(Table 1) Anisotropy of magnetic susceptibility parameters of ODP Hole 134-833B

Ocean Drilling Program (ODP) Sites 832 and 833 were drilled in the intra-arc North Aoba Basin of the New Hebrides Island Arc (Vanuatu). High volcanic influxes in the intra-arc basin sediment resulting from erosion of volcanic rocks from nearby islands and from volcanic activity are associated with characteristic magnetic signals. The high magnetic susceptibility in the sediment (varying on average from 0.005 to more than 0.03 SI) is one of the most characteristic physical properties of this sedimentary depositional environment because of the high concentration of magnetites in redeposited ash flows and in coarse-grained turbidites. Susceptibility data correlate well with the high resolution electrical resistivity logs recorded by the formation microscanner (FMS) tool. Unlike the standard geophysical logs, which have low vertical resolution and therefore smooth the record of the sedimentary process, the FMS and whole-core susceptibility data provide a clearer picture of turbiditic sediment deposition. Measurements of Curie temperatures and low-temperature susceptibility behavior indicate that the principal magnetic minerals in ash beds, silt, and volcanic sandstone are Ti-poor titanomagnetite, whereas Ti-rich titanomagnetites are found in the intrusive sills at the bottom of Site 833. Apart from an increase in the concentration of magnetite in the sandstone layer, acquisition of isothermal and anhysteretic remanences does not show significant differences between sandstone and clayey silts. The determination of the anisotropy of magnetic susceptibility (AMS) in more than 400 samples show that clayey siltstone have a magnetic anisotropy up to 15%, whereas the AMS is much reduced in sandstone layers. The magnetic susceptibility fabric is dominated by the foliation plane, which is coplanar to the bedding plane. Reorientations of the samples using characteristic remanent magnetizations indicate that the bedding planes dip about 10° toward the east, in agreement with results from FMS images. Basaltic sills drilled at Site 833 have high magnetic susceptibilities (0.05 to 0.1 SI) and strong remanent magnetizations. Magnetic field anomalies up to 50 µT were measured in the sills by the general purpose inclinometer tool (GPIT). The direction of the in-situ magnetic anomaly vectors, calculated from the GPIT, is oriented toward the southeast with shallow inclinations which suggests that the sill intruded during a reversed polarity period.

Nd, Sr and Sm isotopic composition and model ages of rocks from the Anginskaya and Talanchanskaya formations, Lake Baikal

The paper presents data on the Nd-Sr systematics of magmatic rocks of the Khaidaiskii Series of the Anginskaya Formation in the Ol'khon region, western Baikal area, and rocks of the Talanchanskaya Formation on the eastern shore of Lake Baikal. Geochemical characteristics of these rocks are identical and testify to their arc provenance. At the same time, the epsilon(t)Nd of rocks of the Khaidaiskii Series in the Ol'khon area has positive values, and the data points of these rocks plot near the mantle succession line in the epsilon(t)Nd-87Sr/86Sr diagram, whereas the epsilon(t)Nd values of rocks of the Talanchanskaya Formation are negative, and the data points of these rocks fall into the fourth quadrant in the epsilon(t)Nd -87Sr/86Sr diagram. This testifies to a mantle genesis of the parental magmas of the Khaidaiskii Series and to the significant involvement of older crustal material in the generation of the melts that produced the orthorocks on the eastern shore of the lake. These conclusions are corroborated by model ages of magmatic rocks in the Ol'khon area (close to 1 Ga) and of rocks of the Talanchanskaya Formation (approximately 2 Ga). The comparison of our data with those obtained by other researchers on the Nd-Sr isotopic age of granulites of the Ol'khon Group and metavolcanics in various structural zones in the northern Baikal area suggests, with regard for the geochemistry of these rocks, the accretion of tectonic nappes that had different isotopic histories: some of them were derived from the mantle wedge and localized in the island arc itself (magmatic rocks of the Anginskaya Formation) or backarc spreading zone (mafic metamagmatic rocks of the Ol'khon Group), while others were partial melts derived, with the participation of crustal material, from sources of various age (metagraywackes in the backarc basin in the Ol'khon Group and the ensialic basement of the island arc in the Talanchanskaya Formation).

