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 Teplá‐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. In addition, 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 article modifies previous models of the tectonic history of the western margin of the Teplá‐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.     

Eocene migmatite formation and diachronous burial revealed by petrochronology in NW Himalaya,Zanskar

In this contribution, we highlight the importance of in-situ monazite geochronology linked to P−T modelling for identification of timescales of metamorphic processes. Barrovian-type micaschists, migmatites and augengneiss from the Gumburanjun dome in the southeastern extremity of the Gianbul dome, NW Himalaya, have been studied in order to correlate the early stages of Himalayan metamorphism at different crustal levels and infer the timing of anatexis. P−T−t paths are constrained through combined pseudosection modelling and in-situ and in-mount monazite and xenotime laser ablation–split-stream inductively coupled plasma-mass spectrometry. Petrography and garnet zoning combined with pseudosection modelling show that garnet-staurolite schists record burial from ~530 to 560°C and 5.5 kbar to ~630 to 660°C and 7 kbar; staurolite-kyanite schists from ~530 to 560°C and 5 kbar to ~670 to 680°C and 7−9 kbar; and garnet-kyanite migmatites from 540−570°C and 5 kbar to ~680 to 750°C and 7−10 kbar, probably also to >750°C and >9 kbar above the muscovite stability field. The decompression paths of garnet-staurolite schists indicate cooling on decompression, while garnet rim chemistry and local sillimanite growth point to a stage of re-equilibration at ~600 to 670°C and 4−6 kbar in some of the staurolite-kyanite schists, and at ~670 to 700°C and 6 kbar in garnet-kyanite migmatites. Some of the staurolite-kyanite schists and garnet-kyanite migmatites also contain andalusite or andalusite-cordierite. Monazite and xenotime were analysed in thin sections in garnet, staurolite and kyanite, and in the matrix; and in mounts. BSE images and compositional maps of monazite (xenotime was too small) show variable internal structures from homogeneous through patchy zoning with embayed to sharp boundaries. Two groups of samples can be identified on the basis of the presence or absence of c. 44 − 37 Ma ages. The first group of samples—two garnet-staurolite schists—recorded only c. 31 − 27 Ma ages in porphyroblasts and no c. 40 Ma ages. The second group (samples of staurolite-kyanite schist, garnet-kyanite migmatites, augengneiss) have both the older, c. 44 − 37 Ma monazite ages in porphyroblasts and younger ages down to c. 22 Ma. These significantly different ranges of ages from porphyroblasts of 44−37 Ma, and 31−27 Ma, are interpreted as the duration of prograde P−T paths in Eocene and Oligocene, and indicate diachronous two-stage burial of rocks. Early migmatization occurred at 38 Ma. The c. 29 Ma is interpreted as the time when rocks from the lower and middle crustal levels were partially exhumed and came in to contact with rocks that were downgoing at this time. Localized monazite recrystallization is as young as 26−24 Ma. The youngest ages of 23−22 Ma are related to leucogranite emplacement.     

Juxtaposition of Barrovian and migmatite domains in the Chinese Altai: a result of crustal thickening followed by doming of partially molten lower crust

