美濃帯西部，舟伏山地域の上部三畳系珪質ミクライト

Abstract

Siliceous micrite crops out in close association with bedded chert and nodular chert in the Middle Jurassic melange of the Mino terrane in the Mt. Funabuseyama area, central Japan. The siliceous micrite and chert form a characteristic lithologic association of a siliceous micrite-chert facies ca. m thick in a bedded chert succession. The siliceous micrite is light gray and occurs as thin, lenticular beds in thin-bedded chert and as irregular-shaped blocks in thick-bedded to massive chert. The siliceous micrite is described as radiolarian limemudstone that includes subordinate thin-shelled bivalves within a limemud matrix. Conodont and radiolarian fossils indicate a Carnian to lower Norian age for the siliceous micrite and associated bedded chert.We interpret the Upper Triassic siliceous micrite as deep-water sediment deposited in a pelagic setting within a mid-oceanic realm. This interpretation implies that short-lived fluctuations of the carbonate compensation depth occurred in Carnian to early Norian time, probably due to the intermittent accumulation of lime mud derived from calcareous nannoplankton.The Upper Triassic siliceous micrite-chert facies differs in containing siliceous micrite from the coeval bedded chert of the oceanic plate stratigraphy of the Mino terrane. We postulate that compared with the Upper Triassic chert, the siliceous micrite-chert facies accumulated on a topographic high at relatively shallow water-depths, probably near the carbonate compensation depth. The Upper Triassic siliceous micrite-chert facies is inferred to have accumulated on the lower slope of a mid-oceanic seamount. We suggest an alternative age of Early Permian or Middle-Late Triassic for the formation of the mid-oceanic seamount. Abstract

Origin of Alkaline Basaltic Intrusive Rocks in an Exhumed Accretionary Complex: Implications for Past Petit‐Spot Volcanism in the Ocean

“…The depositional age of micritic limestone remains unconstrained due to the replacement of radiolarians with recrystallized calcite. However, the mineral assemblage and texture of the micritic limestone resemble Upper Triassic siliceous micrites in Middle Jurassic to Early Cretaceous accretionary complexes of Southwest and Central Japan, which represent an appearance of calcareous nannoplankton in the pelagic region of Panthalassa Ocean during the Late Triassic (Onoue & Yoshida, 2010; H. Sano et al., 2010, 2017; Yoshida & Onoue, 2008). Because terrigenous materials are absent, the BLM may originate from the mixing of micritic limestone and basalt in a submarine landslide on the lower slope of a seamount above the CCD (Figure 12a) (e.g., Matsuda & Ogawa, 1993).…”

Depositional setting of the Upper Triassic siliceous micrite of the Mino-Tamba Belt

“…Funabuseyama. The former three localities are where Sano et al (2010) reported Upper Triassic siliceous micrite. Wakita, 1988;Yamagata, 1989Yamagata, 1989 .…”

“…primitius E. quadrata . Numbered sections are as follows: 1, siliceous micrite of the Kugo Limestone of this study; 2, siliceous micrite-chert facies of the Mino Belt, tentatively named Shiraiwadani Limestone in this paper (after Sano et al, 2010); 3, siliceous micritechert facies of the Tamba Belt, tentatively named Ashimidani Limestone in this paper (after Kosaka and Ishiga, 1996); and 4, radiolarian bedded chert facies of the middle part of the Lower Triassic to Upper Jurassic ocean plate stratigraphy of the Mino and Tamba belts (after Wakita, 2012). Table 3 3 , , 1979; , 1984 Table 3 2010 2010 Jones et al, 1993et al, Ichiyama et al, 2008et al, , 1983, 19832 Ozawa and Nishiwaki, 1992, 2000 Igo, 1989Igo, 2000Igo, , 1989Igo, 2010 Fig.…”