Maekawa, Hirokazu; Shozui, Masaya; Ishii, Teruaki; Saboda, Kristine L; Ogawa, Yujiro (1992): Geochemistry of metamorphic rocks from the Mariana and Izu-Ogasawara forearcs. PANGAEA, https://doi.org/10.1594/PANGAEA.770591, Supplement to: Maekawa, H et al. (1992): Metamorphic rocks from the serpentinite seamounts in the Mariana and Izu-Ogasawara forearcs. In: Fryer, P; Pearce, JA; Stokking, LB; et al. (eds.), Proceedings of the Ocean Drilling Program, Scientific Results, College Station, TX (Ocean Drilling Program), 125, 415-430, https://doi.org/10.2973/odp.proc.sr.125.131.1992
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During Leg 125, two serpentinite seamounts were drilled in the Mariana and Izu-Ogasawara forearcs. Together with abundant serpentinized peridotites, low-grade metamorphic rocks were recovered from both seamounts. The metamorphic rocks obtained from Hole 778A on Conical Seamount on the Mariana forearc contain common blueschist facies minerals, lawsonite, aragonite, blue amphibole, and sodic pyroxene. Approximate metamorphic conditions of these rocks are 150° to 250° C and 5 to 6 kb. These rocks are considered to have been uplifted by diapirism of serpentinite from a deeper portion within the subduction zone. This discovery presents direct evidence that blueschist facies metamorphism actually takes place within a subduction zone and provides new insight about trench-forearc tectonics.
The diagnostic mineral assemblage of the metamorphic rocks from Holes 783A and 784A on Torishima Forearc Seamount, in the Izu-Ogasawara region, is actinolite + prehnite + epidote, with a subassemblage of chlorite + quartz + albite + H2O, which is typical of low-pressure type, prehnite-actinolite facies of Liou et al. (1985). This metamorphism may represent ocean-floor metamorphism within trapped oceanic crust or in-situ metamorphism that occurred at depths beneath the island-arc.
Median Latitude: 22.932845 * Median Longitude: 145.201100 * South-bound Latitude: 19.499000 * West-bound Longitude: 141.738000 * North-bound Latitude: 30.964300 * East-bound Longitude: 146.696000
Date/Time Start: 1986-03-26T10:30:00 * Date/Time End: 1989-03-31T10:00:00
125-778A * Latitude: 19.499000 * Longitude: 146.666000 * Date/Time Start: 1989-02-22T06:30:00 * Date/Time End: 1989-02-24T19:25:00 * Elevation: -3924.0 m * Penetration: 107.6 m * Recovery: 22.8 m * Location: North Pacific Ocean * Campaign: Leg125 * Basis: Joides Resolution * Device: Drilling/drill rig (DRILL) * Comment: 13 cores; 107.6 m cored; 0 m drilled; 21.2 % recovery
125-779A * Latitude: 19.512500 * Longitude: 146.696000 * Date/Time Start: 1989-02-28T10:00:00 * Date/Time End: 1989-03-04T03:00:00 * Elevation: -3958.0 m * Penetration: 317.2 m * Recovery: 73.24 m * Location: North Pacific Ocean * Campaign: Leg125 * Basis: Joides Resolution * Device: Drilling/drill rig (DRILL) * Comment: 37 cores; 317.2 m cored; 0 m drilled; 23.1 % recovery
125-783A * Latitude: 30.964300 * Longitude: 141.788000 * Date/Time Start: 1989-03-24T05:30:00 * Date/Time End: 1986-03-26T10:30:00 * Elevation: -4660.0 m * Penetration: 168.2 m * Recovery: 46.95 m * Location: North Pacific Ocean * Campaign: Leg125 * Basis: Joides Resolution * Device: Drilling/drill rig (DRILL) * Comment: 18 cores; 168.2 m cored; 0 m drilled; 27.9 % recovery