Eclogite trace in evolution of Late Cenozoic alkaline basalt volcanism on the southwestern flank of the Baikal Rift Zone: Geochemical features and geodynamic consequences

Eclogitized material from the oceanic lithosphere are the most likely source of alkaline basalt magmas in the formation of Late Cenozoic volcanic areas on the southwestern flank of the Baikal Rift Zone. Basaltic trachyandesites of the early stage of volcanism (Pg₃² ~ 28–23 Ma) are rich in high field strength elements (HFSE), P₂O₅, F, Zn, Ga, Sr, Sn, and light rare earth elements (LREE); they are characterized by high values of the following ratios: Fe/Mn = 72–77, Sm/Yb = 7.7–8.5, Sr/Y = 57–63, and Ga/Sc = 2.1–2.3. At this stage, magmas are formed under conditions with a 2–8% degree of partial melting of the mantle substrate enriched with the material of the eclogite source (50–70%) (Cpx/Grt = 1.5–1.7). Basaltoid magmas of the final stage of volcanism (N₁³–N₂¹ ~ 6–4 Ma) are formed from melting (1.5–4%) of a less fertilized mantle (Cpx/Grt = 2.1–3.1, Fe/Mn = 62–71, Sm/Yb = 3.5–4.6, Sr/Y = 29–44, Ga/Sc = 1.0–1.4). The directed variations of the compositions of the successive basaltoid magmas, which were formed in the Late Cenozoic, create an “eclogite trace” in this area.     

The Sr,Nd, and Hf isotopic geochemistry of rocks of the gabbro–diorite–tonalite association from the Eastern Segment of the Middle Urals as an indicator of the age of the continental crust in this area

According to isotopic analysis of rocks of the Reft gabbro–diorite–tonalite complex (Middle Urals), gabbro and related diorite and dikes and vein-shaped bodies of plagiogranitoids, crosscutting gabbro, are similar to the depleted mantle substance in ε_Nd(T) = 8.6–9.7 and ε_Hf(T) = 15.9–17.9. Their model Hf ages are correlated with the time of crystallization. Here, the tonalites and quartz diorites constituting most of the Reft massif are characterized by lower values: ε_Nd(T) = 3.7–6.0, ε_Hf(T) = 11.1–12.7, and T _DM values significantly exceeding the age datings. This is evidence that Neoproterozoic crustal rocks were a source of parental magma for these rocks. The primary ⁸⁷Sr/⁸⁶Sr ratio in rocks of both groups is highly variable (0.70348–0.70495). The data obtained allow us to reach the conclusion that the Reft gabbro–diorite–tonalite complex was formed as a result of nearly synchronous processes occurring in the crust and the mantle within a limited area.     

Les argiles bleues du Cambrien inférieur de Saint-Pétersbourg et leur fissuration. Implications pour des stockages souterrains

The Lower Cambrian Saint Petersburg blue clays are composed of predominant illite and chlorite, sometimes accompanied by kaolinite. The <0.1 μm fraction has a high content of illite–smectite mixed layers. Particle-size distribution is more than 50% of clay particles and about 30% of silts. These blue clays correspond to plastic (and soft) clays; they may be compared to the Callovian clays of Bure (France), where storage of natural waste is envisaged. To cite this article: M. Arnould et al., C. R. Geoscience 334 (2002) 1135–1140.

Résumé

Les « argiles bleues » du Cambrien Inférieur de Saint-Pétersbourg sont constituées d'illite dominante et de chlorite, avec parfois présence de kaolinite. Des interstratifiés illite/montmorillonite sont très abondants dans la phase <0,1 μm. Du point de vue granulométrique, outre plus de 50% de particules argileuses, il existe une phase silteuse de l'ordre de 30%. Du point de vue pétrophysique, ce sont des argiles plastiques, de consistance molle. Elles sont subhorizontales et ont jusqu'à 116 m d'épaisseur. Outre l'absence de métamorphisme, malgré leur âge, leur caractère le plus remarquable est leur réseau de fracturation, bien observable en carrière. Les joints sont nets, sans remplissage ni cimentation. Les plans verticaux sont particulièrement développés. Le volume unitaire des blocs de matrice ne dépasse guère 1 m³. Des traces d'oxydation témoignent de circulations d'eau. Ces argiles silteuses anciennes peuvent aider à mieux connaı̂tre et comprendre les argilites silteuses épigénétiques calloviennes du site de Bure (Haute-Marne, France), où un stockage souterrain de déchets nucléaires est envisagé. Pour citer cet article : M. Arnould et al., C. R. Geoscience 334 (2002) 1135–1140.     

Thermotectonic evolution of an extensional dome: the Cenozoic Osogovo–Lisets core complex (Kraishte zone,western Bulgaria)

The Kraishte region of Bulgaria is located at the junction of the Balkanides and Hellenides-Dinarides tectonic belts. Fission-track analysis on both apatites and zircons documents the Cenozoic exhumation of a Precambrian basement bounded by low-angle detachments. Late Eocene–Oligocene extension began prior to 47 Ma and was dominantly in a top-to-the-southwest direction, confirmed by the sense of younging of apatite and zircon ages. This crustal extension controlled the formation of half-graben sedimentary basins on the hanging walls of the detachments. Thermal modelling of these hanging wall units provides evidence for heat transfer across the detachments from a relatively warm rising footwall. From 32 to 29 Ma, pervasive magmatic activity resulted in the emplacement of rhyolitic to dacitic subvolcanic bodies and dykes, along with intrusion of the Osogovo granite. The results give evidence for extension in the southern Balkan older than, and separated from, the Miocene to Quaternary Aegean extension. This might reflect transtension during northeastward extrusion and rotation of continental fragments around the western boundary of Moesia. Eocene–Oligocene extension seems to have been controlled by the distribution of earlier thickening all around the Carpatho-Balkanic orocline, which is reflected by the Cretaceous emplacement of the Morava Nappe in the Kraishte.     

The outermost ob associations in M33

The blink survey ofUBVR plates of the Tautenburg Schmidt telescope reveals about 20 new stellar associations. A few suspected associations are located at 8 kpc from the center of M33. The new associations fit well into the spiral structure of the galaxy.     

Present-day hydrothermal activity within the Kurile-Kamchatka Island arc and its relation to volcanicity

Bulletin of Volcanology -     

Variations of modal composition of granitic rocks in some regions around the pacific

Granitic rocks from the Andes and from Japan, Kurile Islands, and Kamchatka have been investigated in their modal composition determined. Principal components analysis of the data reveals that there is three main factors influencing variation of modal composition of rocks that can be interpreted in forms of petrological processes. There is a similarity in results of the investigation of rocks from Japan, Kamchatka, and Andean regions.     

Cosmic-ray energetics in the supernova remnant Cassiopeia A

Based on the observed radio spectrum for the supernova remnant Cassiopeia A, we have established that it represents synchrotron radiation of relativistic electrons with a nonpower-law energy spectrum in the form of Kaplan-Tsytovich’s standard distribution. The total density of relativistic electrons is 10^?3 cm^?3, only 20% of which form the radio spectrum. The particle number ratio of the proton-nuclear and electron cosmicray components inside the shell differs significantly from the mean Galactic ratio (100) and probably does not exceed unity.