Petrology and geochemistry of volcanic rocks dredged from the Okinawa Trough, an active back-arc basin.

Abstract

Volcanic rocks dredged from the Okinawa Trough, an active back-arc basin behind the Ryukyu island arc (IA)-trench system associated with a subduction of the Philippine Sea plate, encompass a wide variety of petrology and geochemistry, ranging from basalts to rhyolites through andesites. The basalts, consisting of highly vesicular pillow lavas, are moderately porphyritic with phenocrysts of olivine, plagioclase and minor clinopyroxene. Their glass and bulk rock compositions , especially their large-ion lithophile element (LILE) abundances, are transitional between the Ryukyu IA basalts (LILE enriched) and normal-type mid-ocean ridge basalts (N-MORBs, LILE-depleted) . The rhyolites are moderately porphyritic, including phenocrysts of plagioclase and orthopyroxene with or without clinopyroxene and hornblende, and have the close geochemical affinity with the Ryukyu IA rhyolites . It follows that the rhyolites were derived from an IA-type source material similar to that for the Ryukyu IA volcanics, but the basalts were derived from a less LILE-enriched source material approaching that for N-MORBs. Available K-Ar ages indicate the rhyolitic volcanism prior to the basaltic one . It is, therefore, most likely that the magma source region beneath the Okinawa Trough has changed with time from an IA type to a N-MORB type during the back-arc rifting. On the other hand , the andesites are highly porphyritic with phenocrysts of plagioclase, orthopyroxene and clinopyroxene , and have the K Ar age similar to that of the basalts. Most importantly, the andesites exhibit many lines of textural and compositional evidence for mixing between basaltic melt, and rhyolitic partial melt of pre-existing IA lower crust that was presumably heated by the former melt. This again suggests the structural transition of the magma source region beneath the Okinawa Trough, which may be most characteristic of the in itial stage of back-arc basin volcanism in general.

Role of subducted sediments in the genesis of Kurile-Kamchatka island arc basalts: Sr isotopic and elemental evidence.

“…The basalt has a similar geo chemistry (e.g., 17 ppm Ba) to MORB, such as found in hole 304, and is unlikely to be a back arc tholeiite as proposed by Stewart et al (1973) (cf., Ishizuka et al, 1990). The low 87Sr/86Sr (0.70279), which is identical to the value given in von Drach et al (1986), is also consistent with MORB.…”

Primitive Inselbogen- und ozeanische Laven von Hunter R�cken und der Hunter BruchZone: die Bedeutung der Zusammensetzung von Glas, Olivin und Spinell

“…Bryan, 1979;Ikeda and Yuasa, 1989;Ishizuka et al, 1990;Fryer et al, 1990;Hochstaedter et al, 1990a, b) and from ophiolites (e.g. Coleman and Peterman, 1975;Gerlach et al, 1981;Coish et al, 1982;Pearcy et al, 1990).…”

Actively forming Kuroko-type volcanic-hosted massive sulfide (VHMS) mineralization at Iheya North, Okinawa Trough, Japan

“…1a). A rift-related bimodal basaltic-rhyolitic suite, accompanied by minor intermediate rocks, characterizes the middle Okinawa Trough (Ishizuka et al, 1990;Shinjo et al, 1999;Shinjo and Kato, 2000;Zeng et al, 2010). Thick (~1000 m) sequences of organic-rich terrigenous sediments sourced from the Yangtze and Yellow rivers overlie basement lithologies .…”