Vol 57, No 2 (2019)

The paper presents the very first data on concentrations of major and trace elements; Sr, Nd, and Pb isotopic ratios of rocks; and the composition of olivine phenocrysts of 38-Ma basalts recovered by Hole 513a (DSDP Leg 71) in the South Atlantic. The bulk-rock samples and the chilled glasses are mildly magnesian (7–8 wt % MgO) and bear elevated FeO and low Na₂O concentrations, as is typical of MORB of the TOR-1 type. Olivine phenocrysts (Fo_84.5–88) in these rocks contain concentrations of trace elements (Ni, Mn, Cr, and Zn) that are typical of classic MORB, which are produced by partial melting of mantle peridotite. The rocks are strongly depleted in incompatible elements [(La/Sm)_n ~ 0.6] but have elevated Ba/Nb, K/Nb, and Pb/Ce ratios and Cu, Ag, and Au concentrations that are 1.5–4 times higher than in typical depleted MORB (N-MORB) and in most rift basalts in the South Atlantic. Isotope compositions of the basalts (average ratios ²⁰⁶Pb/²⁰⁴Pb ~ 18.0; ²⁰⁷Pb/²⁰⁴Pb ~ 15.6, ²⁰⁸Pb/²⁰⁴Pb ~ 38.0, ¹⁴³Nd/¹⁴⁴Nd ~ 0.5130, and ⁸⁷Sr/⁸⁶Sr ~ 0.7040) are close to those in modern tholeiites from the southern MAR segment (SMAR) north of the Agulhas Fracture Zone. The data indicate that the magmas were derived from a strongly depleted mantle source that contained a minor (~3%) admixture of an enriched component, which is discernible in the magmas of the Discovery hotspot. The composition of the source, which is more depleted than DM, and the high degrees of melting of this source explain why the basalts from DSDP Hole 513a are enriched in chalcophile elements. It is believed that spreading magmatism at 45°–48° S in SMAR as far back as 40 Ma was already affected by the Discovery hotspot. This hotspot might be related to the Tristan plume system, and its origin and long-lasting influence on spreading magmatism in the South Atlantic are regarded as evidence of the extensive effect of the Tristan plume.

The salt-bearing and suprasalt units at the Verkhnekamskoe salt deposit contain newly formed Ti oxides and zircon. The minerals may have been produced at the decomposition of clay material and release of colloid Ti and Zr hydroxides during diagenesis, catagenesis, and supergenesis because of sulfate reduction and an acidic environment caused by Fe²⁺ oxidation in the sylvite and carnallite. Anatase was found in both the suprasalt units and the salt-bearing ones, whereas rutile occurs exclusively in the bottom part of the salt-bearing unit. The possible reasons for this distribution may have been intensification of the catagenetic transformations with depth and the composition of the coexisting sulfate or chloride brines, respectively. The anatase typically forms pinacoidal crystals, which give way to dipyramidal ones in the carnallite zone (whose solutions are the most acidic). The rutile shows twins of three types, whose origin was caused by the growth of the mineral in the fine-grained material of dehydrated Ti hydroxide colloid. The Ti oxides contain V, Cr, Zr, and (Al, Si, and Fe) admixtures, which reflect the geochemistry of the erosion regions which provided the aluminosilicate material.

The Cambrian strata in Tarim basin is mainly evaporate platform facies dolomite. Due to the lack of trilobite fossils, it is hardly to achieve isochronous stratigraphic division and correlation. The Xiaoerbulake outcrop and Fang 1 well are typical surface and subsurface Cambrian sections in Tarim Basin respectively. Shayilike, Awatage and Xiaqiulitage Groups were regarded as the complete Upper -Middle Cambrian previously which composed of dolomite. The carbon isotope values range from –15.5 to 2‰, and three negative excursions are found (N1, N2 and N3). Comparing with the Global Standard Chronostratigraphic Scale for the Middle and Upper Cambrian System, the fifth stage and the Drumian of the third Series, the ninth and the tenth Stage of Furongian Series can be recognized, while the Guzhangian and Paibian are lost. As a result, the Cambrian Formation in Tarim Basin can be divided accurate to stage scale.

Rock–Eval pyrolysis data were obtained on a collection of peat samples at various diagenesis stages (from depths of 0–350 cm) from the Gurskoe deposit, lower Amur area, and on producers of organic matter (OM). The pyrograms of the peat OM are superpositions of three peaks: (1) a low-temperature one, with temperatures of hydrocarbon release lower than 300°C, (2) an intermediate-temperature peak, with hydrocarbons released at 300–400°C, and (3) a high-temperature one, corresponding to hydrocarbon release at temperatures above 400°C. The peaks correspond to the lipid, labile, and kerogen OM components, respectively. The diagenetic transformation of OM is associated with a gradual decrease in the contents of the lipid and labile components and an increase in the content of the kerogen component. The process is the most intense in the upper part of the vertical section.