(Table 1) Characteristics of silicified sediments at DSDP Site 61-462 according to host-rock lithology, opal-CT-to-quartz ratio, and the origin of the quartz phase

This study deals with the mineralogical variability of siliceous and zeolitic sediments, porcellanites, and cherts at small intervals in the continuously cored sequence of Deep Sea Drilling Project Site 462. Skeletal opal is preserved down to a maximum burial depth of 390 meters (middle Eocene). Below this level, the tests are totally dissolved or replaced and filled by opal-CT, quartz, clinoptilolite, and calcite. Etching of opaline tests does not increase continously with deeper burial. Opal solution accompanied by a conspicuous formation of authigenic clinoptilolite has a local maximum in Core 16 (150 m). A causal relationship with the lower Miocene hiatus at this level is highly probable. Oligocene to Cenomanian sediments represent an intermediate stage of silica diagenesis: the opal-CT/quartz ratios of the silicified rocks are frequently greater than 1, and quartz filling pores or replacing foraminifer tests is more widespread than quartz which converted from an opal-CT precursor. As at other sites, there is a marked discontinuity of the transitions from biogenic opal via opal-CT to quartz with increasing depth of burial. Layers with unaltered opal-A alternate with porcellanite beds; the intensity of the opal-CT-to-quartz transformation changes very rapidly from horizon to horizon and obviously is not correlated with lithologic parameters. The silica for authigenic clinoptilolite was derived from biogenic opal and decaying volcanic components.

Vassallo, Ángel. Obra reunida. Buenos Aires: Editorial Las Cuarenta, 2012, t. I, 462 pp.; t. II, 366 pp.; t. III, 496 pp.

Fil: Velarde Cañazares, Marcelo.

Benigno B. Lugones, Crónicas, folletines y otros escritos (1879-1884). Buenos Aires, Biblioteca Nacional, 2012, Col. 'Los Raros', edición crítica y estudio preliminar de Diego Galeano, 462 páginas

Fil: Giaccio, Laura. Universidad Nacional de La Plata. Facultad de Humanidades y Ciencias de la Educación; Argentina.

Benigno B. Lugones, Crónicas, folletines y otros escritos (1879-1884). Buenos Aires, Biblioteca Nacional, 2012, Col. 'Los Raros', edición crítica y estudio preliminar de Diego Galeano, 462 páginas

Fil: Giaccio, Laura. Universidad Nacional de La Plata. Facultad de Humanidades y Ciencias de la Educación; Argentina.

Isotopic composition of igneous rocks from DSDP Site 462

The tholeiitic basalts and microdolerites that comprise the Cretaceous igneous complex in the Nauru Basin in the western equatorial Pacific have moderate ranges in initial 87Sr/86Sr (0.70347 - 0.70356), initial 143Nd/144Nd (0.51278 - 0.51287), and measured 206Pb/204Pb (18.52 - 19.15), 207Pb/204Pb (15.48 - 15.66) and 208Pb/204Pb (38.28 - 38.81). These isotopic ratios overlap with those of both oceanic island basalts (OIB) and South Atlantic and Indian mid-ocean ridge basalts (MORB). However, the petrography, mineralogy, and bulk rock chemistry of the igneous complex are more similar to MORB than to OIB. Also, the rare earth element contents of Nauru Basin igneous rocks are uniformly depleted in light elements (La/Sm(ch) < 1) indicative of a mantle source compositionally similar to that of MORB. These results suggest that the igneous complex is the top of the original ocean crust in the Nauru Basin, and that the notion that the crust must be 15 to 35 m.y. older based on simple extrapolation and identification of the M-sequence magnetic lineations (Larson et al., 1981, doi:10.2973/dsdp.proc.61.1981; Moberly et al., 1985, doi:10.2973/dsdp.proc.81.1984) may be invalid because of a more complicated tectonic setting. The igneous complex most probably was extruded from an ocean ridge system located near the anomalously hot, volcanically active, and isotopically distinct region in the south central Pacific which has been in existence since c. 120 Ma.

Paleomegnetic of basaltic sequenzes from DSDP Site 462 in the Nauru Basin

The voluminous volcanic eruptions in the Nauru Basin, Western Pacific, have long been regarded as important research targets for tectonic history of the Pacific Plate and for the widespread Cretaceous volcanic activity in the Western Pacific. The Nauru Basin volcanic rocks were recovered at Site 462 by Deep Sea Drilling Project (DSDP) Legs 61 and 89, where more than 600 m of lavas and sills were drilled, thereby making it the deepest penetration into crust of Cretaceous age in the Pacific Ocean. For paleomagnetism, this section represents a unique possibility for averaging out secular variation to obtain a reliable paleolatitude estimate. However, previous paleomagnetic studies have only been subjected to alternating field (AF) demagnetization on several core samples, thus, unable to provide comprehensive understanding on the paleolatitude of the basin. The work reported here aims to determine the Cretaceous paleomagnetic paleolatitude for the Pacific Plate and define the magnetostratigraphy for the basaltic sections drilled in the Nauru Basin. A total of 391 basaltic rock samples were carefully re-sampled from DSDP Sites 462 and 462A. Stepwise thermal and AF demagnetizations have isolated characteristic components in the majority of the samples. The most important findings from this study include: (1) Two normal and one reversed polarity intervals are identified in Site 462, and six normal and six reversed polarity intervals are found in Site 462A, although possible erroneous markings of the opposite azimuth for some reversed polarity cores during the DSDP coring cannot be completely ruled out. (2) Based on previous radiometric ages, the magnetostratigraphic correlations with the Geomagnetic Polarity Time Scale (GPTS) indicate that the lower-basaltic flow unit in Site 462A began to erupt at least before 130 Ma. No correlation is available for the upper-sill unit. (3) Paleosecular variation for the lower-flow unit has been sufficiently averaged out; whereas bias may exist for that of the upper-sill unit; (4) The calculated mean inclination of ~50° for the lower-flow unit yields a paleolatitude of 30.8°S for the Nauru Basin at the time of emplacement. This value is well to the north of suggested location in plate reconstruction models, suggesting that there has been a significant amount of apparent polar wander of the Nauru Basin and Pacific plate since 130 Ma. In addition, the paleolatitude for the Nauru Basin is ~7° further south and the basin's age is more than 10 my older than those of the Ontong Java Plateau (OJP), which suggest that the volcanic eruptions of the lower flows in the Nauru Basin are unlikely related to the emplacement of the Ontong Java Plateau.