c. Grant Agency of the Academy of Sciences of the Czech Republic

4c. Grant Agency of the Academy of Sciences of the Czech Republic

The area is marked by the intersection of the NNW–SSE trending Cheb–Domažlice Graben with the SW–NE trending Ohře (Eger) Rift. Quaternary volcanism is restricted to the Cheb Basin. However, Miocene volcanism contemporaneous with the formation of the graben extends further to the N along the trace of the Marianské Lázně Fault zone into the Vogtland.

Strongly silica-undersaturated alkaline volcanics including olivine melilitites, melilite-bearing and melilite-free olivine nephelinites are characteristic for the W Bohemian/Vogtland areas and built up volcanic necks, dikes, diatremes and scoria cones, respectively. The typical mineralogy of the present rocks comprises abundant olivine, diopside, nepheline and melilite as well as minerals such as perovskite, nosean, hauyne, phlogopite, amphibole that are rich in incompatible and volatile elements. Whole rock geochemistry shows that the rocks are highly enriched in incompatible elements such as HFSE (Zr: 280–450 ppm, Nb: 90–170 ppm, TiO₂: 2.6–3.3 wt. %, P₂O₅: 0.9–1.3 wt. %), LREE (La: 65–110 ppm, Ce 150–220 ppm) and other LILE (Ba: 700–1400 ppm, Sr: 900–1600 ppm). All rock types are uniform in their Sr- and Nd- isotopic signature exhibiting the typical European subcontinental mantle composition (European Asthenospheric Reservoir – EAR/Low Velocity Component – LVC) with only minor effects by assimilation: ⁸⁷Sr/⁸⁶Sr = 0.7032–0.7036, ¹⁴³Nd/^l44Nd = 0.51282–0.51287. Rocks enriched in Sr are macroscopically characterized by crustal xenoliths (schists and granites). Trace element ratios (Gd/Yb: 4.6–5.9) suggest small degrees of melting at rather deep levels >80 km in a mantle source enriched in fluids and LILE.

Many of the rocks give evidence of hydrothermal alteration, though others still contain fresh glass. The degree of alteration is determined by the proximity to geodynamically active zones. Carbonate often occurs as a secondary mineral, but seems to be present as a primary phase (carbonate melt droplets) in some of the rock as well.

The abundance of silica-undersaturated melilite-rich rocks in the Vogtland/W Bohemia area may give evidence for carbonate metasomatism of the subcontinental mantle in the area where the Mariánské Lázně Fault zone deeply dissects the lithosphere.

Based on a revision of a set of 115 occurrences of (ultra)mafic mantle xenoliths from the Czech part of the Bohemian Massif, the following results were achieved (Ulrych & Adamovič 2004). Xenoliths occur mostly in lava flows, less commonly in vents. Their host rocks correspond mostly to nepheline basanites to olivi­ne nephelinites. Olivine-bearing xenoiths (harzburgites > spinel lherzolites and peridotites) prevail over pyroxene-bearing xenoliths (clinopyroxenites >> wehrlites). Minerals are dominated by magne­sian olivine (Fo_89-91) and orthopyroxene (En_88–91) chromian diopside (0.9–1.4 wt% Cr₂O₃,), and (Cr, Al)-spinels (l00 Cr/Cr+Al = 15–54). The rise of volcanic rocks carrying the xenoliths occurred especially along faults of E–W and ESE–WNW course, always in the times of their normal extension, probably independently on the structures typical for the Ohře Rift graben limitation; structures rather striking NW–SE were involved in the Pliocene and Pleistocene. The structures used were formed (and partly reactivated) in the Subhercynian and Laramide phases of the Alpine orogeny in the latest Cretaceous, Paleocene and earliest Eocene.

