Equilibrium pressure-temperature (P-T) conditions were estimated for kyanite-bearing eclogite from Nove´Dvory, Czech Republic, by using garnet-clinopyroxene thermometry and garnet-clinopyroxenekyanite-coesite (or quartz) barometry. The estimated P-T conditions are 1050-1150°C, 4.5-4.9 GPa, which are mostly the same as previously estimated values for garnet peridotite from Nove´Dvory (1100-1250°C, 5-6 GPa). Such very high-P conditions, which correspond to about 150-km depth, have been obtained for some garnet peridotites in the Gfo¨hl Unit of the Bohemian Massif, but pressure conditions of eclogites associated with the garnet peridotites have not been so well constrained. This is the first substantial finding of eclogite that gives such very high-P conditions in the Gfo¨hl Unit of the Bohemian Massif. The Gfo¨hl Unit mainly consists of felsic granulite or migmatitic gneiss, but these rock types do not display high-P (>2.5 GPa) evidence. It is unclear whether both the peridotite body and surrounding felsic rocks in the Gfo¨hl Unit were buried to very deep levels, but at least some garnet peridotites and associated eclogites in the Gfo¨hl Unit have ascended from about 150-km depth.
An isochemical kelyphite (orthopyroxene+spinel+plagioclase) that has nearly the same bulk chemical composition as the precursor garnet was found within a matrix of ordinary kelyphites (orthopyroxene+clinopyroxene+spinel±amphibole) in garnet peridotites from the Czech part of the Moldanubian Zone. It was shown that the kelyphitization of garnet took place in three stages: (1) the garnet-olivine reaction, accompanied by a longrange material transfer across the reaction zone, and (2) the isochemical breakdown of garnet, essentially in a chemically-closed system, and finally, ( 3) an open-system hydration reaction producing a thin hydrous zone (amphibole+spinel+plagioclase), which is located between the isochemical kelyphite and relict garnet. The presence of relict garnet suggests that this breakdown reaction of the second stage did not proceed to a completion probably being hindered by the formation of the hydrous zone at the reaction front. It was found by electron back-scattered diffraction method that orthopyroxene and spinel do not show any topotaxic relationship in the first type of kelyphite; whereas they show locally topotaxic relationship in the isochemical kelyphite. The transition from the first type to the second type of kelyphite is discussed on the basis of the detailed observations in the transition zone between the two kelyphites. More widespread occurrence of isochemical kelyphite is expected to occur in orogenic peridotites as well as from xenoliths brought by volcanics. ver. 8.8 Isochemical kelyphite Obata et al.
The shallow oxidized asthenosphere may contain a small fraction of potassic silicate melts that are enriched in incompatible trace elements and volatiles. Here, to determine the chemical composition of such melt, we analysed fossilized melt inclusions, preserved as multiphase solid inclusions, from an orogenic garnet peridotite in the Bohemian Massif. Garnet-poor (2 vol.%) peridotite preserves inclusions of carbonated potassic silicate melt within Zn-poor chromite (<400 ppm) in the clinopyroxene-free harzburgite assemblage that equilibrated within the hot mantle wedge (Stage 1, > 1180 °C at 3 GPa). The carbonated potassic silicate melt, which has a major element oxide chemical composition of K2O = 5.2 wt.%, CaO = 17 wt.%, MgO = 18 wt.%, CO2 = 22 wt.%, and SiO2 = 20 wt.%, contains extremely high concentrations of large ion lithophile elements, similar to kimberlite melts. Peridotites cooled down to ≅800 °C during Stage 2, resulted in the growth of garnet relatively poor in pyrope content, molar Mg/(Mg + Fe + Ca + Mn), (ca. 67 mol.%). This garnet displays a sinusoidal REE pattern that formed in equilibrium with carbonatitic fluid. Subsequently, subduction of the peridotite resulted in the formation of garnet with a slightly higher pyrope content (70 mol.%) during the Variscan subduction Stage 3 (950 °C, 2.9 GPa). These data suggest the following scenario for the generation of melt in the mantle wedge. Primarily, infiltration of sediment-derived potassic carbonatite melt into the deep mantle wedge resulted in the growth of phlogopite and carbonate/diamond. Formation of volatile-bearing minerals lowered the density and strength of the peridotite. Finally, phlogopite-bearing carbonated peridotite rose as diapirs in the mantle wedge to form carbonated potassic silicate melts at the base of the overriding lithosphere.
