Geochemistry of oceanic arc basement rocks of Sumisu Rift and Ohmachi Seamount

Prehnite-pumpellyite facies metamorphism is described in the oceanic-arc basement rocks of Ocean Drilling Program Leg 126, Site 791 in the Sumisu Rift, western Pacific. Chemical variations of pumpellyite, epidote, chlorite, and prehnite are examined and paragenetic relations discussed. The metamorphism took place during the pre-rifting stage of an intraoceanic arc. During the backarc rifting stage, the geothermal gradient of the area was not as high as that of a spreading mid-oceanic ridge.

Chemical composition of zoobenthos and bottom sediments from the gas-hydrate seep area near the Paramushir Island, Kuril Island arc

Bioaccumulation of metals by zoobenthos was investigated during cruise 11A of R/V Akademik Mstislav Keldysh in the vicinity of a gas-hydrate seep off Paramushir Island in the Sea of Okhotsk. Atomic absorption studies of concentrations of Al, Fe, Mn, Ni, Cu and Zn in zoobenthos (polychaetes, bivalves, ophiurans and echinoderms) collected from depths of 700-800 m indicated that their concentrations in individuals near the seep were not significantly different from those in individuals from other communities. Obtained results indicate that sea urchins and holothurians (non-sorting bottom-feeders) can separate mineral fraction of ingested bottom material.

Surface sediment samples from several fore-arc basins west and southwest of the Indonesian Archipelago, analyzed by planktonic foraminifera, stable oxygen and carbon isotopic signals and opal and CaCO3 contents in bulk sediment

A total of 69 surface sediment samples from several fore-arc basins located west and southwest of the Indonesian Archipelago was analyzed with respect to the faunal composition of planktonic foraminifera, the stable oxygen and carbon isotopic signal of a surface-dwelling (Globigerinoides ruber) and a thermocline-dwelling (Neogloboquadrina dutertrei) species, and the opal and CaCO3 contents in bulk sediment. Our results show that the distribution pattern of opal in surface sediments corresponds well to the upwelling-induced chlorophyll concentration in the upper water column and thus, represents a reliable proxy for marine productivity in the coastal upwelling area off S and SW Indonesia. Present-day oceanography and marine productivity are also reflected in the tropical to subtropical and upwelling assemblages of planktonic foraminifera in the surface sediments, which in part differ from previous studies in this region probably due to different coring methods and dissolution effects. The average stable oxygen isotopic values (d18O) of G. ruber in surface sediments vary between 2.9 per mill and 3.2 per mill from basin to basin and correspond to the oceanographic settings during the SE monsoon (July-October) off west Sumatra, whereas off southern Indonesia, they reflect the NW monsoon (December-March) or annual average conditions. The d18O values of N. dutertrei show a stronger interbasinal variation between 1.6 per mill and 2.2 per mill and correspond to the upper thermocline hydrology in July-October. In addition, the difference between the shell carbon isotopic values (d13C) of G. ruber and N. dutertrei (Delta d13C) appears to be an appropriate productivity recorder only in the non-upwelling areas off west Sumatra. Consequently, joint interpretation of the isotopic values of these species is distinctive for different fore-arc basins W and SW of Indonesia and should be considered in paleoceanographic studies.

Geochemistry of lavas from the Lau-Tonga arc and back-arc system

Detailed comparison of mineralogy, and major and trace geochemistry are presented for the modern Lau Basin spreading centers, the Sites 834-839 lavas, the modern Tonga-Kermadec arc volcanics, the northern Tongan boninites, and the Lau Ridge volcanics. The data clearly confirm the variations from near normal mid-ocean-ridge basalt (N-MORB) chemistries (e.g., Site 834, Central Lau Spreading Center) to strongly arc-like (e.g., Site 839, Valu Fa), the latter closely comparable to the modern arc volcanoes. Sites 835 and 836 and the East Lau Spreading Center represent transitional chemistries. Bulk compositions range from andesitic to basaltic, but lavas from Sites 834 and 836 and the Central Lau Spreading Center extend toward more silica-undersaturated compositions. The Valu Fa and modern Tonga-Kermadec arc lavas, in contrast, are dominated by basaltic andesites. The phenocryst and groundmass mineralogies show the strong arc-like affinities of the Site 839 lavas, which are also characterized by the existence of very magnesian olivines (up to Fo90-92) and Cr-rich spinels in Units 3 and 6, and highly anorthitic plagioclases in Units 2 and 9. The regional patterns of mineralogical and geochemical variations are interpreted in terms of two competing processes affecting the inferred magma sources: (1) mantle depletion processes, caused by previous melt extractions linked to backarc magmatism, and (2) enrichment in large-ion-lithophile elements, caused by a subduction contribution. A general trend of increasing depletion is inferred both eastward across the Lau Basin toward the modern arc, and northward along the Tongan (and Kermadec) Arc. Numerical modeling suggests that multistage magma extraction can explain the low abundances (relative to N-MORB) of elements such as Nb, Ta, and Ti, known to be characteristic of island arc magmas. It is further suggested that a subduction jump following prolonged slab rollback could account for the initiation of the Lau Basin opening, plausibly allowing a later influx of new mantle, as required by the recognition of a two-stage opening of the Lau Basin.

