Garnet-clinopyroxene intermediate granulites in the St. Leonhard massif of the Bohemian Massif: ultrahigh-temperature metamorphism at high pressure or not?

摘要

Garnet-clinopyroxene intermediate granulites occur as thin layers within garnet-kyanite-K-feldspar felsic granulites of the St. Leonhard granulite body in the Bohemian Massif. They consist of several domains. One domain consists of coarser-grained coexisting ternary feldspar, clinopyroxene, garnet, quartz and accessory rutile and zircon. The garnet has 16-20% grossular, and the clinopyroxene has 9% jadeite and contains orthopyroxene exsolution lamellae. Reintegrated ternary feldspar and the Zr-in-rutile thermometer give temperatures higher than 950 degrees C. Mineral equilibria modelling suggests crystallization at 14 kbar. The occurrence and preservation of this mineral assemblage is consistent with crystallization from hot dry melt. Between these domains is a finer-grained deformed matrix made up of diopsidic clinopyroxene, orthopyroxene, plagioclase and K-feldspar, apparently produced by reworking of the coarser-grained domains. Embedded in this matrix, and pre-dating the reworking deformation, are garnet porphyroblasts that contain clinopyroxene, feldspar, quartz, rutile and zircon inclusions. In contrast with the garnet in the coarser-grained domains, the garnet generally has > 30% grossular, the included clinopyroxene has 7-27% jadeite and the Zr content of rutile indicates much lower temperatures. Some of these high-grossular garnet show zoning in Fe/(Fe + Mg), decreasing from 0.7 in the core to 0.6 and then increasing to 0.7 at the rim. These garnet are enigmatic, but with reference to appropriate pseudosections are consistent with localized new mineral growth from 650 to 850 degrees C and 10 to 17 kbar, or with equilibration at 20 kbar and 770 degrees C, modified by two-stage diffusional re-equilibration of rims, at 10-15 and 8 kbar. The strong pervasive deformation has obscured relationships that might have aided the interpretation of the origin of these porphyroblasts. The evolution of these rocks is consistent with formation by igneous crystallization and subsequent metamorphism to high-T and high-P, rather than an origin by ultrahigh-T metamorphism. Regarding the petrographic complexity, combination of the high grossular garnet with the ternary feldspar to infer ultrahigh-T metamorphism at high pressure is not justified.