Jana Kotková

Diamond and coesite discovered in Saxony-type granulite: Solution to the Variscan garnet peridotite enigma

Diamond and coesite were discovered in-situ as inclusions in garnet, kyanite and zircon in high-pressure granulites from northern Bohemian Massif. These continental crustal rocks were therefore subducted to depths of c. 140 km, which also explains their common association with mantle garnet-bearing peridotites. Models involving crustal thickening for these high-pressure granulites need to be significantly modified. Whole Variscan belt with numerous HP granulite occurrences can represent a large ultrahigh-pressure terrain.

Zircon dating of North Bohemian granulites, Czech Republic: further evidence for the Lower Carboniferous high-pressure event in the Bohemian Massif

U-Pb zircon and rutile multigrain ages and 207Pb/206Pb zircon evaporation ages are reported from high-pressure felsic and metapelitic granulites from northern Bohemia, Czech Republic. The granulites, in contrast to those from other occurrences in the Bohemian Massif, do not show evidence of successive HT/MPLP overprints. Multigrain size fractions of nearly spherical, multifaceted, metamorphic zircons from three samples are slightly discordant and yield a U-Pb Concordia intercept age of 348 ± 10 Ma, whereas single zircon evaporation of two samples resulted in 207Pb/206Pb ages of 339 ± 1.5 and 339 ± 1.4 Ma, respectively. A rutile fraction from one sample has a U-Pb Concordia intercept age of 346 ± 14 Ma. All ages are identical, within error, and a mean age of 342 ± 5 Ma was adopted to reflect the peak of HP metamorphism. Because rutile has a lower closing temperature for the U-Pb isotopic system than zircon, the results and the P-T data imply rapid uplift and cooling after peak metamorphism. The above age is identical to ages for high-grade metamorphism reported from the southern Bohemian Massif and the Granulite Massif in Saxony. It can be speculated that all these granulites were part of the same lower crustal unit in early Carboniferous, being separated later due to crustal stacking and subsequent late Variscan orogenic collapse.

Formation and evolution of high-pressure leucogranulites: Experimental constraints and unresolved issues

Abstract High-pressure (HP) leucogranulites of the Bohemian Massif are interpreted as the metamorphosed equivalents of HP leucogranites produced by deep crustal melting. This is supported by their preserved mineral assemblages (Grt-Ky-mesoperthite), bulk rock chemistry, P-T estimates, and garnet and accessory phase trace element abundances. Following melting and peak metamorphism, the leucogranulites have been exhumed from lower crustal depths to their present position at the highest structural level of the Gfohl Nappe. The nearisothermal decompression (ITD) P-T path and available geochronological data imply high exhumation rates. The dry character of the leucogranulites reflects the water-undersaturated conditions that prevailed during formation of the precursor leucogranitic melts and their subsequent recrystallization in the middle and lower crust. Compositions of the leucogranulites are displaced towards the Qz-Or join in the Qz-Ab-Or ternary diagram, which corresponds to experimental results for water undersaturated melting. Trace element and REE abundances in whole rocks, garnets and accessory phases are consistent with muscovite and biotite dehydration melting coupled with K-feldspar fractionation or separation as the principal controls on the chemical evolution of the rocks. The melting reactions and protoliths potentially involved in the generation of these HP leucogranite melts are evaluated in the light of available experimental data for water-saturated and dry melting of crustal rocks.

Crystal chemistry and origin of grandidierite, ominelite, boralsilite, and werdingite from the Bory Granulite Massif, Czech Republic

Abstract A mineral assemblage involving grandidierite, ominelite, boralsilite, werdingite, dumortierite (locally Sb,Ti-rich), tourmaline, and corundum, along with the matrix minerals K-feldspar, quartz, and plagioclase, was found in a veinlet cutting leucocratic granulite at Horní Bory, Bory Granulite Massif, Moldanubian Zone of the Bohemian Massif. Zoned crystals of primary grandidierite to ominelite enclosed in quartz are locally overgrown by prismatic crystals of boralsilite and Fe-rich werdingite. Boralsilite also occurs as separate cross-shaped plumose aggregates with Fe-rich werdingite in quartz. Grandidierite is commonly rimmed by a narrow zone of secondary tourmaline or is partially replaced by the assemblage tourmaline + corundum ± hercynite. Grandidierite (XFe = 0.34-0.71) exhibits dominant FeMg-1 substitution and elevated contents of Li (120-1890 ppm). Boralsilite formula ranges from Al15.97B6.20Si1.80O37 to Al15.65B5.29Si2.71O37 and the formula of werdingite ranges from (Fe,Mg)1.44Al14.61B4.00Si3.80O37 to (Fe,Mg)1.22Al14.86B4.25Si3.55O37. Dumortierite and Sb,Ti-rich dumortierite occur as zoned crystals with zones poor in minor elements (≤0.12 apfu Fe+Mg) and zones enriched in Sb (≤0.46 apfu) and Ti (≤0.25 apfu). Secondary tourmaline (XFe = 0.44-0.75) of the schorlmagnesiofotite- foitite-olenite solid solution occurs as a replacement product of grandidierite, rarely boralsilite. Other accessory minerals in the veinlet include monazite-(Ce), ilmenite, rutile, ferberite, srilankite, löllingite, arsenopyrite, and apatite. Formation of the borosilicate-bearing veinlet post-dates the development of foliation in the host granulite and is related to the decompressional process. The assemblage most probably originated from a H2O-poor system at T ~ 750 °C and P ~ 6-8 kbar. Textural relations as well as geological position of the borosilicate veinlet suggest that it represents the earliest intrusion related to pegmatites in the Bory Granulite Massif. Younger granitic pegmatites in the area are characterized by high contents of B, Al, P, Fe, and minor concentrations of W, Ti, Zr, Sc, and Sb. All pegmatite types probably formed within a short time period of ~5 Ma.

Kumdykolite from the ultrahigh-pressure granulite of the Bohemian Massif

Abstract 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 mm 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.

Solid solutions in the system acanthite (Ag2S)–naumannite (Ag2Se) and the relationships between Ag-sulfoselenides and Se-bearing polybasite from the Kongsberg silver district, Norway, with implications for sulfur–selenium fractionation

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

Anatexis during High-pressure Crustal Metamorphism: Evidence from Garnet–Whole-rock REE Relationships and Zircon–Rutile Ti–Zr Thermometry in Leucogranulites from the Bohemian Massif

{a_{{{\text{S}}^{2 - }}}}/{a_{{\text{S}}{{\text{e}}^{2 - }}}}

Evidence for high‐temperature diffusional creep preserved by rapid cooling of lower crust (North Bohemian shear zone, Czech Republic)

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