Abstract
AbstractThe International Ocean Discovery Program Expedition 350 drilled between two Izu rear‐arc seamount chains at Site U1437 and recovered the first complete succession of rear‐arc rocks. The drilling reached 1806.5 m below seafloor. In situ hyaloclastites, which had erupted before the rear‐arc seamounts came into existence at this site, were recovered in the deepest part of the hole (∼15–16 Ma). Here it is found that the composition of the oldest rocks recovered does not have rear‐arc seamount chain geochemical signatures, but instead shows affinities with volcanic front or some of the extensional zone basalts between the present volcanic front and the rear‐arc seamount chains. It is suggested that following the opening of the Shikoku back‐arc Basin, Site U1437 was a volcanic front or a rifting zone just behind the volcanic front, and was followed at ~ 9 Ma by the start of rear‐arc seamount chains volcanism. This geochemical change records variations in the subduction components with time, which might have followed eastward moving of hot fingers in the mantle wedge and deepening of the subducting slab below Site U1437 after the cessation of Shikoku back‐arc Basin opening.
Highlights
The Izu arc (Figure 1a) is a classic oceanic arc characterized by two types of basalt magmas (Tamura et al, 2005, 2007) and bimodal volcanism that produces basalt and rhyolite magmas (Shukuno et al, 2006; Tamura & Tatsumi, 2002), and magma types have been correlated to crustal structures (Kodaira, Sato, Takahashi, Ito, et al, 2007; Kodaira, Sato, Takahashi, Miura, et al, 2007; Tamura et al, 2009; Tamura, Sato, Fujiwara, Kodaira, & Nichols, 2016)
Site U1437 of the International Ocean Discovery Program (IODP) Expedition 350 is located between the Manji and Enpo rear-arc seamount chains
We present whole rock chemical compositions collected from the deepest part of the drilling site, which we interpret as not derived from the Izu rear-arc volcanism in its current form
Summary
The Izu arc (Figure 1a) is a classic oceanic arc characterized by two types of basalt magmas (Tamura et al, 2005, 2007) and bimodal volcanism that produces basalt and rhyolite magmas (Shukuno et al, 2006; Tamura & Tatsumi, 2002), and magma types have been correlated to crustal structures (Kodaira, Sato, Takahashi, Ito, et al, 2007; Kodaira, Sato, Takahashi, Miura, et al, 2007; Tamura et al, 2009; Tamura, Sato, Fujiwara, Kodaira, & Nichols, 2016). The Izu arc is characterized by Miocene rear-arc seamount chains developed at high angle to the arc (Ishizuka, Uto, & Yuasa, 2003, Kodaira et al, 2008; Figure 1b). Site U1437 of the International Ocean Discovery Program (IODP) Expedition 350 is located between the Manji and Enpo rear-arc seamount chains. At this site, the recovery of an in situ volcanic succession subsequently covered by > 1400 m of sediments provides the first information on the magmatism before the formation of the rear-arc seamount chains. This study reveals important information on the previous stage of Izu rear-arc volcanism and on the complexity of arc and rear-arc magmatism
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