Drilling constraints on bimodal volcanism and subsequent formation of contrasted uppermost crustal compositions at the middle Okinawa Trough

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http://abstractsearch.agu.org/meetings/2016/FM/OS41C-1973.html
Author(s): Yamasaki, T.; Takaya, Y.; Mukae, Nobuhiro; Nagase, T.; Tindell, Thomas; Totsuka, Shuhei; Uno, Y.; Yonezu, K.; Nozaki, T.; Ishibashi, J. I.; Kumagai, H.; Maeda, L.
Author Affiliation(s): Primary:
National Institute of Advanced Industrial Science and Technology, Geological Survey of Japan, Tsukuba, Japan
Other:
University of Tokyo, Tokyo, Japan
Kyushu University, Fukuoka, Japan
Tohoku University, Sendai, Japan
University of the Ryukyus, Okinawa, Japan
Japan Agency for Marine-Earth Science and Technology, Kanagawa, Japan
Volume Title: AGU 2016 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2016; American Geophysical Union 2016 fall meeting, San Francisco, CA, Dec. 12-16, 2016. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: The Okinawa Trough (OT) is a young and actively spreading back-arc basin, extending behind the Ryukyu arc-trench system in the southeastern margin of the East China Sea. The OT is believed to be in an initial rifting stage (starting from 6-9 Ma), prior to the normal/stable seafloor spreading which constitutes the main stage of back-arc basin formation. Two drilling cruises - the IODP Exp. 331 and SIP CK14-04 D/V Chikyu Cruise (Exp. 907) in 2010 and 2014 - were conducted at the Iheya North Knoll, middle OT. The Iheya North Knoll is a domal volcanic complex consisting of small volcanic bodies. On these cruises, pumiceous gravel and altered rhyolitic rocks, as well as hemi-pelagic sediments, hydrothermal clay and Kuroko-type ores, were recovered from the upper ≈200 m of the crust. From Feb. 11, 2016 to Mar. 17, 2016, the SIP CK16-01 (Exp. 908) D/V Chikyu cruise was conducted at Iheya North Knoll and the sediment-covered rifting center of the Iheya-Minor Ridge area, middle OT. The Iheya-Minor ridge area is also an active hydrothermal field, located ≈25 km southeast of the Iheya North Knoll. In this area, basaltic rocks are widely distributed, and drilling has confirmed that the basaltic materials continue to ≈120 m below the seafloor. From an igneous petrological point of view, the volcanic rocks in the Okinawa Trough are characterized by bimodal basaltic and rhyolitic compositions, with a compositional gap between SiO2 = 56-66 wt%. The origin of the rhyolitic rock has been interpreted as magmatic differentiation of basaltic magma. However, the existence of an active basalt-hosted hydrothermal field in the Iheya-Minor ridge area suggests the presence of hot basaltic rocks at a shallow position in the crust, and reaching recharged seawater at this depth. Furthermore, the composition of felsic rocks just after the compositional gap (SiO2 = 67 wt%) is very similar to that of the minimum melt of a granitic system, and experimental partial melt of hydrous basalt. Therefore, the contrast in the uppermost crustal composition between very close (≈25 km) areas can reasonably be explained by re-melting of hydrothermally-altered basaltic rocks and production of felsic magma at the upper crustal level, and direct eruption of basaltic magma at the seafloor.
Year of Publication: 2016
Research Program: IODP Integrated Ocean Drilling Program
Key Words: 07 Marine Geology and Oceanography; Cores; Crust; Expedition 331; Integrated Ocean Drilling Program; Marine sediments; North Pacific; Northwest Pacific; Okinawa Trough; Pacific Ocean; Sediments; Upper crust; West Pacific
Coordinates: N274700 N274800 E1265500 E1265300
Record ID: 2018001660
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