This paper reports on priderite (potassium titanate) and burbankite (alkali Sr-Ca-REE-Ba carbonate) from an orogenic garnet peridotite body enclosed in high-pressure garnet-kyanite-bearing quartzo-feldspathic Gföhl granulite in the Bohemian Massif of the Variscan belt. The garnet peridotite contains ubiquitous phlogopite and was interpreted to be derived from the mantle wedge formed at the convergent plate margin. The earliest generation of chromian spinel, surrounded by kelyphitized garnet, ubiquitously contains multiphase solid inclusions (MSIs), which are mainly composed of phlogopite, dolomite, calcite, apatite, graphite, monazite, thorianite, and sulfides, and priderite and burbankite are newly identified as rare accessory minerals in such MSIs. Most of these MSIs contained significant amounts of carbonates. The presence of peculiar accessory minerals in MSIs characterizes the nature of parental melts. The formation of priderite requires an ultrapotassic condition, which is usually defined by K 2 O >3 wt% and K 2 O/Na 2 O >2 in bulk composition, and high Cr 2 O 3 content in priderite (15-18 wt%) suggests that it was formed as a reaction product between a melt inclusion and a host chromite. Burbankite contains significant amounts of Na 2 O and K 2 O (~3 wt%) and REE concentration (>31 wt%). The formation of burbankite requires a per-alkaline condition -K 2 O + Na 2 O > Al 2 O 3 in mol-and requires more sodic composition. The presence of priderite and burbankite in MSIs suggests that some of them crystallized from ultrapotassic melts, whereas others crystallized from sodic peralkaline melts. Such alkalicarbonate melts could be present in the mantle wedge peridotite before its' incorporation into the granulite.
Thorianite (ThO 2 ) was found in the phlogopite -bearing spinel -garnet peridotite (Plešovice peridotite) occurring as decimeter -size lenticular body in the Gföhl granulite that forms the metamorphic core of the Variscan orogenic belt. Thorianite occurs as a member of multiphase solid inclusions, consisting of phlogopite + carbonates + apatite + graphite + rutile + monazite + thorianite, in chromian spinel. U -Th -Pb ages of the thorianite give a good concentration at 333.8 ± 4.5 Ma, which overlaps the U -Pb ages of zircon extracted from the host Gföhl granulite, probably reflecting the Variscan high -pressure high -temperature metamorphism during the continentcontinent collision.
Methods to estimate the pressure-temperature histories of garnet peridotite and eclogite in the ultrahigh-pressure metamorphic belts: A review of geothermobarometers and their geological applicationsUltrahigh-pressure (UHP) metamorphic rocks, represented by coesite-or its pseudomorphbearing eclogites, have been found mainly from continent-continent collision orogenic belts, and garnet peridotite bodies are also known to occur in such UHP belts. The UHP eclogites and garnet peridotite bodies/layers/lenses are commonly enclosed within metamorphic rocks derived from continent crustal materials composed by moderate to low pressure metamorphic minerals, although they should have been located under deep mantle depths (>50 km). Therefore, elucidation of juxtaposition processes between the mantle material and the host crustal material is one of main subjects for the petrology in the UHP belts. Delineating of pressure-temperature (P T) paths of these UHP garnet peridotite bodies can give us indispensable constraints to clarify the juxtaposition process of mantle and crustal materials in the continent-collision settings and the exhumation processes of deeply subducted rocks with higher density than the crustal rocks. In this paper, we summarize the commonly used methods to determine P T histories of the UHP garnet peridotite bodies (i.e., geo-thermometer and barometer) and discuss P T paths of UHP rocks in collision type orogenic belts, and their tectonic signiˆcance.
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