Rare eclogite-mafic granulite in felsic granulite in Blansky les: precursor of intermediate granulite in the Bohemian Massif?

摘要

Mafic granulite, generated from eclogite, occurs in felsic granulite at Klet, Blansky les, in the Bohemian Massif. This is significant because such eclogite is very rare within the felsic granulite massifs. Moreover, at this locality, strong interaction has occurred between the mafic granulite and the adjacent felsic granulite producing intermediate granulite, such intermediate granulite being of enigmatic origin elsewhere. The mafic granulite involves garnet from the original eclogite, containing large idiomorphic inclusions of omphacite, plagioclase and quartz, as well as rutile. The edge of the garnet is replaced by a plagioclase corona, with the garnet zoned towards the corona and also the inclusions. The original omphacite–quartz–?plagioclase matrix has recrystallized to coarse-grained polygonal (‘equilibrium’-textured) plagioclase-diopsidic clinopyroxene–orthopyroxene also with brown amphibole commonly in the vicinity of garnet. Somewhat larger quartz grains are embedded in this matrix, along with minor ilmenite, rutile and zircon. Combining the core garnet composition with core inclusion compositions gives a pressure of the order of 18 kbar from assemblage and isopleths on a P-T pseudosection, with temperature poorly constrained, but most likely >900 °C. From this P-T pseudosection, the recrystallization of the matrix took place at ~12 kbar, and from Zr-in-rutile thermometry, at relatively hot conditions of 900–950 °C. It is largely at these conditions that the eclogite/mafic granulite interacted with the felsic granulite to make intermediate granulite (see next paper).