Hegemonía y contrahegemonía en la América Latina de hoy. Apuntes hacia una nueva época

El despliegue sobre el presente latinoamericano de la problemática gramsciana de la hegemonía, exige tanto una elucidación de yerros y simplificaciones en la utilización de la categoría original, como un repaso histórico de la evolución social y cultural del continente. La estructura de clase, los modos de la lucha social y política, la configuración organizacional y la subjetividad de los luchadores, se han modificado profundamente en estos años, enmarcadas por una ofensiva de alcance estratégico del gran capital. Las concepciones gramscianas pueden (deben) constituir un auxilio fundamental a la hora de volver a pensar en una perspectiva de transformación social radical, que eluda el doble riesgo de la adaptación al orden existente, en un extremo, y un «choque frontal¼ contra el mismo que cambie eficacia real por radicalismo programático. Se encontrarán allí herramientas para examinar críticamente el pasado y el presente del pensamiento y las prácticas contestatarias de nuestro continente; y proyectarlas en dirección a su perspectiva futura.

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.

Hegemonía y contrahegemonía en la América Latina de hoy. Apuntes hacia una nueva época

El despliegue sobre el presente latinoamericano de la problemática gramsciana de la hegemonía, exige tanto una elucidación de yerros y simplificaciones en la utilización de la categoría original, como un repaso histórico de la evolución social y cultural del continente. La estructura de clase, los modos de la lucha social y política, la configuración organizacional y la subjetividad de los luchadores, se han modificado profundamente en estos años, enmarcadas por una ofensiva de alcance estratégico del gran capital. Las concepciones gramscianas pueden (deben) constituir un auxilio fundamental a la hora de volver a pensar en una perspectiva de transformación social radical, que eluda el doble riesgo de la adaptación al orden existente, en un extremo, y un «choque frontal¼ contra el mismo que cambie eficacia real por radicalismo programático. Se encontrarán allí herramientas para examinar críticamente el pasado y el presente del pensamiento y las prácticas contestatarias de nuestro continente; y proyectarlas en dirección a su perspectiva futura.

Petrology and geochemistry of Quaternary tephras of the Izu-Bonin Arc

Quaternary marine tephras in the Izu-Bonin Arc offer significant information about explosive volcanic activities of the arc. Visual core descriptions, petrographic examinations, and chemical and grain-size analyses were conducted on tephras of backarc, arc, and forearc origin. Tephras are black and white and occur in simple and multiple modes with mixed and nonmixed ashes of black and white glass shards. The grain size distributions of the tephras are classified into three categories: coarse, white pumiceous, and fine white and black well-sorted types. The frequency of occurrence of the white and black tephras differs within the tectonic settings of the arc. Chemically, the Quaternary tephras in this region belong to low-alkali tholeiitic series with lower K2O and TiO2 than normal ordinary arc volcanic materials. Several tephras from different sites along the forearc correlate with each other and with tephras in the Shikoku Basin site and with Aogashima volcanics. These volcanic ashes resemble those in other backarc rifting areas, such as in the Fiji, Okinawa (Ryukyu), and Mariana regions.

Sr, Nd and Pb isotopic composition of volcanic and volcaniclastic samples of ODP Leg 134

A suite of volcanic and volcaniclastic rocks selected from Ocean Drilling Program Leg 134 Sites 832 and 833 in the North Aoba Basin (Central New Hebrides Island Arc) has been analyzed for Sr, Nd, and Pb isotopes to investigate the temporal evolution of the arc magmatism. This arc shows two unusual features with respect to other western Pacific arcs: 1) subduction is eastdirected; and 2) a major submarine ridge, the d'Entrecasteaux Zone, has been colliding almost perpendicularly with the central part of the arc since about 3 Ma. Volcanic rocks from the upper parts of both holes, generated during the last 2 m.y., show higher 87Sr/86Sr and significantly lower 206Pb/204Pb and 143Nd/144 Nd values compared to those volcanics erupted before the collision of this ridge, as represented by samples from the lower section of both holes, or remote from the collisional region, in the southern part of the arc. These isotopic differences in the respective mantle sources cannot be interpreted in terms of geochemical input into the mantle wedge induced by the collision itself. Rather, they require long term (>500 m.y.) enrichment processes. The enriched mantle source could be, on a regional scale, a DUPAL-type reservoir with strong similarities to the source of Indian Ocean basalts. Isotopic analyses of drilled rocks from the DEZ show that the anomalous, enriched mantle component is not derived from this feature. We currently cannot identify a source for this enriched component, but note that it also exists in Lau Basin backarc volcanics, lavas from the West Philippine Sea, and also some lavas from the Mariana-Izu-Bonin arc.

