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The southern Mariana Arc-Trench system is rapidly deforming, resulting in unusual interactions between arc and back-arc basin (BAB) magmatic systems. New geochemical data for volcanoes in this region are presented and explored. Tracey Seamount, an extinct submarine volcano about 30 km northwest of Guam, is the southernmost stratovolcano of the Mariana Arc. Tracey is built about 125 km above the subducted slab and has erupted a bimodal suite of typical arc mafic and felsic lavas as recently as 0.527 ± 0.023 Ma (40Ar/39Ar age). An unusual cluster of small basaltic volcanoes, informally termed the Alphabet Seamount Volcanic Province (ASVP), is found about where the next arc volcano to the southwest of Tracey Seamount should have grown. Samples from six of these volcanoes were studied here. At least two ASVP volcanoes were recently active, as shown by hydrothermal activity. The lack of magmatic focusing to build a single stratovolcano where the ASVP is situated reflects strong extension in the BAB. Construction of northern ASVP volcanoes is controlled by east-west extension accompanying opening of the Mariana Trough. In contrast, southern ASVP volcanoes are affected by north-south extension due to rapid rollback of a narrow slab of Pacific seafloor that is subducted along the east-west trending Challenger Deep segment of the Mariana Trench to the south. ASVP lava compositions are distinct from Tracey Seamount and other Mariana Arc lavas, instead showing affinities with Mariana Trough BAB basalt (BABBs): they are mafic, tholeiitic, low-K2O and LREE-depleted, with low 87Sr/86Sr, but show a subtle gradient from somewhat more arc-like lavas closer to the trench to BABBs farther west. The unusual tectonic setting of ASVP provides a unique perspective on how different arc magma batches reflect melting of mantle with strong compositional gradients which are mixed together beneath long-lived arc volcanoes but here rise to form scattered small ASVP volcanoes. [ABSTRACT FROM AUTHOR]

The article presents a study regarding the forearc ophiolites during the Late Cretaceous period in Iran. It says that the reconstruction of the Neotethyan Ocean was based from ophiolitic sutures such as the Late Cretaceous Ophiolite Belt of Southwest Asia (LCOBSWA), which is less known in the Zagros region of Iran. Moreover, the igneous rocks and sediments suggest that the ophiolithic belt made an approximately 3000-kilometers (km) long path of forearc oceanic lithosphere.

West Rota Volcano (WRV) is a recently discovered extinct submarine volcano in the southern Mariana Arc. It is large (25 km diameter base), shallow (up to 300 m below sealevel), and contains a large caldera (6 × 10 km, with up to 1 km relief). The WRV lies near the northern termination of a major NNE-trending normal fault. This and a second, parallel fault just west of the volcano separate uplifted, thick frontal arc crust to the east from subsiding, thin back-arc basin crust to the west. The WRV is distinct from other Mariana Arc volcanoes: (i) it consists of a lower, predominantly andesite section overlain by a bimodal rhyolite-basalt layered sequence; (ii) andesitic rocks are locally intensely altered and mineralized; (iii) it has a large caldera; and (iv) WRV is built on a major fault. Submarine felsic calderas are common in the Izu and Kermadec Arcs but are otherwise unknown from the Marianas and other primitive, intraoceanic arcs. 40Ar–39Ar dating indicates that andesitic volcanism comprising the lower volcanic section occurred 0.33–0.55 my ago, whereas eruption of the upper rhyolites and basalts occurred 37–51 thousand years ago. Four sequences of rhyolite pyroclastics each are 20–75 m thick, unwelded and show reverse grading, indicating submarine eruption. The youngest unit consists of 1–2 m diameter spheroids of rhyolite pumice, interpreted as magmatic balloons, formed by relatively quiet effusion and inflation of rhyolite into the overlying seawater. Geochemical studies indicate that felsic magmas were generated by anatexis of amphibolite-facies meta-andesites, perhaps in the middle arc crust. The presence of a large felsic volcano and caldera in the southern Marianas might indicate interaction of large normal faults with a mid-crustal magma body at depth, providing a way for viscous felsic melts to reach the surface. [ABSTRACT FROM AUTHOR]

Oxygen is the most abundant element in the earth, and isotopic analysis of this element in island arc lavas potentially provides sensitive constraints on the proportion of oxygen recycled from subducted material, relative to that extracted from the mantle. Here we report on 225 new oxygen isotopic analyses of whole-rock and glass samples, and clinopyroxene separates, from lavas collected from the southernmost 1500 km of the Izu–Bonin–Mariana (IBM) convergent margin. Whole-rock samples clustered around a mean of 6.11 ± 0.47‰, whereas Mariana Trough glasses and mafic melts, calculated to be in equilibrium with mafic phenocrysts, clustered narrowly around a mean of 5.7‰. These data demonstrate that unequivocal identification of magmatic oxygen requires analysis of fresh glass or mafic minerals, and that the source of southern IBM Arc melts is entirely, or almost entirely, in equilibrium with normal mantle oxygen. If the elemental enrichments characteristic of the subduction component originate in subducted materials, these oxygen isotopic data are most consistent with the interaction of a small amount of sediment melt (<4%; mostly less than 1%) with mantle peridotite to yield the hybrid mantle that melts to form IBM Arc magmas. [ABSTRACT FROM AUTHOR]

Abstract Late Miocene (7–9 Ma) basaltic rocks from the Monbetsu-Kamishihoro graben in northeast Hokkaido have chemical affinities to certain back-arc basin basalts (referred to herein as Hokkaido BABB). Pb-, Nd- and Sr-isotopic compositions of the Hokkaido BABB and arc-type volcanic rocks (11–13 Ma and 4–4.5 Ma) from the nearby region indicate mixing between the depleted mantle and an EM II-like enriched component (e.g. subducted pelagic sediment) in the magma generation. At a given 87Sr/86Sr, Hokkaido BABB have slightly lower 143Nd/144Nd and slightly less radiogenic 206Pb/204Pb compared with associated arc-type lavas, but both these suites are difficult to distinguish solely on the basis of isotopic compositions. These isotopic data indicate that while generation of the Hokkaido BABB involves smaller amounts of the EM II-like enriched component than do associated arc lavas, Hokkaido BABB are isotopically distinct from basalts produced at normal back-arc basin spreading centers. Instead, northeast Hokkaido BABB are more similar to basalts erupted during the initial rifting stage of back-arc basins. The Monbetsu-Kamishihoro graben may have developed in association with extension that formed the Kurile Basin, suggesting that opening of the basin continued until late Miocene (7–9 Ma). [ABSTRACT FROM AUTHOR]

Focuses on the scientific investigations made in the Ize-Bonin-Mariana arc system in Tokyo, Japan. Examples of intra-oceanic convergent margins (IOCM); Importance of IOCM in understanding the operations of subduction zones; Use of active hydrothermal systems in determining the formation of ore deposits.