Petrogenetic significance of orthopyroxene‐free garnet + clinopyroxene + plagioclase ± quartz‐bearing metabasites with respect to the amphibolite and granulite facies

Abstract

Orthopyroxene‐free garnet + clinopyroxene + plagioclase ± quartz‐bearing mineral assemblages represent the paragenetic link between plagioclase‐free eclogite facies metabasites and orthopyroxene‐bearing granulite facies metabasites. Although these assemblages are most commonly developed under P–T conditions consistent with high pressure granulite facies, they sometimes occur at lower grade in the amphibolite facies. Thus, these assemblages are characteristic but not definitive of high pressure granulite facies. Compositional factors favouring their development at amphibolite grade include Fe‐rich mineral compositions, Ca‐rich garnet and plagioclase, and Ti‐poor hornblende. The generalized reaction that accounts for the prograde development of garnet + clinopyroxene + plagioclase ± quartz from a hornblende + plagioclase + quartz‐bearing (amphibolite) precursor is Hbl + Pl + Qtz=Grt + Cpx + liquid or vapour, depending on whether the reaction occurs above or below the solidus. There are significant discrepancies between experimental and natural constraints on the P–T conditions of orthopyroxene‐free garnet + clinopyroxene + plagioclase ± quartz‐bearing mineral assemblages and therefore on the P–T position of this reaction. Semi‐quantitative thermodynamic modelling of this reaction is hampered by the lack of a melt model and gives results that are only moderately successful in rationalizing the natural and experimental data.

An ultra-high-pressure metamorphic condition and isothermal decompression path obtained from a garnet-pyroxenite in the Horní Bory granulite of the Bohemian Massif

“…Stage 3 is defined by the following kelyphite -forming reaction: Grt + Cpx + Qtz = Opx + Pl (e.g., Pattison, 2003). Previous experimental work on the anorthite -enstatite system (Kushiro and Yoder, 1966) and the pseudosection calculation analyses on pyroxenitic rock from the Erzgebirge (Schmädicke, 2000) suggested that this reaction takes place in a narrow pressure range (1.4 -1.6 GPa) at relatively high T conditions (700 -1000 °C).…”

Garnet-clinopyroxene amphibolite from the Takahama metamorphic rocks, western Kyushu, SW Japan: evidence for high-pressure granulite facies metamorphism

“…The assemblage, garnet + clinopyroxene + plagioclase, characterizes a transition between the granulite and eclogite facies in terms of continuous reactions that produce garnet at the expense of orthopyroxene and plagioclase with increasing pressure (e.g., O'Brien and Rötzler, 2003;Pattison, 2003). This assemblage with or without hornblende and quartz occurs in the terranes not only of high pressure granulite facies but also of amphibolite facies (e.g., Pattison, 2003).…”

“…This assemblage with or without hornblende and quartz occurs in the terranes not only of high pressure granulite facies but also of amphibolite facies (e.g., Pattison, 2003). Figure 9 compares the estimated condition with the P T estimates of metabasites possessing these assemblages for granulite, transitional amphibolite granulite and amphibolite occurrences compiled by Pattison (2003). The estimated condition is similar to those of garnet + clinopyroxene + plagioclase ± hornblende ± quartz assemblages found in terranes of the granulite facies in the world.…”

Pan-African granulites of central Dronning Maud Land and Mozambique: A comparison within the East-African-Antarctic orogen

“…The granulite-facies pressure-temperature domain is subdivided into low-, intermediate-and high-P fields based on the coexistence of orthopyroxene, clinopyroxene and garnet in mafic lithologies (e.g., Spear, 1993). High-P granulites are defined by the mineral assemblage Grt +Cpx +Pl ±Hbl ±Qtz (Pattison, 2003; abbreviations follow Kretz, 1983). Opx coexists with this paragenesis in the intermediate-P field, while Grt disappears in the low-P field (Spear, 1993).…”