Petrology and geochemistry of the Běstvina granulite body metamorphosed at eclogite facies conditions, Bohemian Massif

Abstract

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.

Geochemistry and petrology of high-pressure kyanite-garnet-albite-K-feldspar felsic gneisses and granulites from the Kutná Hora Complex, Bohemian Massif

“…The Běstvina Unit is dominated by felsic granulites of calc-alkali leucogranite and granite composition, but samples 5 and 6 from the set studied by Vrána et al (2005) are albite-K-feldspar felsic granulites equilibrated under eclogite-facies conditions.…”

“…The data for MP/LT retrogression calculated by Nahodilová et al (2006) from the matrix biotite, garnet rims and the recrystallized grains of plagioclase, K-feldspar and quartz yielded P-T conditions of 712 ± 39 °C and 1.06 ± 0.18 GPa, which could be compared with the event M 3 . The values calculated for the M 2 event in HP migmatites of the Malín Unit can be compared with P-T data obtained for a migmatitic gneiss from the Běstvina Unit (P = 1.4 GPa, T = 660 °C: Vrána et al 2005).…”

“…The samples were selected from a large sample set studied in connection with geological mapping of the Kutná Hora area on the scale of 1 : 25 000 by one of the authors (Štědrá et al 2009). In addition, we used several granulite samples from the Běstvina Unit characterised by Vrána et al (2005).…”

Metamorphic history of skarns, origin of their protolith and implications for genetic interpretation; an example from three units of the Bohemian Massif

“…The metamorphic evolution of the upper part of the KHC (Běstvina, Malín and Plaňany Unit) corresponds to the HP/HT-HP/MT events (Vrána et al 2005) followed by HP/MT tectonometamorphic overprint. The lower part of the KHC is composed of the Kouřim Nappe (fine-grained leucocratic migmatites and orthogneisses) and further, in the marginal part, of the Micaschist Unit (metapelites).…”

High (ultrahigh) pressure metamorphic terrane rocks as the source of the detrital garnets from the Middle Jurassic sands and sandstones of the Cracow Region (Cracow-Wieluń Upland, Poland)

“…Representative analyses of the garnets from various rocks (kimberlites, garnet peridotites, UHP eclogites, eclogites, HP granulites, LP granulites, retrograde eclogites), were taken from the following papers: garnets from HP granulites in the Góry Sowie Mts. (O'Brien et al 1997); garnets from peridotites, eclogites and granulites from the Bohemian Massif (Messiga and Bettini 1990;Nakamura et al 2004;Seifert and Vrána 2005;Vrána et al 2005;Medaris et al 2006a,b;Janoušek et al 2006Janoušek et al , 2007Racek et al 2008); garnets from HP and UHP eclogites and garnet peridotites from the Norway Western Gneiss Region (Krogh Ravna and Terry 2004); garnets from kimberlites (Schulze 1997); garnets from eclogites with inclusions of diamond (Schulze 1997); garnets from HP granulites, from UHP eclogites with inclusions of coesite and from garnet peridotites from the Saxonian Erzgebirge and Granulitgebirge (Massonne and Bautsch 2004). The resulting triangular diagram was divided to three sectors -A, B, C, with the latter subdivided into the sectors C1 and C2 (see Text-figs 6, 7, white fields): (A) garnets from high pressure (HP) and ultrahigh pressure (UHP) conditions; (B) garnets from HP eclogite and HP granulite facies conditions; and (C) garnets from amphibolite facies conditions: Subsector C1 represents a transitional subgroup metamorphosed under transitional P-T conditions between the granulite and amphibolite facies conditions and Subsector C2 represents a subgroup of amphibolite facies conditions.…”