Evidence for partial melting of eclogite from the Moldanubian Zone of the Bohemian Massif, Czech Republic

Abstract

Eclogites from the Moldanubian Zone of the Bohemian Massif (Czech Republic) have experienced ultrahighpressure (UHP) and ultrahigh-temperature (UHT) metamorphic conditions (P > 4.0 GPa, T > 1000°C). One eclogite sample collected from Nové Dvory, eastern part of Czech Republic, contains leucocratic pockets. The mineral assemblage of the melanocratic part of the eclogite is garnet + omphacite + rutile + later-stage minerals (biotite + amphibole + plagioclase). The leucocratic pockets mainly consist of plagioclase and include moderate amounts of garnet and clinopyroxene + small amounts of biotite and amphibole. Garnet and clinopyroxene grains in the leucocratic pockets display faceted shapes whereas those in the melanocratic part show irregular shapes. These are consistent with the idea that garnet and clinopyroxene grains in the leucocratic pockets had been surrounded by melt, as melt can provide free space for the formation of such faceted shapes. The major and trace element composition of garnet grains in the melanocratic part are the same as those in the leucocratic pockets. Clinopyroxene also shows the same major and trace element composition for irregular shape grains in the melanocratic part as faceted grains in the leucocratic pockets. If the melt had originated from the gneiss surrounding the peridotite body with eclogite lenses or layers, the composition of clinopyroxene in the leucocratic pockets would have been completely different from that in the melanocratic part. In addition, the trace element composition of melt estimated from clinopyroxene composition resembles that of TonaliteTrondjhemite -Granodiorite (TTG), which can be produced by partial melting of eclogite. Thus, these observational and analytical data suggest that the leucocratic pockets were originally melt that was internally produced by partial melting of the eclogite.

Cryptic metasomatic agent measured in situ in Variscan mantle rocks: Melt inclusions in garnet of eclogite, Granulitgebirge, Germany

“…As already made clear in the petrographic descriptions, these ‘classic’ metasomatic phases are present in the matrix together with leucocratic pockets filled with plagioclase (Figure e,f) but these represent a later modal metasomatism that post‐dates the metamorphic peak and is unrelated to the preserved MI. Leucocratic pockets have been already reported in the Nové Dvory eclogite where they contain plagioclase, a moderate amount of garnet, clinopyroxene and minor biotite and amphibole (Miyazaki et al, ). Clinopyroxene in the latter has been interpreted as having crystallized from a melt with composition close to a high‐pressure TTG.…”

Mineralogical heterogeneity of UH<i>P</i> garnet peridotite in the Moldanubian Zone of the Bohemian Massif (Nové Dvory, Czech Republic)

“…However, the relationships between the occurrences of these peridotite types require considerably more research. In addition, eclogite is closely associated with Nové Dvory peridotite, which has been shown to undergo an increase in pressure before the peak P metamorphic stage (Miyazaki et al, 2016). The occurrence of spinel and amphibole inclusions in garnet as well as our thermobarometric estimations using inclusion clinopyroxene and host garnet data indicate that the Nové Dvory peridotite body has existed as medium P/T spinel peridotite and then experienced prograde metamorphism equilibrating at higher P-T conditions followed by the transformation into garnet peridotite after which it was exhumed with significant cooling to crustal levels.…”

Multiple origins of UHP eclogites in a garnet peridotite block (Nové Dvory, Czech Republic) and short duration of heating

“…1100 °C for eclogite (e.g., Nakamura et al, 2004;Medaris et al, 2006;Faryad, 2009;Miyazaki et al, 2016), with some peridotite blocks showing peak P conditions of 3-7 GPa (e.g., Medaris et al, 2006;Faryad, 2009;Naemura et al, 2009Naemura et al, , 2011Muriuki et al, 2020). These data indicate that the estimated P-T conditions even within the Gföhl Unit are highly variable depending on differences in both the locality and lithological type.…”

“…In the study of Obata et al (2006), three eclogite samples that were collected from one peridotite block (Nové Dvory, Czech Republic) showed positive Sr and Eu anomalies in bulk-rock chemical compositions. The peak metamorphic P-T conditions of kyanite (Ky)-bearing and Ky-free eclogite samples collected from the same peridotite block (Nové Dvory) have been estimated as 4.5-4.9 GPa and 1050-1150 °C (Nakamura et al, 2004) and 4.3 GPa and 1050 °C (Miyazaki et al, 2016), respectively, with both results showing UHP conditions. However, petrological studies of the three samples analyzed by Obata et al (2006) have not yet been performed, nor have the metamorphic P-T conditions of these samples been determined.…”