Petrology and Geochemistry of Pliocene-Pleistocene Volcanic Rocks from the Izu Arc, Leg 126

Abstract

The igneous geochemistry of lavas and breccias from the basement of Sites 790 and 791, and pumice clasts from the Pliocene-Pleistocene sedimentary section of Sites 788, 790, 791, and 793 were studied. Arc volcanism became silicic about 1.5 m.y. before the inception of rifting in the Sumisu Rift at 2 Ma, but eruption of these silicic magmas reflects changes in stress regime, especially during the last 130,000 yr, rather than crustal anatexis. Arc magmas have had a larger proportion of slab-derived components since the inception of rifting than before, but are otherwise similar. Rift basalts and rhyolites are derived from a different source than are arc andesites to rhyolites. The rift source has less slab-derived material and is an E-MORB-like source, in contrast to an N-MORB-type source overprinted with more slab-derived material beneath the arc. Rift magma types, in the form of rare pumice and lithic clasts, preceded the rift, and the earliest magmas that erupted in the rift already differed from those of the arc. The earliest large rift eruption produced an exotic explosion breccia ("mousse") despite eruption at >1800 mbsl. Although this rock type is attributed primarily to high magmatic water content, the clasts are more MORB-like in trace element and isotopic composition than are modern Mariana Trough basalts. After rifting began, arc volcanism continued to be predominantly silicic, with individual pumice deposits containing clasts that vary in composition by about 5 wt% SiO 2 , or about as much as in historical eruptions of submarine Izu Arc volcanoes. The overall variations in magma composition with time during the inception of arc rifting are broadly similar in the Sumisu Rift and Lau Basin, though newly tapped OIB-type mantle seems to be present earlier during basin formation in the Sumisu than Lau case.

Depositional setting, provenance, and tectonic-volcanic setting of Eocene–Recent deep-sea sediments of the oceanic Izu–Bonin forearc, northwest Pacific (IODP Expedition 352)

“…The La versus Th versus Sc discrimination plot (Figure 12 (d)) highlights the similarities of many of the Oligocene-early Miocene, generally up-slope sediments, with typical basalt-andesite compositions of the Izu-Bonin arc (e.g. Fryer et al 1990;Hochstaedter et al 1990Hochstaedter et al , 2001Gill et al 1992;Taylor and Nesbitt 1998;Kuritani et al 2003;Yokoyama et al 2003Yokoyama et al , 2006Tamura et al 2005Tamura et al , 2007Tollstrup et al 2010). In contrast, the mid-Pliocene to Holocene downslope sediments are more similar to volcanic ash from Honshu (e.g.…”

A trapped Philippine Sea plate origin for MORB from the inner slope of the Izu–Bonin trench

“…The rare-earth elements (REE) and trace elements Ba, Th, Hf, Ta, U, Pb, Rb and Cs were also analyzed at Washington State University by a Sciex Elan model 250 ICP-MS equipped with a Babington nebulizer, watercooled spray chamber and Brooks mass £ow controller. The precision on in-house standards [8,13], basalts from the Quaternary arc [14,15] and basalts from the Sumisu Rift [17].…”

“…This was followed by Nd puri¢cation from other REE on cation resin using 2-methyllactic acid. [13,18,19], (E) Sumisu Rift [17] and (F) active arc basalts from the Shichito Ridge and Torishima knoll [15,17]. Normalization values are from Taylor and McLennan [39].…”

“…3. They are compared to forearc basement recovered at Site 786 of ODP Leg 125 [8,13], basalts from the Quaternary IzuB onin arc [14,15], Paci¢c plate MORB [16] and basalts from the Sumisu Rift [17] Chondrite-normalized REE patterns for the inner trench slope samples (Fig. 4) show depletion of the light REE and Ba, with values of La/Yb CN from 0.4 to 0.6.…”

“…The samples do not have the typical enrichment of largeion lithophile elements (such as Ba) that is so characteristic of island arc magmas, including basalts from the Quaternary Izu^Bonin arc [14,15], boninites from the forearc basement [13,18,19] and Neogene sills from the Mariana and Izu^Bo-nin forearcs [7]. They also do not share REE characteristics with backarc basin basalts of the Sumisu Rift [17] or the Shikoku Basin [5,20]. The REE patterns do, however, show similarities to Mesozoic Paci¢c MORB [16] and MORB from the West Philippine Basin [20], although the Paci¢c plate samples show £atter patterns with generally higher abundances for a given MgO content than the trench slope samples (Fig.…”