Martin Racek

Metamorphic inheritance of Rheic passive margin evolution and its early Variscan overprint in the Teplá-Barrandian Unit, Bohemian Massif

Rift-related regional metamorphism of passive margins is usually difficult to observe on the surface, mainly due to its strong metamorphic overprint during the subsequent orogenic processes that cause its exposure. However, recognition of such a pre-orogenic evolution is achievable by careful characterization of the polyphase tectono-metamorphic record of the orogenic upper plate. A multidisciplinary approach, involving metamorphic petrology, P–T modelling, structural geology and in-situ U-Pb monazite geochronology using laser-ablation split-stream inductively coupled plasma mass spectrometry, was applied to unravel the polyphase tectono-metamorphic record of metapelites at the western margin of the Tepla-Barrandian domain in the Bohemian Massif. The study resulted in discovery of three tectono-metamorphic events. The oldest event M1 is LP–HT regional metamorphism with a geothermal gradient between 30 and 50°C km−1, peak temperatures up to 650°C and of Cambro-Ordovician age (c. 485 Ma). The M1 event was followed by M2-D2, which is characterized by a Barrovian sequence of minerals from biotite to kyanite and a geothermal gradient of 20–25°C km−1. D2-M2 is associated with a vertical fabric S2, and was dated as Devonian (c. 375 Ma). Finally, the vertical fabric S2 was overprinted by a D3-M3 event that formed sillimanite- to chlorite- bearing gently inclined fabric S3 also of Devonian age. The high geothermal gradient of the M1 event can be explained as the result of an extensional, rift-related tectonic setting. Additionally, restoration of the deep architecture and polarity of the extended domain before the Devonian history–together with the supracrustal sedimentary and magmatic record–lead us to propose a model for formation of an Ordovician passive continental margin. The subsequent Devonian evolution is interpreted as horizontal shortening of the passive margin at the beginning of Variscan convergence, followed by detachment-accommodated exhumation of lower-crustal rocks. Both Devonian shortening and detachment occurred in the upper plate of a Devonian subduction zone. The tectonic evolution presented in this paper modifies previous models of the tectonic history of the western margin of the Tepla-Barrandian domain, and also put constraints on the evolution of the southern margin of the Rheic ocean from the passive margin formation to the early phases of Variscan orogeny. This article is protected by copyright. All rights reserved.

Slawsonite-celsian-hyalophane assemblage from a picrite sill (Prague Basin, Czech Republic)

Abstract The first European occurrence of slawsonite is reported from a picrite sill within Upper Ordovician strata of the Prague Basin near the village of Rovina, Czech Republic. The rare slawsonite forms an interstitial phase in association with abundant celsian and hyalophane, replacing the original calcic plagioclase (bytownite). A study of this curious natural slawsonite-celsian-hyalophane assemblage provides a valuable insight into feldspar stability and petrogenesis.Whole-rock geochemical signatures of the picrite sill and underlying doleritic basalt intrusion show conspicuous enrichment in Sr and Ba superimposed on normal basaltic multielement patterns. These two elements were most likely introduced by intergranular fluids during diffusional seafloor metasomatism (rodingitization and serpentinization) of the picrite. Strontian and barian feldspars precipitated directly from BaO-SrO-H2Obearing fluid, which caused decomposition of plagioclase to vuagnatite, aqueous SiO2 and Al2O3 at T ≤ 350 °C. Subsequently, vuagnatite decomposed to hydrogrossular and excess SiO2 was consumed by serpentinization of olivine. At the expense of aqueous Al2O3, serpentine reacted to chlorite closing the picrite alteration at 320-160 °C. Pressure did not exceed 0.5 GPa. The in situ EDS analyses indicate that the chemical composition of the slawsonite is Sl91Cn3An3Ab (core) to Sl82Cn3An4Ab9Or2 (rim), the celsians range from Cn96.9An0.3Ab0.2Or2Sl0.6 to Cn76.3An4.7Ab3Or15.7Sl0.3, and the hyalophanes vary from Cn72.2An-0.8Ab5.1Or21.1Sl0.2 to Cn57.3An0.8Ab3.1Or38.5Sl0.3.

Role of strain localization and melt flow on exhumation of deeply subducted continental crust

A section of anatectic felsic rocks from a high-pressure (>13 kbar) continental crust (Variscan Bohemian Massif) preserves unique evidence for coupled melt flow and heterogeneous deformation during continental subduction. The section reveals layers of migmatitic granofels interlayered with anatectic banded orthogneiss and other rock types within a single deformation fabric related to the prograde metamorphism. Granofels layers represent high strain zones and have traces of localized porous melt flow that infiltrated the host banded orthogneiss and crystallized granitic melt in the grain interstices. This process is inferred from: (1) gradational contacts between orthogneiss and granofels layers; (2) grain size decrease and crystallographic preferred orientation of major phases, compatible with oriented growth of crystals from interstitial melt during granular flow, accommodated by melt-assisted grain boundary diffusion creep mechanisms; and (3) pressuretemperature equilibria modeling showing that the melts were not generated in situ. We further argue that this porous melt flow, focused along the deformation layering, significantly decreases the strength of the crustal section of the subducting continental lithosphere. As a result, detachment folds develop that decouple the shallower parts of the layered anatectic sequence from the underlying and continuously subducting continental plate, which triggers exhumation of this anatectic sequence. LITHOSPHERE GSA Data Repository Item 2017403 https:// doi .org /10 .1130 /L666 .1