The classical xenoliths of the supposed lower crust origin include alkali pyroxenite and ijolite xenoliths occurring in ca 37–30 Ma nephelinite of the Loučná–Oberwiesenthal Volcanic Centre in Krušné hory/Erzgebirge region (Ulrych et al. 2005). The latter is located on the uplifted shoulder of the Ohře/Eger Rift within the Variscan basement of central Europe. The alkali pyroxenites and transitional xenoliths are abundant whereas ijolite xenoliths are rare. The host nephelinite is chemically evolved (Mg# = 47–46), and the entrained alkali c1inopyroxenite xenoliths (Mg# = 64–47) and transitional to ijolite xenoliths (Mg# = 69–25) show a range of compositions from little to highly evolved. The alkali pyroxenite xenoliths probably represent fragments of an intracrustal, possibly layered alkaline complex, overprinted by a late magmatic pegmatoid phase of ijolite composition. This late stage metasomatic process may account for a spectrum of rocks ranging from alkali pyroxenite to ijolite. Initial ε Nd values of +2.3 in clinopyroxene samples from the host nephelinite and +3.1 to +3.0 in clinopyroxene from the xe­noliths indicate similar, yet different sources. The initial ⁸⁷Sr/⁸⁶Sr ratios of 0.70361 to 0.70365 and initial ε_Nd values of +3.0 and +3.1 for the alkali pyroxenite xenoliths are consistent with mantle sources of HIMU-affinity. A similar source may be inferred from the isotopic composition of the host nephelinite yielding an initial ⁸⁷Sr/⁸⁶Sr of 0.70368 and initial ε_Nd value of +2.3.

The crystal structure of diopside from an alkali pyroxenite xenolith from the Loučná–Oberwiesenthal Volcanic Centre with the formula (Ca_0.95 Na_0.04) (Mg_0.65 Fe_0.13 ²⁺ Fe_0.10³⁺Ti_0.10 Al_0.01) (Si_1.69 Al_0.31) O₆ and the lattice parameters a = 9.773(2), b = 8.886(2), c = 5 .308(1) [A] and β = 105.89(3) [^o] was re­fined to an R-value of 0.025 for 1174 reflections (Ulrych et al. 2006). The mean interatomic distances are: within the Mel-O6 octahedron <2.067> A, within the Me2–O8 polyhedron <2.498> A. The last value reflects the occupation of this atomic position by significant amounts of Fe²⁺ and Ti⁴⁺. The enlargement determined for the band length to 1.657 Å is in accordance with the site population for this position: (Si_1.69 Al_0.31) The molar ratio Fe^2+/Fe³determined by Mössbauer spectroscopy is equal to 0.786. The Al^IV deficiency in T-sites of clinopyroxene of rims is negligible (up to 0.019 a. p. f. u.) restricted to sporadic local electron microprobe analyses. The presence of Fe³⁺ in the T-position of Si- and Al-poor clinopyroxenes was not confirmed by X-ray structural analyses because of its low quantity. Neverthe­less, the Mössbauer spectroscopy measurements (isomer shift of 0.36 mm.s^-1) imply that Fe³⁺ is present only in the Me1–O6 positions.

Neogene basanite lavas of Kozákov volcano, located along the Lusatian Fault in the northeastern Czech Republic, contain abundant anhydrous spinel lherzolite xenoliths that provide an exceptionally continuous sam­pling of the upper two-thirds of central European lithospheric mantle. The xenoliths yield a range of two-pyroxene equilibration temperatures from 680 °C to 1070 °C, and are estimated to originate from depths of 32–70 km, based on a tectonothermal model for basaltic underplating associated with Neogene rifting (Ackerman et al. 2007). The sub-Kozákov mantle is layered, consisting of an equigranular upper layer (32–43 km), a protogranular intermediate layer that contains spinel-pyroxene symplectites after garnet (43–67km), and an equigranular lower layer (67–70km). Negative correlations of wt. % TiO₂, Al₂O₃, and CaO with MgO and clinopyroxene mode with Cr-number in the lherzolites record the effects of partial fusion and melt extraction; Y and Rb contents of clino­pyroxene and the Cr-number in spinel indicate 5 to 15% partial melting. Subsequent metasomatism of a depleted lherzolite protolith, probably by a silicate melt, produced enrichments in the large ion litho­phile elements, light rare earth elements and high field strength ele­ments, and positive anomalies in primitive mantle normalized trace element patterns for P, Zr, and Hf. Although there are slight geochemical discontinuities at the boundaries between the three textural layers of mantle, there tends to be an overall decrease in the degree of depletion with depth, accompanied by a decrease in the magnitude of metasomatism. Clinopyroxene separates from the intermediate proto­granular layer and the lower equigranular layer yield ¹⁴³Nd/¹⁴⁴Nd values of 0.51287–0.51307 (ε_Nd = +4.6 to +8.4) and ⁸⁷Sr/⁸⁶Sr values of 0.70328–0.70339. Such values are intermediate with respect to the Nd-Sr isotopic array defined by anhydrous spinel peridotite xenoliths from central Europe and are similar to those associated with the present-day low-velocity anomaly in the upper mantle beneath Europe. The geochemical characteristics of the central European lithospheric mantle reflect a complex evolution related to Devonian to Early Carboniferous plate convergence, accretion, and crustal thickening, Late Carboniferous to Permian extension and gravita­tional collapse, and Neogene rifting, lithospheric thinning, and magmatism.

