Metamorphic rocks of the Samerberg complex, Eastern Alps: 2. P-T paths and the problem of a geodynamic model

Abstract

This paper continues the study of Perchuk et al. (2004), who reported a detailed petrographic description of peridotites and host gneisses composing the ultrahigh-pressure Samerberg complex. Part 2 presents the calculation of the P-T parameters of local equilibria and derivation of P-T paths: counterclockwise for spinel-gamet metaultrabasic rocks and decompression cooling for the host two-mica gneisses and migmatites. A possible clinoenstatite-orthoenstatite transition (characteristic twins in orthopyroxene) suggests high-temperature but shallow-depth, almost near-surface crystallization (quenching) of the ultrabasic protolith. During the subsequent burial of the already quenched rocks to great depths, metamorphic temperature increased but remained below the quenching temperature, which must be reflected in a clockwise P-T loop. Since the subsidence rate was higher than the rates of chemical reactions, complete reequilibration of the rocks upon heating was reached at maximum depths. Using these data, a geodynamic model was proposed for the formation and exhumation of the complex. It was found, in particular, that the P-T path of the gneisses closely reproduces the P-T path of the Erzgebirge gneiss dome in the western Bohemian Massif. This suggests that the thermal and dynamic Hercynian evolution of the entire Bohemian Massif can be adequately described by the model of lower crustal delamination.