Metabasic rocks in the Varied Group of the Moldanubian Zone, southern Bohemia - their petrology, geochemical character and possible petrogenesis

Abstract

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 . 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.

Tectonometamorphic features of geological units along the northern periphery of the Moldanubian Zone

“…The Varied Unit is built by paragneisses with abundant intercalations of marbles, calc-silicate rocks, quartzites, graphitic gneisses and amphibolites. Varied unit had sedimentary protoliths mainly of Early Palaeozoic age (Fiala et al 1995, Drábek andStein 2003;Janoušek et al 2008). The Gföhl Unit is composed of lower-crustal and upper mantle components (granulites, migmatitic orthogneisses, with lenses of peridotites and eclogites) derived from heterogeneous sources.…”

Manganese-rich garnet-quartz rocks and gneisses in the Bohemian part of the Moldanubian Zone: lithostratigraphic markers

“…Calcite marble within several metres of the Mn-rich layer is represented by a silicate-rich variety carrying aggregates of minerals rich in Fe 3+ such as andradite, epidote, accessory magnetite, in addition to diopside . Relations of rock types in the quarry were characterized by Orlov (1931), geology of the broader area by Suk et al (1977), mineralogy in the quarry and nearby caves by Litochleb et al (1997Litochleb et al ( , 2004, M o l d a n u b i a n B a t h o l i t h C e n tr a l B o h e m ia n P lu to n ic C o m p le x Bouška et al (1985) while geochemistry and petrology of amphibolites was studied by Janoušek et al (2008). A tendency to local enrichment in Mn is independently documented by rare occurrence of the violet variety of tremolite in marble ("hexagonite", containing up to 0.29 wt.…”

“…In the context of increasing abundance of geochronological information supporting the Cambro-Ordovician age of the Varied Group in the Czech part of the Moldanubian Zone (Fiala et al 1995;Drábek and Stein 2003;Janoušek et al 2008;Košler et al 2011), the occurrences of Mn-rich garnet-quartz rocks (coticules) appear as yet another indication of its Lower Palaeozoic age, particularly with reference to coticule localities in NW Europe and the eastern United States.…”

“…The Monotonous Group, composed dominantly of biotite and sillimanite-biotite paragneisses, with widely scattered orthogneiss bodies, subordinate quartzite, calc-silicate gneiss and amphibolite, is assumed to be mainly Neoproterozoic in age, as the protoliths to the orthogneisses were dated between c. 550 Ma and 508 Ma (Vrána and Kröner 1995;Košler et al 1996). The Varied Group is probably Cambro-Ordovician (Fiala et al 1995;Drábek and Stein 2003;Janoušek et al 2008;Košler et al 2011). The Varied Group contains abundant layers of marble, calc-silicate gneiss, graphite gneiss, quartzite and amphibolite in addition to dominating paragneisses.…”

“…According to Janoušek et al (2008), several amphibolite samples from the Chýnov quarry have a relatively high ratio Fe 2 O 3 /FeO = 0.7-1.0 (wt. % values).…”