Petrology and geochemistry of the Izu-Bonin Forearc tephras

Numerous marine tephra layers cored at Sites 792 and 793 in the Izu-Bonin forearc region offer additional information about the timing and spatial characteristics of arc volcanism and the evolution of island arcs. Explosive volcanism along the Izu-Bonin Arc, with maxima just before rifting of the arc at ~40 and 5-0 Ma, produced black and white tephras of variable grain sizes and chemical compositions. Most of the tephras belong chemically to low-K and low-alkali tholeiitic rock series with a few tephra of the high-K and alkalic rock series. Most of the tephras (low-K series) were derived from the Izu-Bonin Arc, although a few were produced far to the west of the Izu-Bonin Arc (e.g., from the Ryukyu Arc). Black tephras may have come from nearby sources, such as Aogashima, Sumisu, and Torishima islands. The high-K series of tephras, within the sediments younger than 3 Ma, may reflect thickening of the island-arc crust.

Geochemistry of felsic rocks and minerals of ODP Hole 135-841B

A felsic volcanic series (605-825 mbsf) overlain by upper Eocene shallow-water sediments (500-605 mbsf) and basalticandesitic sills that intruded into sediments of Holocene to Miocene age (0-500 mbsf) was drilled in the forearc region of the Lau Basin at a water depth of 4810 m. The volcanic sequence at Site 841 includes altered and mineralized calc-alkaline rhyolites and dacites, dacitic tuffs, lapilli tuffs, flow breccias, and welded tuffs. These rocks formed subaerially or in a very shallow-water environment suffering a subsidence of >5000 m since Eocene times. Calculations of gains and losses of the major components during alteration show most pronounced changes in the uppermost 70 m of the volcanic sequence. Here, Al, Fe, Mg, and K are enriched, whereas Si and Na are strongly depleted. Illite, vermiculite, chlorite, and hematite predominate in this part of the hole. Throughout the section, quartz, plagioclase, kaolinite, and calcite are present. Sulfide mineralization (up to 10 vol%) consisting mainly of disseminated pyrite (with minor pyrrhotite inclusions) and marcasite together with minor amounts of chalcopyrite is pervasive throughout. Locally, a few sulfide-bearing quartz-carbonate veins as well as Ti-amphibole replacement by rutile and then by pyrite were observed. Strong variations in the As content of sulfides (from 0 to 0.69 wt%) from the same depth interval and local enrichments of Co, Ni, and Cu in pyrite are interpreted to result from fluctuations in fluid composition. Calculations of oxygen and sulfur fugacities indicate that fO2 and fS2 were high at the top and lower at the bottom of the sequence. Sulfur isotope determinations on separated pyrite grains from two samples give d34S values of +6.4ë and +8.4ë, which are close to those reported from Kuroko and Okinawa Trough massive sulfide deposits and calc-alkaline volcanic rocks of the Japanese Ryukyu Island Arc. Calculated chlorite formation temperatures of 265°-290°C at the top of the sequence are consistent with minimum formation temperatures of fluid inclusions in secondary quartz, revealing a narrow range of 270°-297°C. Chlorite formation temperatures are constant downhole and do not exceed 300°C. The presence of marcasite and 4C-type pyrrhotite indicates a formation temperature of <= 250°C. At a later stage, illite was formed at the top of the volcanic series at temperatures well below 200°C.

