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The Lu–Hf isotope system and Sr–Nd–Hf–Os isotope systematics of mantle rocks are capable of unravelling the early processes in collision belts, especially in a hot subduction context where the Sm–Nd and U–Pb systems in crustal rocks are prone to resetting owing to high temperatures and interaction with melts during exhumation. To improve models of the Devonian–Carboniferous evolution of the Bohemian Massif, we investigated in detail mafic and ultramafic rocks (eclogite, pyroxenite, and peridotite) from the ultrahigh-pressure and ultrahigh-temperature Kutná Hora Crystalline Complex (KHCC: Úhrov, Bečváry, Doubrava, and Spačice localities). Petrography, multiphase solid inclusions, major and trace element compositions of rocks and minerals, and radiogenic isotopic data document contrasting sources and protoliths as well as effects of subduction-related processes for these rocks. The Úhrov peridotite has a depleted composition corresponding to the suboceanic asthenospheric mantle, whereas Bečváry and Doubrava peridotites represent lithospheric mantle that underwent melt refertilization by basaltic and SiO2-undersaturated melts, respectively. Multiphase solid inclusions enclosed in garnet from Úhrov and Bečváry peridotites represent trapped H2O ± CO2-bearing metasomatizing agents and Fe–Ti-rich melts. The KHCC eclogites either formed by high-pressure crystal accumulation from mantle-derived basaltic melts (Úhrov) or represent a fragment of mid-ocean ridge basalt-like gabbroic cumulate (Spačice) and crustal-derived material (Doubrava) both metamorphosed at high P – T conditions. The Lu–Hf age of 395 ± 23 Ma obtained for the Úhrov peridotite reflects garnet growth related to burial of the asthenospheric mantle during subduction of the oceanic slab. By contrast, Spačice and Doubrava eclogites yield younger Lu–Hf ages of ∼350 and 330 Ma, respectively, representing mixed ages as demonstrated by the strong granulite-facies overprint and trace element zoning in garnet grains. We propose a refined model for the Early Variscan evolution of the Bohemian Massif starting with the subduction of the oceanic crust (Saxothuringian ocean) and associated oceanic asthenospheric mantle (Úhrov) beneath the Teplá–Barrandian at ≥380 Ma, which was responsible for melt refertilization of the associated mantle wedge (Bečváry, Doubrava). This was followed by continental subduction (∼370–360 Ma?) accompanied by the oceanic slab break-off and incorporation of the upwelling asthenospheric mantle into the Moldanubian lithospheric mantle and subsequent coeval exhumation of mantle and crustal rocks at ∼350–330 Ma. [ABSTRACT FROM AUTHOR]

Results of the project "Mongol Altai-50"of the Czech Development Cooperation with Mongolia, were successfully defended before the Mongolian Stratigraphic Commission in September 2016, in Ulan Bator. The project was carried out during 2013-2016, and experts from four countries participated in its implementation. The project was located in western Mongolia in the Hovd Department and the districts (soums) of Munhkhairkhan, Mankhan, and Zereg (Fig. 1). The western part of the Mongolian Altai Range has a mostly character of stepped plateaus, some of which rise above 4000 m a.s.l. There is a different morphology in the lower and peripheral parts of the mountain rejuvenated by deep faults or in places with former glaciation. Rocky ridges and deep gorges and canyons originated due to intensive feedback erosion (Figs. 2a-h). Five base geological maps at a scale of 1 : 50,000 (sheets L_46_28V, L_46_28G, L_46_29V, L_46_29G and L_46_30V) covering an area of 1770 km2 were compiled during the project implementation. The field-work in Mongolia was realised during three main campaigns in 2013-2015, each year 2-3 months long (Figs. 3 a-d). The project also had an educational significance, which involved training of 19 young Mongolian geologists in the field (Figs. 4 a-d). The Final Report (Žáček et al. 2016a) presents 43 new geochronological data, new insights into petrology, geochemistry, mineralogy, structural geology, sedimentology and palaeontology, and also includes a chapter on economic geology, and 50 applied maps showing results of exploration geochemistry. A set of geological maps represents an instructive E-W profile across an area on the contact of Lake, Altai and Hovd terranes (zones), which are part of the Central Asian orogenic belt (CAOB, Badarch et al. 2002), Fig. 5. These terranes are a collage of Lower Palaeozoic volcanic arcs, accretionary wedges, and back-arc basins separated from one another by deep-seated faults. The generally NW-SE trending and relatively steeply dipping lithological contacts, deformational fabrics and morphological pattern are common geological feature of a western Mongolian Altai. Structural record in the rocks is a result of polyphase development of three ductile deformational phases portrayed in varying intensity. The youngest deformation phase is associated with penetrative hydrothermal event, which produced abundant epidote and chlorite, ubiquitous in whole research area. Previous maps of the area on the scale 1 : 200,000 were given by Luvsandanzan et al. (1999) and Baatarhuyag and Gansukh (1999), general and local stratigraphy by Ariunchimeg et al. (2012). The Lake Zone in the E of the research area is formed by mid-Cambrian volcano-sedimentary Baatar Formation. It is intruded in several stages by the Togtokhinshil Igneous Complex, the age of which was newly determined to be Middle Ordovician (gabbros and diorites), and Upper Devonian (granites), Soejono et al. (2016). Thick sequence of folded Lower Palaeozoic sediments prevails in the Hovd and Altai zones. The sediments are ascribed to Zuun Nuruu (Ordovician), Tsetseg (Silurian) and Sagsai (Lower Devonian) formations (see Fig. 5). The age of these sediments decreases generally westwards, but the stratigraphic record is complicated by strong deformation when the same part of the sequence may be repeated. The sediments occur in several developments: deep basinal (shales), common flysh, shallower calcareous flysh, minor volcano-sedimentary, and shallow psammitic. Fossils are rare and poorly preserved, and usually indicate a broader age. The following groups of fossils were found: Graptoloidea, Anthozoa, Crinoidea, Brachiopoda, Bryozoa, Cephalophoda and abundant ichnofossils.(...) [ABSTRACT FROM AUTHOR]

