Element and isotope composition of selected volcanics from the Bicol arc, Philippines

Pliocene to recent volcanic rocks from the Bulusan volcanic complex in the southern part of the Bicol arc (Philippines) exhibit a wide compositional range (medium- to high-K basaltic-andesites, andesites and a dacite/rhyolite suite), but are characterised by large ion lithophile element enrichments and HFS element depletions typical of subduction-related rocks. Field, petrographic and geochemical data indicate that the more silicic syn- and post-caldera magmas have been influenced by intracrustal processes such as magma mixing and fractional crystallisation. However, the available data indicate that the Bicol rocks as a group exhibit relatively lower and less variable 87Sr/86Sr ratios (0.7036-0.7039) compared with many of the other subduction-related volcanics from the Philippine archipelago. The Pb isotope ratios of the Bicol volcanics appear to be unlike those of other Philippine arc segments. They typically plot within and below the data field for the Philippine Sea Basin on 207Pb/204Pb versus 206Pb/204Pb and 208Pb/204Pb versus 206Pb/204Pb diagrams, implying a pre-subduction mantle wedge similar to that sampled by the Palau Kyushu Ridge, east of the Philippine Trench. 143Nd/144Nd ratios are moderately variable (0.51285-0.51300). Low silica (<55 wt%) samples that have lower 143Nd/144Nd tend to have high Th/Nd, high Th/Nb, and moderately low Ce/Ce* ratios. Unlike some other arc segments in the Philippines (e.g. the Babuyan-Taiwan segment), there is little evidence for the involvement of subducted terrigenous sediment. Instead, the moderately low 143Nd/144Nd ratios in some of the Bicol volcanics may result from subduction of pelagic sediment (low Ce/Ce*, high Th/Nd, and high Th/Nb) and its incorporation into the mantle wedge via a slabderived partial melt.

K-Ar dating of magmatic rocks from the Urup Island, Kuril Island Chain

An isotope-geochronological study of Neogene-Quaternary igneous rocks from the Urup Island (Greater Kuril Ridge) was carried out. It was established that magmatic activity in the island developed during the last 10 my and it was not interrupted by long inactive periods. K-Ar data obtained along with results of diatomic analysis are in good agreement with the regional stratigraphic scheme of Paleogene and Neogene deposits and the intraregional correlation scheme of magmatic rocks in the Kuril Islands, which are developed for the State Geologic Map, scale 1:200 000 (Second edition). In the present-day territory of the Urup Island, the earliest Late Miocene - Early Pliocene (10.5-4.5 Ma) magmatic stage was associated with formation of the Rybakovsky andesite volcanic complex, which is represented by an effusive series (Rybakovskaya Suite) and subvolcanic rocks. Actually at the same time (6.6-4.7 Ma), but at a great depth, intrusive bodies of the Prasolovsky plagiogranite-diorite plutonic complex were intruded. The Pliocene stage of magmatism in the Urup Island is characterized by formation of rocks of the Kamuysky dacitic volcanic complex (4.0-2.1 Ma). This complex is locally represented only by subvolcanic acidic bodies, and its occurrence in the island is limited. During the Pliocene - Early Neopleistocene stage of magmatism (3.0-0.8 Ma) the Fregatsky andesibasalt volcanic complex was formed in the Urup Island. This complex includes effusive series (Fregatskaya unit) and subvolcanic bodies. Quaternary time in the Urup Island is characterized by eruptive activity in subaerial conditions with formation of effusive-pyroclastic intermediate-basic rocks of the Bogatyrsky Middle Neopleistocene - Holocene complex (<0.5 Ma). Rocks of this complex formed stratovolcano cones. Pyroclastic rocks of the Rokovsky dacitic volcanic complex were erupted simultaneously. The mentioned magmatic complexes of the Urup Island well correlate with the distinguished magmatic complexes within the bounds of contiguous insular blocks of the Greater Kuril Arc and confirm uniform geologic history of magmatic development of the region.

Age and chemical composition of dredged igneous rocks from the Vityaz Ridge, Pacific slope of the Kuril Island Arc

Original results of igneous rock studies are presented. The rocks were dredged during a marine expedition (cruise 37 of R/V Akademik M.A. Lavrent'ev in August-September, 2005) in the region of the submarine Vityaz Ridge and the Kuril Arc outer slope. Several age complexes (Late Cretaceous, Eocene, Late Oligocene, Miocene, and Pliocene-Pleistocene) are recognizable on the Vityaz Ridge. These complexes are characterized by a number of common geochemical features since all of them represent formations of island arc calc-alkali series. At the same time, they also have individual features reflecting different geodynamic settings. The outer slope of the Kuril Arc demonstrates submarine volcanism. Pliocene-Pleistocene volcanic rocks dredged here are similar to volcanites of the Kuril-Kamchatka Arc frontal zone.

Chemistry of Pliocene-Pleistocene pumice from the Izu-Bonin Arc

Thick pumice deposits were found in the cored sequences of forearc, arc, and backarc sites of Leg 126 in the Izu-Bonin Arc. These deposits, composed of fragmental rhyolite pumice with the chemical composition of low-alkali tholeiites, are products of arc volcanism. Pumice deposits constitute more than half of the thickness of the sediment fill of the Sumisu Rift, a backarc rift of the Izu-Bonin Arc. They comprise five thick pumiceous beds separated by thin hemipelagic units; as such, they record four major episodes or pulses of explosive, rhyolitic volcanism during the last 0.15 Ma, separated by quiescent intervals that each lasted about 30-60 k.y. The thick pumiceous beds were deposited in the rift mainly by sediment gravity flows during and immediately after the eruption of arc volcanos, which were probably submarine. Initiation of rifting was also preceded in the Pliocene by submarine rhyolitic volcanism, as seen in samples from the top of the eastern rift flank. Thick pumice beds correlative with those in the backarc also occur in the forearc basin to the east.

(Table 1) K, Rb and Sr concentrations and Sr isotopic composition of sediments and igneous rocks from DSDP Hole 30-286

To evaluate the possible contribution of ocean floor sediments during the genesis of the volcanism of Vanuatu (New Hebrides) active margin, we have determined the balance of Sr isotopes and K, Rb and Sr contents for the stratigraphic column of site 286 (leg 30, DSDP). This site is located on the oceanic plate that will be subducted. Analyses have been performed on sedimentary and igneous rocks, before and after acid leaching. The Sr isotopic data do not support the occurrence of some continental component in arc magmas of this active margin which is really intraoceanic. It is demonstrated that the d'Entrecasteaux fracture zone results from the intense fracturing of typical oceanic crust. The analyses of the volcanogenic components of the sediments show a change in the source of volcanoclastic detritus from the Loyalty islands in the Eocene to the volcanic arc of Vanuatu (New Hebrides) during Pliocene and Quaternary times. The determined balance of Sr isotopes and of K, Rb, Sr contents, may be used for calculation of multicomponent melting mixing models for the origin of Vanuatu arc magmas, but we emphasize that in these models the Sr isotopes cannot be considered as an appropriated tracer of sediment contribution.