Geochemistry of the volcanic basement of ODP Hole 134-831B

The basement of Bougainville Guyot drilled at Site 831 consists of andesitic hyalobreccias derived from a submarine arc volcano. The volcanic sequence has been dated by K/Ar at approximately 37 Ma. The 121 m of andesitic hyalobreccias drilled in Hole 831B have been divided into five subunits of two types: one appears to be primary, and the other contains evidence of reworking and a subaerial clastic input. Variations are attributed to fluctuations in water depth. The distinctive hyalobreccias consist of andesitic blebs with chilled margins and peripheral fractures set in a chaotic greenish matrix that is mainly altered glass, with crystals similar to those in the blebs or clasts. Their formation is attributed to violent reaction of andesitic magma discharged into seawater, in perhaps the submarine equivalent of fire-fountaining. There was limited reworking by currents and debris flows on the flanks of the submarine volcano. The andesite shows no significant compositional variation in phenocryst phases throughout the drilled sequence and contains phenocrysts of plagioclase (An88-43), clinopyroxene (Ca44Mg46Fe10-Ca41Mg40Fe19), orthopyroxene (Ca4Mg79Fe17-Ca3Mg58Fe39), and titanomagnetite. There is a systematic change in volcanic composition with height in the section, from more mafic andesites at the base, to overlying more acid andesites, and strong evidence exists that magma mixing may have played a significant role in the genesis of these lavas. The andesites have affinities with the low-K arc tholeiite series. Trace element and isotopic systematics for these rocks indicate very minor involvement of a LILE- and 87Sr-enriched slab-derived fluid in their petrogenesis. This accords with the previous suggestion that Bougainville Guyot forms part of an Eocene proto-island arc developed along the southern side of the d'Entrecasteaux Zone, above a southward-dipping subduction zone.

Paleomagnetism and bedding orientations at DSDP Sites 87-582, 87-583 and 87-584

Paleomagnetic measurements of sediment samples provide the magnetostratigraphy at Deep Sea Drilling Sites 582, 583, and 584 in the Nankai Trough and the Japan Trench. Drastic changes in the rate of sediment accumulation are documented by the magnetostratigraphic and biostratigraphic correlations. The changes in the accumulation rate correspond to the supply of sediments and variations in the accretionary process, which are directly related to the tectonic cycles in the geologic evolution of the Japanese island arc. Faults and folds within the drilled sedimentary sequences are oriented by paleomagnetic declination. Their directions and stress patterns are related to the relative plate motion along the trough and trench. The original remanent magnetization of the sediment was modified and remagnetized in the tectonic process of accretion by physical deformation, faulting, and intrusion of dewatering veinlets.

Petrology and geochemistry of volcanic rocks from ODP Sites 134-827, 134-829 and 134-830

Igneous rocks recovered from Ocean Drilling Program (ODP) Leg 134 Sites 827, 829, and 830 at the toe of the forearc slope of New Hebrides Island Arc were investigated, using petrography, mineral chemistry, major and trace element, and Sr, Nd, and Pb isotopic analyses. Basaltic and andesitic clasts, together with detrital crystals of plagioclase, pyroxenes, and amphiboles embedded in sed-lithic conglomerate or volcanic siltstone and sandstone of Pleistocene age, were recovered from Sites 827 and 830. Petrological features of these lava clasts suggest a provenance from the Western Belt of New Hebrides Island Arc; igneous constituents were incorporated into breccias and sandstones, which were in turn reworked into a second generation breccia. Drilling at Site 829 recovered a variety of igneous rocks including basalts and probably comagmatic dolerites and gabbros, plus rare ultramafic rocks. Geochemical features, including Pb isotopic ratios, of the mafic rocks are intermediate between midocean ridge basalts and island arc tholeiites, and these rocks are interpreted to be backarc basin basalts. No correlates of these mafic rocks are known from Espiritu Santo and Malakula islands, nor do they occur in the Pleistocene volcanic breccias at Sites 827 and 830. However, basalts with very similar trace element and isotopic compositions have been recovered from the northern flank of North d'Entrecasteaux Ridge at Site 828. It is proposed that igneous rocks drilled at Site 829 represent material from the North d'Entrecasteaux Ridge accreted onto the over-riding Pacific Plate during collision. An original depleted mantle harzburgitic composition is inferred for a serpentinite clast recovered at 407 meters below seafloor (mbsf) in Hole 829A. Its provenance is a matter of speculation. It could have been brought up along a deep thrust fault affecting the Pacific Plate at the colliding margin, or analogous to the Site 829 basaltic lavas, it may represent material accreted from the North d'Entrecasteaux Ridge.