We report the first occurrence of kumdykolite, a high-temperature analog of albite, in the European Variscan belt. It was discovered in an ultrahigh-pressure, diamond-bearing quartzofeldspathic granulite from the northern Bohemian Massif. It is associated with phlogopite and quartz in a multiphase solid inclusion within garnet, considered to represent a trapped fluid or melt phase. Micro-Raman analysis and mapping along with BSE revealed the presence of a sub-equant, elongated grain of kumdykolite reaching 20 µm in length. WDX analysis has shown that kumdykolite contains 2 wt% CaO, probably indicating significant miscibility with the Ca-end-member svyatoslavite. Similar to the case of microdiamond inclusions, the kumdykolite-bearing multiphase inclusion is located in the Ca- and Mg-rich central part of the garnet and thus must have been trapped at P > 4 GPa. The inclusion minerals, however, crystallized upon decompression and cooling during the exhumation. Kumdykolite preservation thus provides independent evidence for high temperature of the original trapped fluid, or melt, crystallization, and rapid cooling of the rocks. Our results imply that kumdykolite and other feldspar modifications stable at elevated pressures and temperatures may be common phases in quartzofeldspathic granulites and need to be searched for. [ABSTRACT FROM AUTHOR]

Sulfoselenides [Ag2(S,Se)] and Se-bearing polybasite have been discovered at the Kongsberg silver district. The selenium-bearing minerals occur in two samples from the northern part of the district, forming either single or polyphase inclusions together with chalcopyrite within native silver. The Ag-sulfoselenides show large chemical variations, covering nearly the complete compositional range between acanthite (Ag2S) and naumannite (Ag2Se). For the data presented here, there is no local maximum at the composition Ag4SSe attributed to the distinct phase called aguilarite, suggesting that this composition can be considered as one of many possible along the monoclinic Ag2S-Ag2S0.4Se0.6 solid solution series rather than a specific mineral phase. We present a model explaining the variations in the Se-content of Ag2(S,Se) as a result of gradual de-sulfidization of the rock under oxidizing conditions. During this process, sulfur from the Ag2S-component of Ag2(S,Se) oxidized and dissolved in the fluid phase as SO42−, resulting in the formation of native silver. The activity ratio aS2-/aSe2- of the system gradually decreased due to the removal of SO42−, which resulted in the stabilization of a sulfoselenide with higher selenium content. As a result of reaction progress, grains of Ag2(S,Se) became gradually enclosed in newly formed native silver, and therefore isolated from further reactions with the grain-boundary fluid. Grains isolated early during the process show low content of Se reflecting high aS2-/aSe2- of the equilibrium fluid, while grains showing high Se reflect the composition of late low aS2-/aSe2- fluids. Analyses of Se-bearing polybasite show that selenium is preferentially partitioned into Ag2(S,Se) compared to polybasite. The model presented here demonstrates how oxidation of sulfoselenides leads to fractionation of sulfur and selenium. [ABSTRACT FROM AUTHOR]

Orogenic garnet peridotites of diverse origins and histories in the Bohemian Massif attest to a variety of mantle processes, including partial melting, cryptic metasomatism, and modal metasomatism (refertilization), all of which are recorded by Saxothuringian garnet peridotite from the T-7 borehole in northern Bohemia. The T-7 peridotite consists of interlayered garnet lherzolite, harzburgite, and phlogopite-garnet pyroxenite lenses that yield peak temperatures and pressures of 1030–1150 °C and 36.1–48.0 kbar. Olivine crystallographic preferred orientations exhibit [axial](010) slip, corresponding to a pure shear component of deformation under relatively low flow stress conditions. Some lherzolite samples are fertile, resembling primitive mantle in major and trace element composition, but other lherzolites are slightly depleted in incompatible major elements, HREE, and HFSE, and slightly enriched in LREE. Harzburgite is depleted in incompatible major elements, HREE, and HFSE, but enriched in LREE. Harzburgite adjacent to pyroxenite has been refertilized, containing phlogopite, less olivine, more orthopyroxene, and more garnet than distal harzburgite. The T-7 peridotite compositions are the result of variable degrees of partial melting in the spinel stability field, followed by cryptic metasomatism and modal metasomatism by transient basaltic melts in the garnet field. Trace elements, Sr and Nd isotopes, and occurrence of phlogopite reflect a subduction component in the metasomatising melts. Partial melting of the T-7 peridotite was a Proterozoic event, as indicated by Rhenium depletion model ages (T RD ); the age of cryptic and modal metasomatism is unconstrained, but is thought to be related to Variscan subduction and amalgamation of the Bohemian Massif. [ABSTRACT FROM AUTHOR]