AVČR

The aim of the project is characterisation of Re-Os and Lu-Hf isotopic systems on pyroxenites, peridotites from the Bohemian Massif

The basis of the international Czech–Spanish project is a detailed correlation of selected sedimentary successions of the late Silurian and early Devonian in peri-Gondwana (Prague Synform, Spanish Central Pyrenees and Carnic Alps). The correlation is based on application of several methods in the sections: the detailed biostratigraphic framework is supplemented by multiple chemo-physical measurements (i.e., gamma-ray spectrometry and magnetic susceptibility) in order to avoid discrepancies in correlation of the peri-Gondwanan successions. A substantial part of the project is focused on conodont biostratigraphy which is fundamental for time correlation.
The highest precision has been attained in the Lochkovian where numerous cosmopolitan conodont time-marks occur. These are mostly represented by taxa belonging to the genera Ancyrodelloides and Lanea with minor widespread of other relevant taxa as Flajsella, Masaraella, Pedavis and Kimognathus. In three key areas of European peri-Gondwanan sections (Spanish Central Pyrenees; Prague Synform and Carnic Alps) the Icriodus
and Pelekysgnathus record is also remarkable and can help in increasing the detail of correlations. In 2013 the basic correlation of the major Lochkovian conodont time-marks was finished and a draft of a refined biostratigraphical subdivision for the key areas was made. This preliminary subdivision became a basis for correlation with even more distant regions, e.g., Southern Urals and Northeast Asia. The biozonation established for the Pyrenees and Prague Synform has been successfully tested in the Southern Urals (Mavrinskaya & Slavík 2013). According to the plan, the project leaders organized two field campaigns in 2013. First field work in the Prague Synform was focussed on the examination of classical Lower Devonian sections and conodont sampling. The following field campaign in the Pyrenees concentrated on hi-res biostratigraphic correlation between th Czech and Spanish sections. Two Lower Devonian sections (Segre II near Seu d’Urgell and Compte 1 near Baro and Sort) were sampled for magnetic susceptibility, and gamma-ray spetrometric data were obtained. All the samples for MS were already meassured. Evaluation of the results and further fieldwork is planned for 2014.
MAVRINSKAYA T. & SLAVÍK L. (2013): Correlation of Early Devonian (Lochkovian–early Pragian) conodont faunas of the South Urals (Russia). – Bulletin of Geosciences, 88, 2: 283–296.

A new lithotype of peridotite, phlogopite- and apatite-bearing spinel–garnet peridotite, associated with leucocratic granulite, has been recognized at the Plešovice quarry in the Gföhl Unit within the Moldanubian Zone of the Bohemian Massif, Czech Republic. There are three equilibrium stages in the Plešovice peridotite. The existence of Stage I, the precursor spinel ± garnet peridotite stage, is supported by the presence of an aluminous (Al2O3 3.0 wt. %) orthopyroxene megacryst in the matrix. The minimum temperature of Stage I was estimated to be 1,020 ± 15 °C. Stage II is defined by the cores of relatively large (<3 mm long) grains of olivine, low-Al orthopyroxene
(Al2O3 1.3–1.7 wt. %), clinopyroxene, and chromian spinel [Cr/(Cr + Al) = 0.50–0.57], along with relatively small (<1 mm long) Ba-rich phlogopite (BaO = 1.0–4.0 wt. %), Sr-rich apatite (SrO 1.7 wt. %) and rare potassic (K2O 0.9–1.2 wt. %) amphibole. Garnet generally occurs as large spheroidal grains (up to 20 mm in diameter). It contains inclusions of olivine, orthopyroxene, chromian spinel, and phlogopite, all of which have similar compositions to their matrix counterparts. Therefore, garnet appears to be in equilibrium with the matrix phases at Stage II. Application of appropriate geothermobarometers to the assemblage at Stage II yielded temperatures of 850–1,030 °C and pressures of 2.3–3.5 GPa. Stage III is defined by aluminous orthopyroxene (Al2O3 2.1–4.0 wt. %), aluminous clinopyroxene and aluminous spinel along with pargasitic amphibole and
Ba-rich phlogopite in kelyphite; temperature conditions at this stage were estimated to be 730–770 (± 27) °C at 0.8–1.5 GPa. Multiphase solid inclusions, mainly composed of phlogopite, dolomite, apatite and calcite with minor amounts of chlorite and magnesiohornblende, are present only within large grains of chromian spinel, which are surrounded by kelyphites. The idiomorphic outline of the multiphase solid inclusions suggests that frozen remnants of carbonatite melts or supercritical fluids were trapped in the spinel. The mineral assemblage in the multiphase solid inclusions suggests relatively low-P and low-T conditions (T <750 °C; P <1.6 GPa) for its crystallization. Furthermore, the timing of the crystallization of the multiphase solid inclusions appears to predate Stage II, as most multiphase solid inclusions are completely surrounded by the host chromian spinel. These
data suggest that the Plešovice peridotite experienced cooling after Stage I and was then transformed to spinel–garnet peridotite by subsequent subduction processes.