Stratigraphic Sequences and Magmatic Cycles of the Tianchi Volcano, Changbaishan

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The Junjianshan Basalts has been subdivided into 3 units as the Toudao, Baishan and Laofangzixiaoshan Basalts to reconstruct the process of the shield-forming of the Tianchi Volcano, after the plateau-forming eruption. Here we define the Toudao F as the thick and long reached basaltic lava flows on the north and northwest part of the Tianchi Shield. As a representative profile we present one at Yaoshui along the Toudao River from which K-Ar ages of 2.29, 2.35, 5.02 and 5.82 Ma are dated. The basalts of the Baishan F compose the main terrain of the Tianchi Shield. These lava flows vary in thickness and ages, such as 1.28 and 1.39Ma. The Xiaobaishan Trac hyandecites-Trachytes, distributed mainly on the southern Tianchi Trachyte Cone and the northern base of the Cone, are representative of trachytic episodes of the eruption before 0.81, 0.92 and 1.19Ma. Basalts from the Laofangzixiaoshan F, distributed on the northeast, northwest and southwest sides of the Tianchi Lava Shield, make up a part of the surface of the Tianchi Shield, on which several scoria cones and flat craters are situated. The K-Ar ages for the Laofangzixiaoshan F are generally very young compared with the Toudao and Baishan Formations; such as the northeastern lava flow ages of 0.23, 0.31, 0.34, 0.35 and 0.47Ma and the southwestern lavas dated as 0.4 and 0.42Ma. These lava flows are equivalent to the parasitic Laohudong basalts in 0.34 and 0.32Ma. Materials erupted in the Baitoushan stage constructed the strata cone with a largest thickness of about 600m and are grouped into 3 members; Lower, Middle and Upper Members of the depositional sequence. The Holocene volcanic strata of the Tianchi volcano are listed as Bingchang, Qixiangzhan, Baiyunfeng and Baguamiao Formations. The Bingchang F, of welded pyroclastic rocks of trachyte and comendite, locates at the depressions of Bingchang, Gudisenlin and Baishanqiao. The Qixiangzhan F is a typical clastogenic lava flow originating from satellite comenditic volcanism near the top of the Tianchi Cone, while the Baiyunfeng F is of comenditic plinian fallout and ignimbrite deposits that are widely distributed on the surface of the Tianchi volcano. The Baguamiao F is represented as dark trachytic fallout and ignimbrite deposits mantling the surface and thickening in the valleys. Eruptions from the Tianchi Volcano are reported to have taken place in 1668, 1702 and 1903 AD. The Tianchi Volcano is a completely evolved magma system showing evolutionary trends as the volcanism varies with time. We note that the mechanism of magma evolutionary exhibits a successive differentiation trend and magma mixing process. This mechanism is definitely different from the formal bi-modal distribution of the magma composition. In the early cycle of trachytic magma production, a trachyte sample is obtained from the Daewoo Hotel borehole. In the middle cycle, basaltic magmas evolved into the trachytes and trachyandesites of the Xiaobaishan Formation. The late trachyte cycle comprised the main edifice of the Tianchi stratovolcano and had a distinct explosive regime change from trachyte to comendite.論文Article
東北大学の論文
2007-03-30

Stratigraphic Sequences and Magmatic Cycles of the Tianchi Volcano, Changbaishan

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