Mass balance during retrogression of eclogite-facies minerals in the Rongcheng eclogite, eastern Sulu ultrahigh-pressure terrane, China

Abstract

Eclogite from Rongcheng in the easternmost part of the Sulu ultrahigh-pressure metamorphic terrane has broken down (at least partially) with the formation of coronae of spinel + anorthite around kyanite, symplectites of pargasite + plagioclase around garnet, and symplectites of diopside + plagioclase after omphacite. Textural evidence and microprobe analysis indicate that this breakdown was generated by discontinuous reactions. QuantiÞ cation of material transfer and textural information suggest that the reaction mechanism involved local exchange of components among the three major minerals (omphacite, garnet, and kyanite) during dissolution and discontinuous precipitation, coupled with exchange between eclogites and their host rocks. The symplectite formed after omphacite gained Al and minor Si and Na from its surroundings, and lost Mg, Ca, and minor Fe to its surroundings. Formation of the symplectite around garnet was accompanied by gain of Ca, Al, Si, Na, and H 2 O, and by loss of Mg and Fe 2+ to its surroundings. Coronae around kyanite gained Ca, Fe, Mg, and minor Si, and provided Al to their surroundings. Material exchange between eclogite and host rocks occurred as well: eclogite gained minor Al, Na, and H 2 O from the host rocks, and provided Mg, Fe, and Ca to the host rocks. Pressure and temperature estimation using THERMOCALC yielded a lower temperature (655 ± 70 °C) and higher pressure (1.3 ± 0.2 GPa), which values contrast with estimates made by other authors. Strong deformation may have created a path for ingress of hydrous ß uids, then resulted in a localized lower T and/or higher P.

Petrology and P–T path of high-pressure granulite from the Dulan area, North Qaidam Mountains, northwestern China

“…In general, symplectite of clinopyroxene + plagioclase can be formed by many decompressional reactions involving clinopyroxene (omphacite), garnet and kyanite (Holland, 1980;Wikström, 1970;Mysen and Griffin, 1973;Godard and Mabit, 1998). For the Dulan HP granulite, the symplectic clinopyroxene + plagioclase appear mostly between or around coarse-grained clinopyroxenes indicating that they were produced by the breakdown of clinopyroxene ± quartz through the following typical decompressional reactions (Holland, 1980;Yang et al, 2004): Holdaway (1971). GADS and GASP reactions were calculated by THERMOCALC v.3.1 (Holland and Powell (1998).…”

Identification of repeated Alpine (ultra) high-pressure metamorphic events by U–Pb SHRIMP geochronology and REE geochemistry of zircon: the Rhodope zone of Northern Greece

“…Hornblende + plagioclase kelyphites around garnet are also a known feature in overprinted eclogites and form by a reaction of omphacite + garnet. For details on textural features, especially symplectites, in retrograded eclogites, the reader is referred to e.g., Heinrich (1986) or Yang et al (2004) and references therein. Rutile is almost always surrounded by titanite.…”

Mineral evolution of a garnet‐pyroxenite nodule within eclogite, eastern Sulu ultrahigh‐pressure metamorphic terrane, East China

“…2). Both kinds of eclogites had been partially retrograded, which led to the formation of a corona around the garnet and kyanite, and symplectite after clinopyroxene (Yang et al. , 2004).…”

“…, 2004). The kyanite‐rich eclogite shows advanced retrogression and looks very much like gneiss; it may easily be mistaken for garnet‐bearing gneisses (Yang et al. , 2004).…”