Geochronology of alkali volcanism in Oki-Dogo Island, Southwest Japan; geochemical evolution of basalts related to the opening of the Japan Sea

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Authors:Uto, Kozo; Takahashi, Eiichi; Nakamura, Eizo; Kaneoka, Ichiro
Author Affiliations:Primary:
Geological Survey of Japan, Ibaraki, Japan
Other:
University of Tennessee, United States
Tohoku University, Japan
Tokyo Institute of Technology, Japan
Okayama University, Japan
Volume Title:Basaltic rocks of various tectonic settings; selected papers from the 29th international geological congress
Volume Authors:Sato, Hiroaki, prefacer; Misra, Kula C.; Fujimaki, Hirokazu
Source:Geochemical Journal, 28(6), p.431-449; 29th international geological congress, Kyoto, Japan, Aug. 1992, prefaced by Hiroaki Sato, Kula C. Misra and Hirokazu Fujimaki. Publisher: Geochemical Society of Japan, Nagoya, Japan. ISSN: 0016-7002
Publication Date:1994
Note:In English. 58 refs.; illus., incl. 2 tables, geol. sketch maps
Summary:Geochronological and geochemical studies were performed on alkali volcanic rocks from Oki-Dogo Island ∼ 60 km off the coast of SW Japan inside the Japan Sea. There are big differences both in age and in geochemistry between the oldest alkali rocks and the younger rocks; the former having significant Nb- and Ta-depletion and Ba- and K2O-enrichment erupted at 19-18 m.y. when the opening of the Japan Sea was ongoing, while the latter with no Ta-depletion were active between 5.5 and 6.0 m.y. considerably after the cessation of the rifting activity. Syn-extensional rocks should not have erupted in the active subduction zone but are probably originated from or at least contaminated by the lithosphere beneath the early Proterozoic crust. The lithosphere is considered to have suffered mantle metasomatism to form phlogopite selectively enriched in Ba and K2O by fluids, which had been related from previous subductions at the Asian continental margin. Partial melting of such lithospheric mantle during the active rifting was essential for the generation of Ba- and K2O-enrichment alkali basalt magmas. The younger basaltic rocks are probably originated either from the asthenosphere or the deeper plume mantle. A suite of volcanic rocks from mugearite through trachyte to alkali rhyolite gave concordant ages of 5.5-5.4 m.y. These ages suggest that a previously reported Rb-Sr whole-rock age of 6.8 ± 0.3 m.y. does not represent the age of eruption and is probably disturbed by magma mixing between basic and acidic end members. [Authors' abstract] [H.Mi.]
Sections:Geochemistry
Subsections:Igneous rocks
Subjects:Absolute age; Alkali basalts; Basalts; Cenozoic; Chemical composition; Chemical ratios; Dates; Igneous rocks; K/Ar; Major elements; Metals; Miocene; Neogene; Rare earths; Tertiary; Trace elements; Upper Miocene; Volcanic rocks; Volcanism; Asia; Dogo Island; Far East; Honshu; Japan; Japan Sea; North Pacific; Northwest Pacific; Oki Islands; Pacific Ocean; Shimane Japan; West Pacific
Coordinates:N361000 N362000 E1332500 E1331000
Abstract Numbers:98M/3932
Record ID:1995048754
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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