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To examine the processes and histories of arc and of volcanism associated with backarc rifting, 130 samples containing igneous glass shards were taken from the Pliocene-Quaternary succession on the rift flank (Site 788) and the Quaternary fill in the basin fill of the Sumisu Rift (Sites 790 and 791). These samples were subsequently analyzed at the University of Illinois at Chicago and Shizuoka University. The oxides determined by electron probe do not account for the total weight of the material; differences between summed oxides and 100% arise from the water contents, probably augmented by minor losses that result from alkali vaporization during analysis. Weight losses in colorless glasses are up to 9%; those in brown glasses (dacites to basalts) are no more than 4.5%; shards from the rift-flank (possibly caused by prolonged proximity to the seafloor) generally have higher values than those from the rift-basin fill. How much of the lost water is magmatic, and how much is hydrated is uncertain; however, although the shards absorb potassium, calcium, and magnesium during hydration in the deep sea, they do so only to a minor extent that does not significantly alter their major element compositions. Therefore, the electron-probe results are useful in evaluating the magmatism recorded by the shards. Preand syn-rift Izu-Bonin volcanism were overwhelmingly dominated by rhyolite explosions, demonstrating that island arcs may experience significant silicic volcanism in addition to the extensive basaltic and basaltic andesitic activity, documented in many arcs since the 1970s, that occurs in conjunction with the andesitic volcanism formerly thought to be dominant. Andesitic eruptions also occurred before rifting, but the andesitic component in our samples is minor. All the preand syn-rift rhyolites and andesites belong to the low-alkali island-arc tholeiitic suite, and contrast markedly with the alkalic products of Holocene volcanism on the northernmost Mariana Arc that have been attributed to nascent rifting. The Quaternary dacites and andesites atop the rift flank and in the rift-basin fill are more potassic than those of Pliocene age, as a result of assimilation from the upper arc crust, or from variations in degrees of partial melting of the source magmas, or from metasomatic fluids. All the glass layers from the rift-flank samples belong to low-K arc-tholeiitic suites. Half of those in the Pliocene succession are exclusively rhyolitic; the others contain minor admixtures of dacite and andesite, or andesite and either basaltic andesite or basalt. In contrast, the Quaternary (syn-rift) volcaniclastics atop the rift-flank lack basalt and basaltic andesite shards. These youngest sediments of the rift flank show close compositional affinities with five thick layers of coarse, rhyolitic pumice deposits in the basin fill, the two oldest more silicic than the younger ones. The coarse layers, and most thin ash layers that occur in hemipelagites below and intercalated between them, are low-K rhyolites and therefore probably came from sources in the arc. However, several thin rhyolitic ash beds in the hemipelagites are abnormally enriched in potassium and must have been provided by more distal sources, most likely to the west in Japan. Remarkably, the Pliocene-Pleistocene geochemistry of the volcanic front does not appear to have been influenced by the syn-rift basaltic volcanism only a few kilometers away. Rare, thin layers of basaltic ash near the bases of the rift-basin successions are not derived from the arc. They deviate strongly from trends that the arc-derived glasses display on oxide-oxide plots, and show close affinities to the basalts erupted all over the Sumisu Rift during rifting. These basalts, and the basaltic ashes in the basal rift-basin fill, are compositionally similar to those erupted from mature backarc basins elsewhere. INTRODUCTION A primary objective of Ocean Drilling Program (ODP) Leg 126 was to explicate the processes and history of arc volcanism, and especially of volcanism as ociated with backarc rifting, by recovering and examining sediments and rocks that fill the rif basins, as ll as adjacent pre-rift rocks. Over the past two decades, extensive analyses of materials sampled by the Deep Sea Drilling Project (DSDP) have shown that volcanic glasses are fairly faithful indicators of magma compositions. In the deep-sea environment, although glasses are readily hydrated and absorb potassium, calcium, and magnesium, they do so only to a minor extent that does not significantly alter their major element compositions, as seen by Scheidegger et al. (1978) in ' Taylor, B., Fujioka, K., et al., 1992. Proc. ODP, Sci. Results, 126: College Station, TX (Ocean Drilling Program). 2 Department of Geological Sciences, M/C 186, University of Illinois at Chicago, P. O. Box 4348, Chicago, IL 60680, U.S.A. Marine Geology Department, Geological Survey of Japan, 1-1-3 Higashi, Ibaraki 305, Japan. 4 Institute of Geosciences, Shizuoka University, 836 Oya, Shizuoka, 422, Japan. Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano, Tokyo 164, Japan (present address: Japan Marine Science and Technology Center, 2-14 Natsushima, Yokosuka, Kanagowa 238, Japan). the similarity of atomic-absorption data (which provide bulk analyses) and electron probe data (which analyze shard interiors) for glasses sampled by DSDP Leg 19 in the northwest Pacific. Were alteration significant, it would progress from s ard margins to their interiors, thereby resulting in ignificant differences in the bulk and interior compositions. Numerous volcanic ash layers several millimeters to several centimeters thick were recovered from Sites 790 and 791 in the basin fill of the Sumisu Rift, and others were sampled from sparsely recovered sedimentary cores taken at Site 788 on the eastern ridge-flank uplift. These thin layers of fine tephra were sampled by investigators from both the United States and Japan, and glass shards from the samples were analyzed for major oxide chemistry by electron microprobe at the Department of Geological Sciences of the University of Illinois at Chicago (UICDoGS) and at the Institute of Geosciences of Shizuoka University (IGSU). In addition, fine fractions from five thick layers of pumice in the basin fill that are the subject of other reports (Nishimura et al., 1991; Nishimura et al., this volume) were also analyzed, although less systematically. We present and discuss the results of these analyses in this report. Figures 1, 2, and 3 show how the backarc sites are spatially and stratigraphically related. Sites 788 and 789 are situated on the eastern foot wall of the rift-flank uplift 1100 m below sea level (mbsl).

Major-Oxide Stratigraphy of Glass Shards in Volcanic Ash Layers of the Izu-Bonin Arc-Backarc Sites (Sites 788/789 and 790/791)