Hegemonía y contrahegemonía en la América Latina de hoy. Apuntes hacia una nueva época

El despliegue sobre el presente latinoamericano de la problemática gramsciana de la hegemonía, exige tanto una elucidación de yerros y simplificaciones en la utilización de la categoría original, como un repaso histórico de la evolución social y cultural del continente. La estructura de clase, los modos de la lucha social y política, la configuración organizacional y la subjetividad de los luchadores, se han modificado profundamente en estos años, enmarcadas por una ofensiva de alcance estratégico del gran capital. Las concepciones gramscianas pueden (deben) constituir un auxilio fundamental a la hora de volver a pensar en una perspectiva de transformación social radical, que eluda el doble riesgo de la adaptación al orden existente, en un extremo, y un «choque frontal¼ contra el mismo que cambie eficacia real por radicalismo programático. Se encontrarán allí herramientas para examinar críticamente el pasado y el presente del pensamiento y las prácticas contestatarias de nuestro continente; y proyectarlas en dirección a su perspectiva futura.

Ascidiofauna in the Southern Ocean

Three Spanish Antarctic research cruises (Ant-8611, Bentart-94 and Bentart-95) were carried out in the South Shetland Archipelago (Antarctic Peninsula) and Scotia Arc (South Orkney, South Sandwich and South Georgia archipelagos) on the continental shelf and upper slope (10-600 m depth). They have contributed to our knowledge about ascidian distribution and the zoogeographical relationships with the neighbouring areas and the other Subantarctic islands. The distribution of ascidian species suggests that the Scotia Arc is divided into two sectors, the South Orkney Archipelago, related to the Antarctic Province, and the South Georgia Archipelago (probably including the South Sandwich Archipelago), which is intermediate between the Antarctic Province and the Magellan region.

Chemical composition of glass inclusions of volcanic rocks from the Bonin Arc

The chemical composition of glass inclusions in phenocrystic plagioclase and pyroxene from Sites 792 and 793, drilled during Ocean Drilling Program Leg 126 in the Bonin Arc, is examined. Immiscible liquid, which is preserved as glass inclusions with unmixed textures in plagioclase, is observed in a high-magnesian andesite, which suggests an important role of liquid immiscibility in the fractionation of high-magnesian andesite. In other andesitic rocks (SiO2 = 57-60 wt%), such unmixed textures of glass inclusions in calcic plagioclase with a similar percentage of An (around 80%) is not found. The degree of fractionation and mixing of liquid are inferred from the glass composition in pyroxene.

Age determination of granite from Himalaya

The Gangdese belt, Tibet, records the opening and closure of the Neo-Tethyan ocean and the resultant collision between the Indian and Eurasian plates. Mesozoic magmatic rocks generated through subduction of the Tethyan oceanic slab constitute the main component of the Gangdese belt, and play a crucial role in understanding the formation and evolution of the Neo-Tethyan tectonic realm. U-Pb and Lu-Hf isotopic data for tonalite and granodiorite from the Xietongmen-Nymo segment of the Gangdese belt indicate a significant pulse of Jurassic magmatism from 184 Ma to 168 Ma. The magmatic rocks belong to metaluminous medium-K calc-alkaline series, characterized by regular variation in major element compositions with SiO2 of 61.35%-73.59 wt%, low to moderate MgO (0.31%-2.59%) with Mg# of 37-45. These magmatic rocks are also characterized by LREE enrichment with concave upward trend in MREE on the chondrite-normalized REE patterns, and also LILE enrichment and depletion in Nb, Ta and Ti in the primitive mantle normalized spidergrams. These rocks have high zircon ?Hf(t) values of + 10.94 to + 15.91 and young two-stage depleted mantle model ages (TDM2) of 192 Ma to 670 Ma. The low MgO contents and relatively depleted Hf isotope compositions, suggest that the granitoid rocks were derived from the partial melting of the juvenile basaltic lower crust with minor mantle materials injected. In combined with the published data, it is suggested that northward subduction of the Neo-Tethyan slab beneath the Lhasa terrane began by the Late-Triassic, which formed a major belt of arc-related magmatism.

