Structural, geochronological and barometric constraints on Variscan deformation in the southwestern Bohemian Massif: the case study of the Untermitterdorf igneous body (Bavarian Forest, Germany)

摘要

The Untermitterdorf igneous body is located south of the Bavarian Pfahl shear zone. It resulted from the intrusion of two magma batches (granodiorite and rhyolite) and gives evidence for two different deformation phases which are distin-guishable from each other in terms of age, temperature conditions and kinematics. The deformation phase referred to as D_2 is responsible for the subvertical, NW-SE striking foliation developed by dextral shear in the granodiorite and the surrounding migmatites. D_3 gave rise to the mylonitic foliation developed by sinistral shear along subvertical, ENE-WSW to ESE–WNW striking planes in the rhyolite. As D_3 took place, the studied rocks were at an intermediate crustal level, as suggested by phengite barometry data. Microfabrics and lattice-preferred orientation patterns point to a deformation temperature of about 500℃. The granodiorite yielded ~(238)U/~(206)Pb and ~(235)UP/~(207)Pb zircon ages between 315 and 325 Ma. The rhyolite yielded ~(238)U/~(206)Pb and ~(235)U/~(207)Pb average ages of 315.0 ± 1.0 Ma and 314.7 ± 1.2 Ma, respectively. The age of the granodiorite represents a time mark for the duration of D_2, whereas the age of the rhyolite implies that at least some pulses of D_3 took place at or later than ～ 315 Ma.