According to Konečný et al. (2006) the studied samples of xenoliths of spinel lherzolite composition from the Kozákov volcano come from the depth of about 50–75 km. Their mineral assemblage preserved subsolidus tempera­tures of 1165–1052 °C from the time of xenolith entrapment. Oxygen fugacity varies from +0.14 to +0.93 log unit relative to fayalite-magnetite-quartz (FMQ) buffer. Major bulk-rock oxides and variations in mineral chemistry indicate a continual depletion trend mainly associated with extraction of basaltic melt from the mantle. Mineralogical features and the absence of highly oxidized lherzolites suggest a negligible degree of modal metasomatic overprint. On the contrary, the LREE upward patterns and U-shaped REE patterns of clinopyroxenes, as well as of the bulk lherzolite compositions are indicators of cryptic metasomatic event(s) in the upper mantle. The U-shape REE patterns corroborates to enrichment mechanisms in the mantle by reactive porous flow and chromatographic fractionation. A possible cryptic metasomatic event(s) could have occurred in pre-Cenozoic times, probably during the Variscan orogeny.

The effects of melt percolation on highly siderophile element (HSE) concentrations and Re-Os isotopic systematics of subcontinental lithospheric mantle are examined for a suite of spinel peridotite xenoliths from the 4 Ma Kozákov volcano (Ackerman et al. 2009).The xenoliths have previously been estimated to originate from depths ranging from ~32 to 70 km and represent a layered upper mantle profile. Prior petrographic and lithophile trace element data for the xenoliths indicate that they were variably modified via metasomatism resulting from the percolation of basaltic melt derived from the asthenosphere. Chemical and isotopic data suggest that lower sections of the upper mantle profile interacted with melt characterized by a primitive, S-undersaturated composition at high melt/rock ratios. The middle and upper layers of the profile were modified by more evolved melt at moderate to low melt/rock ratios. This profile permits an unusual opportunity to examine the effects of variable melt percolation on HSE abundances and Os isotopes.

Most HSE concentrations in the studied rocks are significantly depleted compared to estimates for the primitive upper mantle. The depletions, which are the most pronounced for Os, Ir and Ru in the lower sections of the mantle profile, are coupled with strong HSE fractionations (e. g., OsN/IrN ratios ranging from 0.3 to 2.4). Platinum appears to have been removed from some rocks, and enriched in others. This enrichment is coupled with lithophile element evidence for the degree of percolating melt fractionation (i. e. Ce/Tb ratio).

Osmium isotopic compositions vary considerably from subchondritic to approximately chondritic (γ_Os at 5 Ma from -6.9 to +2.1). The absence of correlations between ¹⁸⁷Os/¹⁸⁸Os and indicators of fertility, as is common in many lithospheric mantle suites, may suggest significant perturbation of the Os isotopic compositions of some of these rocks, but more likely reflect the normal range of isotopic compositions found in the modern convecting mantle. Osmium isotopic compositions correspondingly yield model Re-depletion (TRD) ages that range from essentially modern to ~1.3 Ga.

