Metabasic rocks form an important constituent of the Chýnov and Český Krumlov units belonging to the Varied Group (Moldanubian Zone, south Bohemia).The amphibolites are dominated by amphibolite-facies mineral assemblages of mainly tschermakitic amphibole and plagioclase.Hornblendes show compositional variation with Si ~ 6.5 apfu, Mg/(Mg + Fe) ~ 0.5 and (Na + K) A ~ 0.5 apfu.Garnet with clinopyroxene are subordinate and occur in a few samples only.No relics of previous greenschist-or granulite-facies assemblages have been observed, most likely due to the relatively simple metamorphic history.The petrology indicates rather close correlation of the Chýnov and Český Krumlov units.The similarities include presence of dolomite in carbonate bodies, graphite schists, rocks with marialitic scapolite, locally also Ti-andradite (± magnetite, epidote) oxidic assemblages and thin layers of Mn-rich garnet-quartz rocks.However, there is a major difference in the oxidation state.Most Chýnov amphibolites have Fe 2 O 3 /FeO = 0.70-1.00and their protolith probably experienced an early incipient oxidation.Great deal of the parental basalts thus could have been effusive.The Český Krumlov amphibolites have Fe 2 O 3 /FeO ≤ 0.4, perhaps because they show much closer association with graphite schists that could have been responsible for the reduction of the adjacent rock units.The dataset is dominated by EMORB-like tholeiite basalts interpreted as having been derived by Early Palaeozoic melting of a strongly depleted mantle source (ε 50 Nd 0 = +8.6 to +9.4; T D Nd M = 0.43-0.50Ga).This argues stoutly against Precambrian age of the Varied Group in south Bohemia.The composition of the remaining samples reflects contamination by upper continental crust (ε 50 Nd 0 = +3.1 to +1.3, progressive enrichment in Th, development of a significant negative Nb, and lesser P and Ti anomalies on the NMORB-normalized spiderplots).A much smaller group of amphibolites is characterised by steep REE patterns (La N /Yb N = 5.5-11) and high contents of HFSE (Nb, Ta, Zr and P).It is of a clear OIB affinity, with parental alkali basalt (Nb/Y = 0.7-1.6)generated by a low degree of partial melting of a deep, garnet-bearing asthenospheric mantle source (ε 50 Nd 0 = +4.5 to +6.1; T D Nd M = 0.75-0.83Ga).Metamorphosed doleritic/gabbroic dykes cutting the Palaeoproterozoic Světlík orthogneiss show rather unradiogenic Nd isotopic composition (ε 50 Nd 0 = +0.1 and -3.6; T D Nd M = 1.34 and2.03Ga).This precludes closed-system crystallization from depleted mantle derived melts in Phanerozoic times.The exact age and nature of their parental magma remain enigmatic but any genetic link with the amphibolites in the structurally overlying Český Krumlov Varied Unit seems ruled out.Overall, the most likely tectonic setting of the magmatism was attenuated lithosphere, subjected to an Early Palaeozoic extension, leading eventually to fragmentation of the northern Gondwana margin.The minor OIB component preserved as alkali basalts as well as some contribution to the EMOR-like basaltic magmas was probably added by a rising mantle plume.
The geochemistry and petrology of the Kaplice Unit, a regional zone of muscovite-biotite gneisses 55×8 km in size, was studied.Geochemistry of the gneisses is characterized using 27 samples for major element analyses and 15 samples for minor and trace element analyses.The data are compared with element abundances in gneisses of the adjacent Monotonous and Variegated Units, with the aim to define respective chemical differences.K, Rb, Cs, Ba, H 2 O + and Fe 2 O 3 /FeO are significantly enriched and Na, Ca, Sr, P and U are distinctly depleted in comparison to biotite gneisses of both the Monotonous and the Variegated Units; the ratio Rb/Sr increased by a factor of 3.5.The contents of Ti, Al, Fe t , Mg, Cr, Ce, Y, Hf, Ta and Th are closely comparable, which corresponds to behaviour of relatively immobile elements.Extensive hydration was accompanied by significant oxidation and other chemical changes comparable to synmetamorphic acid leaching.Microstructural relations indicate sillimanite-and K-feldspar-consuming reactions producing muscovite intergrown with biotite in recrystallized quartz-feldspar fabric.Residual sillimanite occurs as minute aggregates armoured by muscovite.Abundant quartz segregation lenses aligned in the dominant S3 foliation indicate high silica activity and carry occassional large crystals of andalusite, rare kyanite, cordierite or rutile.Concurrent reactions in calc-silicate gneiss interbands resulted in replacement of diopside by actinolite or low-Al magnesiohornblende and partial replacement of anorthite by clinozoisite ± minor garnet.Structural and petrological data indicate that the unit represents a regional shear zone (D3 planar foliation sets), functioning as a major conduit with a high, synmetamorphic fluid flux in the NW roof domain of the Moldanubian Pluton in southern Bohemia.Since the intrusion of the Eisgarn pluton post-dates the S3 planar fabric and largely pre-dates extension D4 crenulation, it is possible that the cooling two-mica granite body, structurally underlying the Kaplice Unit, served as a source of fluids, important in the retrograde alteration of paragneisses and the accompanying chemical changes.
