to the Siberian side until 314 Ma in this area. As the basalt of the Gorkhi Fm shows the hornblende K-Ar age of 412.7±8.6 Ma (Nadmid et al., 2022) and the chert preserves Late Silurian conodonts and Early-Middle-Late Devonian radiolarians (Kurihara et al., 2009), the OPS of the Gorkhi Fm records ca. 100 million years of history in the Mongol-Okhotsk Ocean. The basalt's major and trace elemental compositions show the oceanic island alkaline basalt origin, consistent with the previous study in the same area (Tsukada et al., 2013). The intraplate seamount volcanism occurred in the pelagic realm in the Mongol-Okhotsk Ocean, and the subduction of the seamounts at the trench may have caused the synclinal deformation of a central unit of the Gorkhi Fm and the collapse of the accretionary complex to form the characteristic conglomerate beds in it.