Chemical and isotopic compositions of sediments from DSDP Hole 21-204

The highly depleted intra-oceanic Tonga-Kermadec island arc forms an endmember of arc systems and a unique location in which to isolate the effects of the slab flux. High precision TIMS uranium, thorium, strontium, neodymium, and lead isotopes, along with complete major and trace element data, have been obtained on an extensive sample set comprising fifty-eight lavas along the arc as well as nineteen samples of the subducting sediments at DSDP site 204 just to the east of the Tonga-Kermadec trench. Ca/Ti and Al/Ti ratios extend from values appropriate to an N-MORB source in the southern Kermadecs to very high ratios in Tonga interpreted to reflect increasing degrees of depletion of the mantle wedge due to backarc basalt extraction. The isotope data emphasize the need for four components in the petrogenesis of the lavas: (1) the mantle wedge; (2) a component with elevated 207Pb/204Pb towards which the Kermadec and southern Tongan lavas extend; (3) a component characterised by high 206Pb/204Pb, Ta/Nd, and low 143Nd/144Nd observed only in the northernmost Tongan islands of Tafahi and Niuatoputapu; (4) a fluid component characterised by strong enrichments of Rb, Ba, U, K, Ph, and Sr, relative to Th, Zr, and the REE and producing large 238U excesses ((230Th/238U) = 0.8-0.5) in the more depleted lavas. The mantle wedge (Component 1) is isotopically similar to the source of the Lau BABB. Component 2 is average pelagic sediment on the downgoing Pacific plate as observed at DSDP sites 595/596 and in the upper sections of the sediment pile at DSDP site 204. Mass balance calculations indicate that less than 0.5% is recycled into the arc lavas; essentially all the subducted sediment is returned to the upper mantle (~0.03 km**3/yr). Exceptionally low concentrations of Ta and Nb relative to Th and the LREE requires that this sediment component is added as a partial melt which was in equilibrium with residual rutile or ilmenite. Component 3 is identified as volcaniclastics from the Louisville Ridge which comprise the lower 44 m of the sediment section intersected at DSDP site 204. These volcaniclastics are spatially restricted to the vicinity of the Louisville Ridge and provide a unique sediment tracer which can be used to show that it takes 4 Myr from the time of subduction to its first appearance in the arc lava signature. Component 4, the fluid contribution to the lava source is inferred to contribute ~1 ppm Rb, 10 ppm Ba, 0.02 ppm U, 600 ppm K, 0.2 ppm Ph, and 30 ppm Sr. It has 87Sr/86Sr = 0.7035 and 206Pb/204Pb = 18.5 and thus it is inferred to have been derived from dehydration of the subducting altered oceanic crust. U-Th isotope disequilibria reflect the time since fluid release from the subducting slab and a reference line through the lowest (230Th/232Th) lavas constrains this to be 30000-50000 yr. The U-Th and Th-Ra isotope systematics are decoupled, and it is suggested that Th-Ra isotope disequilibria record the time since partial melting and thus indicate rapid channelled magma ascent. Olivine gabbro xenoliths from Raoul are interpreted as cumulates to their host lavas with which they form zero age U-Th isochrons indicating that minimal time was spent in magma chambers. The subduction signature is not observed in lavas from the backarc island of Niuafo'ou. These were derived from partial melting of fertile peridotite at 130-160 km depth with melt rates around 0.0002 kg/m**3/yr.