Jiri Konopasek

Distribution of zinc and its role in the stabilization of spinel in high-grade felsic rocks of the Moldanubian domain (Bohemian Massif)

(Fe–Zn–Mg)-spinel-bearing assemblages in the high-grade rocks from three localities in the Moldanubian domain of the Bohemian Massif (Variscan belt of Central Europe) have been studied to monitor the partition behaviour of zinc along the pressure–temperature path followed by the host rock and to discuss the role of Zn in spinel stabilization. Spinels in two studied samples are characterized by low gahnite content (max. 0.09 Zn p.f.u), they crystallized only under silica-undersaturated conditions and the presence of zinc is thus assumed to be insignificant for their stabilization. One sample contains spinels that are gahnite-rich (up to 0.58 Zn p.f.u) leading to their stabilization within a quartz-bearing matrix. All spinel-bearing domains reflect late re-equilibration corresponding to decompression. However, in samples with preserved relics of a high-pressure mineral assemblage, changes in the zinc content attributed to prograde garnet growth were observed in the garnet cores. In order to monitor the distribution of zinc along the pressure-temperature path, textural observations have been combined with electron-microprobe and laser ablation ICP-MS data. The results point to preferential partitioning of zinc into biotite during the spinel-absent early stages of decompression and suggest that biotite was the main source of zinc for spinel crystallizing at low pressures.

Eclogite-facies metamorphism at the eastern margin of the Bohemian Massif -subduction prior to continental underthrusting?

In garnetiferous micaschists occurring at the northern termination of the Moravian domain, metabasites were found containing relics of eclogite-facies assemblages. In these rocks, minimum pressure conditions of 13 kbar at temperatures of 625–655 °C were followed by decompression to amphibolite-facies conditions. Surrounding metapelites contain Fe-rich chloritoid and Fe-rich staurolite preserved as inclusions in garnet porphyroblasts indicating early PT conditions of 570 °C at pressures of 6–7 kbar. The matrix assemblage shows an increase of temperature and pressure to about 615 °C at 6.5–9 kbar. The PT paths of both metapelite and amphibolitized eclogite are suggested to converge at the peak temperature conditions of the metasediments. The PT evolution of the eclogitic sample and of the surrounding metapelites is different from the PT history of the same rocks occurring in the Moldanubian domain but similar to that of the Moravian metasediments. Thus, the presence of eclogite-facies metamorphism at the eastern margin of the Bohemian massif suggests early subduction and subsequent collision of the Brunovistulian foreland with the internal Variscan orogenic root domain.

Eclogitic micaschists in the central part of the Krušné hory Mountains (Bohemian Massif)

In the central part of the Krusne hory (Erzgebirge) Mountains (Saxothuringian domain of the Bohemian Massif) in the Czech Republic, mafic eclogites are associated with a horizon of garnetiferous micaschists. In these rocks, analysis of stable mineral assemblages together with thermodynamic modelling of their evolution shows that the two-phase AFM assemblage garnet-kyanite in the Měděnec area has equilibrated at 640°C and 22 kbar, whereas the two-phase assemblage garnet-chloritoid in the Srni area represents amphibolite-facies conditions. The matrix mineral assemblages show that subsequent decompression is not associated with temperature increase. The observed metamorphic evolution suggests rapid exhumation of the eastern margin of the Saxothuringian domain after its subduction below an already thickened orogenic root.

Laser ablation ICPMS dating of zircons in Erzgebirge orthogneisses: evidence for Early Cambrian and Early Ordovician granitic plutonism in the western Bohemian Massif

U-Pb isotopic data obtained by laser ablation ICPMS analysis of nine zircons with centre to margin oscillatory growth zones from a K-feldspar-rich augen gneiss in the allochthonous Lower Crystalline Nappe of the Erzgebirge domain of the western part of the Bohemian Massif yield a concordia age of 524 ± 10 Ma (2 sigma). This Early Cambrian age represents the time of magmatic crystallization of the protolith of a representative, from near Měděnec, of the allochthonous “Red gneiss” whose igneous nature is shown by the presence of (deformed) xenoliths. Data from TIMS analysis of zircons with variable proportions of unzoned xenocrystic cores surrounded by oscillatory-zoned overgrowths point to magma derivation from upper Proterozoic, or older, rocks. Data obtained for five zircon grains from another “Red gneiss” in the Lower Crystalline Nappe (in the Klinovec anticline) plot below the concordia with the age of the one point that is near concordant being 519 ± 26 Ma (2 sigma). These data, together with internal features of the zircons, are consistent with Early Cambrian granitic plutonism also in this part of the Erzgebirge but with later Pb loss, possibly associated with considerable fluid movement during thrust nappe development. Another sample of a coarse-grained orthogneiss from the autochthonous St Catherine9s dome yielded a significantly younger Early Ordovician age of 480 ± 10 Ma (2 sigma) calculated from eight zircon analyses. However, three zircon grains from the same sample gave a significantly older near-concordant Late Proterozoic age of ca. 620 Ma. Provided that the age difference of ca . 40 Ma between orthogneisses from Měděnec - Klinovec and St Catherine9s dome holds also for other orthogneisses in the Erzgebirge, zircon U-Pb age data could be used to discriminate between allochthonous and autochthonous units in this region. The ca. 25 Ma difference between the Early Cambrian protolith age of the augen gneiss from near Měděnec determined by the laser ablation ICPMS technique and a previously reported older age of 550 ± 9 Ma for a nearby sample determined by the Pb-Pb evaporation technique is accounted for on the basis of the latter not being adequate for dating zircons with a small xenocrystic component. This study demonstrates the importance of high spatial resolution dating techniques, such as SHRIMP or laser ablation ICPMS, in dating zircons with complex growth history that are common in crustally-derived melts.