Our data provide evidence for large-scale incompatible behavior of HSE during melt percolation as a result of sulfide dissolution, consistent with observations of prior studies. The degree of incompatibility evidently depended on melt/rock ratios and the degree of S-saturation of the percolating melt. The high Pt contents of some of these rocks suggest that the Pt present in this pervasively metasomatized mantle was controlled by a phase unique to the other HSE. Further, high Os concentrations in several samples suggest deposition of Os in a minority of the samples by melt percolation. In these rocks, the mobilized Os was characterized by similar to the ¹⁸⁷Os/¹⁸⁸Os ratios in the ambient rocks. There is no evidence for either the addition of Os with a strongly depleted isotopic composition, or Os with suprachondritic isotopic composition, as is commonly observed under such circumstances.

Ackerman L., Mahlen N., Jelínek E., Medaris G. Jr., Ulrych J., Strnad L. & Mihaljevič M. (2007): Geochemistry and evolution of subcontinental lithospheric mantle in Central Europe: evidence from peridotite xenoliths of the Kozákov volcano, Czech Republic. – Journal of Petrology, 48, 12: 2235–2260.

Ackerman, L., Walker, R.J., Puchtel, I.S., Pitcher, L., Jelínek, E. & Strnad, L. (2009): Effects of melt percolation on highly siderophile elements and Os isotopes in subcontinental lithospheric mantle: a study of the upper mantle profile beneath Central Europe. – Geochimica et Cosmochimica Acta, 73, 8: 2400–2414.

Filip J., Ulrych J., Adamovič J. & Balogh K. (2007): Apatite fission track implication for timing of hydrothermal fluid flow in Tertiary volcanics of the Bohemian Massif. – Journal of the Czech Geological Society, 52, 3–4: 211–220.

Konečný P., Ulrych J., Schovánek P., Huraiová M. & Řanda Z. (2006): Upper mantle xenoliths from the Pliocene Kozákov volcano (NE Bohemia): P–T–fO2 and geochemical constraints. – Geologica Carpathica, 57, 5: 379–396. Bratislava.

Ulrych J. & Adamovič J. (2004): (Ultra)mafic mantle xenoliths in Cenozoic alkaline volcanics of the Bohemian Massif (Czech Republic). – Mineralia Slovaca, 36: 205-215.

Ulrych, J., Dostal, J., Hegner, E., Balogh, K. & Ackerman, L. (2008): Late Cretaceous to Paleocene melilitic rocks of the Ohře/Eger Rift in northern Bohemia, Czech Republic: insights into the initial stages of continental rifting. – Lithos, 101, 1–2: 141–161.

Ulrych, J., Lloyd, F.E., Balogh, K., Hegner, E., Langrová, A., Lang, M., Novák, J.K. & Řanda, Z. (2005): Petrogenesis of alkali pyroxenite and ijolite xenoliths from the Tertiary Loučná–Oberwiesenthal Volcanic Centre, Bohemian Massif in the light of new mineralogical, geochemical and isotopic data. – Neues Jahrbuch für Mineralogie, Abhandlungen, 182, 1: 57–79.

Ulrych, J., Nižňanský, D., Pertlik, F., Giester, G., Ertl, A. & Brandstätter, F. (2006): Clinopyroxene from alkali pyroxenite xenolith, Loučná–Oberwiesenthal Volcanic Centre, Bohemian Massif: crystal chemistry and structure. – Geological Quarterly, 50, 2: 257–264.

Ulrych J., Pivec E., Povondra P. & Rutšek J. (2000): Upper mantle xenoliths in melilitic rocks of the Osečná Complex, North Bohemia. – Journal of the Czech Geological Society, 45, 1: 79–93.

The general character of spore assemblages is the same in both parts of the basin, and some differences concern mainly different stratigraphical ranges of selected spore taxa. These differences were probably influenced by lateral development in various parts of the basin, i. e. they probably were ecologically controlled. Some significant spore taxa occurred in the Czech part for a longer time than in the Polish one (e. g., Tripartites and Rotaspora knoxi) and some others appeared earlier in the Polish part (Florinites, Schulzospora, Bellispores, Tripartites, Dictyotriletes bireticulatus, Crassispora; Fig. 40).