Jadeite in Continental Subduction: Its Preservation, Breakdown and Pseudomorphic Replacement in Erzgebirge UHP Gneisses

ABSTRACT The absence of jadeite in regions where subduction of continental crust has been documented by other high‐pressure (HP) or ultrahigh‐pressure (UHP) minerals is striking. This study presents only the second discovery of jadeite in the crystalline rocks of the Erzgebirge, a region known for its occurrences of microdiamond, coesite and omphacite in eclogite, felsic granulite and other quartzofeldspathic rocks involved in continental subduction. The studied quartzofeldspathic gneiss is composed of quartz, garnet, plagioclase, K‐feldspar, muscovite, biotite, rutile and kyanite. Garnet occurs mainly as large porphyroblastic grains of Grt1 overgrown by thin discontinuous rim of Grt2. Jadeite occurs as inclusions in kyanite and Grt1 garnet, the latter also containing coesite. Domains up to 3 mm in size of fine‐grained plagioclase‐muscovite symplectite, surrounded by plagioclase‐muscovite mosaic and occasionally associated with small garnet grains of Grt2, are interpreted as pseudomorphic replacement of jadeite. The pressure‐temperature () evolution of the samples was reconstructed by comparing the composition and proportion of the observed mineral assemblage with pseudosections constructed in THERMOCALC. Prograde metamorphism is constrained by the presence of garnet and jadeite inclusions in kyanite. Peak conditions reaching > 28 kbar and 600°C–800°C are constrained by the composition of porphyroblastic garnet Grt1 containing coesite. Early exhumation is constrained by the rim composition of Grt1 containing jadeite. Modelling results suggest that exhumation was accompanied by an influx of H 2 O, leading to a significant increase in the proportion of Ti‐bearing muscovite. The subsequent exhumation and cooling are associated with a second generation of garnet Grt2, which partly preceded the breakdown of jadeite into plagioclase‐muscovite symplectite and intergrowths. The late‐stage conditions, constrained by the composition of albitic plagioclase and small garnet Grt2 grains in the pseudomorphs, are estimated at 9–13 kbar and 420°C–530°C. The balanced reaction of jadeite breakdown fits the observed mineral compositions and the plagioclase‐muscovite proportions in the pseudomorphs. Furthermore, the reaction balance reveals that while K and Na are highly mobile, the Al/Si ratio in the former jadeite remains unchanged in the resulting plagioclase‐muscovite pseudomorphs. The jadeite‐ and coesite‐bearing rocks reported in this study broaden the known extent of UHP metamorphism in the Erzgebirge tens of kilometres further to the south, calling for revision of existing geodynamic models of this area.