The high-pressure granulites from the Bìstvina body contain a mineral association garnet-kyanite-biotite-plagioclase-K-feldspar-quartz ± antiperthite.Based on petrology, as well as major-and trace-element whole-rock geochemistry, the studied eight samples can be subdivided into four types: (1) metamorphosed greywacke, (2) felsic granulite with a low CaO/Na 2 O and K 2 O/Na 2 O ratio (type I), (3) felsic granulite of leucogranitic composition (type II), and (4) kyanite-garnet-biotite migmatitic gneiss.These rocks are associated with minor garnet peridotite and a crustal eclogite with a prograde metamorphic record, as described in the literature.Garnet in the quartz-feldspathic rocks typically contains 2435 mol.% Prp and 514 mol.% Grs components with Ca zoning across grains.Using compositions of garnet cores and the associated plagioclase and biotite, the geothermobarometric calculations prove the equilibration under eclogite facies conditions (P = 1.8 2.2 GPa and T = 800920 °C).Two of the samples important for thermobarometry (felsic granulites type I and II) are free of antiperthite.The associated Ky-Grt-Bt migmatitic gneiss recording conditions near 1.4 GPa and 670 °C may represent a foreign component in the Bìstvina granulite body.Although one meta-greywacke sample shows incipient replacement of garnet by biotite + plagioclase, the other samples are free of a decompression recrystallization record.Thus the granulites of the Bìstvina body stand in sharp contrast to the better known granulites in the Moldanubian Zone, by their lack of evidence for an extensive decompression recrystallization.
The Kaplice dyke swarm of biotite granodiorite porphyry and its relationship to the Freistadt granodiorite, Moldanubian Batholith Biotitické granodioritové porfyry v kaplickém zlomovém pásmu a jejich vztah ke granodioritu typu Freistadt (9 figs, 3 tabs) STANISLAV VRÁNA 1 JAROMÍR SLABÝ 2 JIØÍ BENDL 3 1 c/o Czech Geological Survey, Klárov 3, 118 21
The phlogopite pyroxenite with a high content of pyrrhotite forms a body of circular outline about 1 km in diameter at Přední Zvonková in Moldanubian Zone (Šumava/Bohemian Forest/Böhmerwald), Czech Republic.The contours of pyroxenite body are indicated by magnetometry and gravity data.Loose blocks up to 70 cm in size scattered on fields in the flat terrain indicate that the rock is at least partly exposed at the current erosion level.The best preserved sample SV235A is a medium-grained rock containing 27 vol.% phlogopite, 23 % diopside, 25 % enstatite, abundant apatite (7 %), minor plagioclase (3 %, An 39-42 ), and pyrrhotite (3 %).Tendency to sulphide segregation is indicated by several mm pyrrhotite aggregates enclosing individual grains of pyroxenes, phlogopite and apatite.In the mg# vs. MgO diagram the studied pyroxenites plot along the linear trend defined by granitoid durbachites (melanocratic quartz syenites, quartz monzodiorites to melagranites), biotite-amphibole quartz diorites and biotite-amphibole ultramafic rocks (all from the Moldanubian Zone in southern and south-central Bohemia), after the whole-rock compositions of pyroxenites are corrected for the respective content of pyrrhotite.Several pyroxene thermometers yield equilibration temperatures in the range of 970 to 1040 °C for the sample SV235A.The Re-Os and Sr-Nd isotope composition of whole-rock samples indicates important crustal component in the sulphides and silicates.Petrological and geochemical data show a major role of fractional crystallization/crystal accumulation in formation of these rocks but magma mixing process is also recorded.The phlogopite pyroxenite is a rare rock important for understanding genesis of durbachitic magmatism.
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