In situ white mica Rb/Sr geochronology of the Leszczyniec metaigneous complex, West Sudetes: evidence of upper plate deformation at the onset of Variscan collision

The Karkonosze-Izera Massif in the West Sudetes preserves evidence of subduction of the Saxothuringian Ocean beneath the Tepla-Barrandian Domain. Within the massif, the Leszczyniec metaigneous complex (LMC) is identified as the upper allochthon. It exhibits a unique structural history in comparison to the underlying allochthons, suggesting that the LMC records a distinctive tectonic history. To investigate the timing of this history, two orthogneisses were studied from a single outcrop of the LMC for in situ white mica Rb/Sr geochronology. The outcrop bears a southeast-dipping foliation (S1) and a north/northeast plunging stretching lineation (L1), defined by white mica and quartz. Quartz recrystallization textures, white mica chemistry (celadonite content of 0.23-0.47), preservation of igneous plagioclase phenocrysts, and the metamorphic mineral assemblage, all indicate maximum epidote-amphibolite facies conditions. Single-spot Rb/Sr dates were calculated from white mica using initial 87Sr/86Sr values obtained by titanite and epidote. Results are similar for both rocks, providing weighted averages of 352.4 +/- 4.1 Ma (MSWD: 0.6; n: 24) and 349.3 +/- 2.5 Ma (MSWD: 0.4; n: 31). The rocks are interpreted to have the same structural and metamorphic history; thus, a pooled weighted average of 350.1 +/- 5.3 Ma (2 sigma) is reported as the timing of white mica (re)crystallization during S1 and L1 development in epidote-amphibolite facies conditions. This event is bracketed by the timing of blueschist-facies metamorphism for the subjacent middle (c. 364 Ma) and lower (c. 345-341 Ma) allochthons, associated with an east/southeast-plunging L1. Considering the different structural and metamorphic histories of the LMC compared to the subjacent allochthons, it is likely that the complex was extracted from the Tepla-Barrandian upper plate due to subduction erosion prior to collective exhumation and stacking of the Karkonosze-Izera Massif.