Variability of protoliths and pressure-temperature conditions of amphibolites from the Ohmachi Seamount (Izu-Bonin-Mariana arc): evidence of a fossil subduction channel in a modern intra-oceanic arc

Autor: Toru Nagahashi, Yoshihiro Asahara, Hayato Ueda, Tadashi Usuki, Masayo Minami, Takeshi Imayama
Rok vydání: 2020
Předmět:
Zdroj: Mineralogy and Petrology. 114:305-318
ISSN: 1438-1168
0930-0708
DOI: 10.1007/s00710-020-00705-z
Popis: Three types of amphibolites were found around a large serpentinite body in the basement of the Ohmachi Seamount in the Izu-Bonin-Mariana arc: epidote-albite amphibolite, epidote-garnet amphibolite, and garnet-zoisite amphibolite. The epidote-albite amphibolite shows a mid-ocean ridge basalt geochemical affinity based on whole-rock chemistry and Nd and Sr isotopes. The epidote-garnet amphibolite originated in an arc tectonic setting and contains relict glaucophane, showing blueschist-facies metamorphism (MD1) prior to amphibolite-facies recrystallization (MD2) that formed barroisite and paragonite. The Rb–Sr garnet-whole-rock isochron (289 ± 58 Ma) could represent the timing of blueschist-facies metamorphism. In contrast, the Rb-Sr barroisite-paragonite isochron of 67 ± 13 Ma is an errorchron or mixed age due to insufficient equilibrium with the whole-rock Sr isotope composition, with no geologic significance. The garnet-zoisite amphibolite formed in a mid-oceanic ridge setting and preserves relicts of eclogite-facies paragenesis (MR2a), including zoisite, kyanite, omphacite, and magnesiohornblende, followed by amphibolite-facies metamorphism (MR2b) represented by actinolite and then by a later MR3 event that resulted in the formation of edenite and pargasite. The Rb–Sr mineral isochron (244 ± 28 Ma) defined by zoisite + magnesiohornblende/actinolite + edenite/pargasite represents the time at which the rock recrystallized in the amphibolite facies (MR2b) or cooled (MR3) through ~ 500 °C (the closure temperature of amphibole), indicating that eclogite-facies metamorphism occurred before ca. 244 Ma. The two Permo-Triassic isochron ages (289–244 Ma) indicate that the high-pressure metamorphism of the basement in the Ohmachi Seamount occurred by ancient subduction rather than along the modern subduction zone. The different protoliths and P–T conditions of amphibolites exposed in the serpentine melange of the Ohmachi Seamount represent a mixture of rocks exhumed to the Earth’s surface from various depths in the subducted complex, which are interpreted as evidence for a fossil subduction channel preserved in a modern intra-oceanic arc.
Databáze: OpenAIRE
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