Composition of rocks and minerals from the Bolshoy Lyakhovsky Island (New Siberian Islands)

In the southeast of the Bolshoi Lyakhovsky Island there are outcrops of tectonic outliers composed of low-K medium-Ti tholeiitic basic rocks represented by low altered pillow basalts, as well as by their metamorphosed analogs: amphibolites and blueschists. The rocks are depleted in light rare-earth elements and were melted out of a depleted mantle source enriched in Th, Nb, and Zr also contributed to the rock formation. The magma sources were not affected by subduction-related fluids or melts. The rocks were part of the Jurassic South Anyui ocean basin crust. The blueschists are the crust of the same basin submerged beneath the more southern Anyui-Svyatoi Nos arc to depth of 30-40 km. Pressure and temperature of metamorphism suggest a setting of "warm" subduction. Mineral assemblages of the blueschists record time of a collision of the Anyui-Svyatoi Nos island arc and the New Siberian continental block expressed as a counter-clockwise PT trend. The pressure jump during the collision corresponds to heaping of tectonic covers above the zone of convergence 12 km in total thickness. Ocean rocks were thrust upon the margin of the New Siberian continental block in late Late Jurassic - early Early Cretaceous and mark the NW continuation of the South Anyui suture, one of the main tectonic sutures of the Northeastern Asia.

Lithogeochemical and isotope (Rb-Sr, Sm-Nd, Pb-Pb whole rock and Lu-Hf zircon) composition of samples from the Shackleton Range, East Antarctica

Three distinct, spatially separated crustal terranes have been recognised in the Shackleton Range, East Antarctica: the Southern, Eastern and Northern Terranes. Mafic gneisses from the Southern Terrane provide geochemical evidence for a within-plate, probably back-arc origin of their protoliths. A plume-distal ridge origin in an incipient ocean basin is the favoured interpretation for the emplacement site of these rocks at c. 1850 Ma, which, together with a few ocean island basalts, were subsequently incorporated into an accretionary continental arc/supra-subduction zone tectonic setting. Magmatic underplating resulted in partial melting of the lower crust, which caused high-temperature granulite-facies metamorphism in the Southern Terrane at c. 1710-1680 Ma. Mafic and felsic gneisses there are characterised by isotopically depleted, positive Nd and Hf initials and model ages between 2100 and 2000 Ma. They may be explained as juvenile additions to the crust towards the end of the Palaeoproterozoic. These juvenile rocks occur in a narrow, c. 150 km long E-W trending belt, inferred to trace a suture that is associated with a large Palaeoproterozoic accretionary orogenic system. The Southern Terrane contains many features that are similar to the Australo-Antarctic Mawson Continent and may be its furthermost extension into East Antarctica. The Eastern Terrane is characterised by metagranitoids that formed in a continental volcanic arc setting during a late Mesoproterozoic orogeny at c. 1060 Ma. Subsequently, the rocks experienced high-temperature metamorphism during Pan-African collisional tectonics at 600 Ma. Isotopically depleted zircon grains yielded Hf model ages of 1600-1400 Ma, which are identical to Nd model ages obtained from juvenile metagranitoids. Most likely, these rocks trace the suture related to the amalgamation of the Indo-Antarctic and West Gondwana continental blocks at ~600 Ma. The Eastern Terrane is interpreted as the southernmost extension of the Pan-African Mozambique/Maud Belt in East Antarctica and, based on Hf isotope data, may also represent a link to the Ellsworth-Whitmore Mountains block in West Antarctica and the Namaqua-Natal Province of southern Africa. Geochemical evidence indicates that the majority of the protoliths of the mafic gneisses in the Northern Terrane formed as oceanic island basalts in a within-plate setting. Subsequently the rocks were incorporated into a subduction zone environment and, finally, accreted to a continental margin during Pan-African collisional tectonics. Felsic gneisses there provide evidence for a within-plate and volcanic arc/collisional origin. Emplacement of granitoids occurred at c. 530 Ma and high-temperature, high-pressure metamorphism took place at 510-500 Ma. Enriched Hf and Nd initials and Palaeoproterozoic model ages for most samples indicate that no juvenile material was added to the crust of the Northern Terrane during the Pan-African Orogeny but recycling of older crust or mixing of crustal components of different age must have occurred. Isotopically depleted mafic gneisses, which are spatially associated with eclogite-facies pyroxenites, yielded late Mesoproterozoic Nd model ages. These rocks occur in a narrow, at least 100 km long, E-W trending belt that separates alkaline ocean island metabasalts and within-plate metagranitoids from volcanic arc metabasalts and volcanic arc/syn-collisional metagranitoids in the Northern Terrane. This belt is interpreted to trace the late Neoproterozoic/early Cambrian Pan-African collisional suture between the Australo-Antarctic and the combined Indo-Antarctic/West Gondwana continental blocks that formed during the final amalgamation of Gondwana.