Ordovician metasedimentary rocks are the oldest and most extensive sedimentary sequence in the Chinese Altai. They experienced two major episodes of deformation (D1 and D2) resulting in the formation of juxtaposed Barrovian‐type and migmatite domains. D1 is characterized by a penetrative sub‐horizontal fabric (S1), and D2 is marked by upright folds (F2) with NW–SE‐trending axial planes in shallow crustal levels and by sub‐vertical transposition foliations (S2) in the high‐grade cores of large‐scale F2 antiforms. In the Barrovian‐type domain, successive growth of biotite, garnet and staurolite is observed in the S1 fabric. Kyanite included in garnet and plagioclase in the migmatite domain is interpreted to have formed also in S1. In the biotite and garnet zones, the spaced S2 cleavage is marked by biotite and muscovite, and in the staurolite and kyanite zones, the penetrative S2 fabric is characterized by sillimanite, locally with late cordierite. Phase equilibria modelling indicates that the S1 fabric was associated with an increase in pressure and temperature under Barrovian‐type conditions in both domains. The S2 fabric was related to decompression, in which rocks in the biotite and garnet zones well preserve the peak assemblage, and the higher grade rocks in the staurolite and kyanite zones re‐equilibrated to different degrees under high‐temperature/low‐pressure (HT/LP) conditions. The D1 metamorphic history is attributed to the progressive burial related to Early–Middle Palaeozoic crustal thickening and the metamorphism associated with D2 is interpreted to result from exhumation by vertical extrusion. The extrusion of hot rocks was contemporaneous with the formation of gneiss domes accompanied by the intrusion of juvenile magmas at middle crustal levels during the Middle Palaeozoic. Consequently, there is a genetic link between the Barrovian‐type and migmatite domains related to continuous transition of the Barrovian‐type fabric into the HT/LP one during development of domal structures in the southern Altai orogenic belt. This study has a broad impact on the understanding of the thermo‐mechanical behaviour of accretionary orogenic systems worldwide. The lower crustal flow and doming of hot crust, so far reported only in continental collisional settings, seems to be also an integral mechanism responsible for both horizontal and vertical redistribution of accreted material prior to continental collision.     

P–T–t–D record of crustal‐scale horizontal flow and magma‐assisted doming in the SW Mongolian Altai

The Chandman massif, a typical structure of the Mongolian Altai, consists of a migmatite–magmatite core rimmed by a lower grade metamorphic envelope of andalusite and cordierite‐bearing schists. The oldest structure in the migmatite–magmatite core is a subhorizontal migmatitic foliation S1 parallel to rare granitoid sills. This fabric is folded by upright folds F2 and transposed into a vertical migmatitic foliation S2 that is syn‐tectonic, with up to several tens of metres thick granitoid sills. Sillimanite–ilmenite–magnetite S1 inclusion trails in garnet constrain the depth of equilibration during the S1 fabric to 6–7 kbar at 710–780 °C. Reorientation of sillimanite into the S2 fabric indicates that the S1–S2 fabric transition occurred in the sillimanite stability field. The presence of cordierite, and garnet rim chemistry point to decompression to 3–4 kbar and 680–750 °C during development of the S2 steep fabric, and post‐tectonic andalusite indicates further decompression to 2–3 kbar and 600–650 °C. Widespread crystallization of post‐tectonic muscovite is explained by the release of H₂O from crystallizing partial melt. In the metamorphic envelope the subhorizontal metamorphic schistosity S1 is heterogeneously affected by upright F2 folds and axial planar subvertical cleavage S2. In the north, the inclusion trails in garnet are parallel to the S1 foliation, and the garnet zoning indicates nearly isobaric heating from 2.5 to 3 kbar and 500–530 °C. Cordierite contains crenulated S1 inclusion trails and has pressure shadows related to the formation of the S2 fabric. The switch from the S1 to the S2 foliation occurred near 2.5–3 kbar and 530–570 °C; replacement of cordierite by fine‐grained muscovite and chlorite indicates further retrogression and cooling. In the south, andalusite containing crenulated inclusion trails of ilmenite and magnetite indicates heating during the D2 deformation at 3–4 kbar and 540–620 °C. Monazite from a migmatite analysed by LASS yielded elevated HREE concentrations. The grain with the best‐developed oscillatory zoning is 356 ± 1.0 [±7] Ma (²⁰⁷Pb‐corrected ²³⁸U/²⁰⁶Pb), considered to date the crystallization from melt in the cordierite stability ~680 °C and 3.5 kbar, whereas the patchy BSE‐dark domains give a date of 347 ± 4.2 [±7] Ma interpreted as recrystallization at subsolidus conditions. The earliest sub‐horizontal fabric is associated with the onset of magmatism and peak of P–T conditions in the deep crust, indicating important heat input associated with lower crustal horizontal flow. The paroxysmal metamorphic conditions are connected with collapse of the metamorphic structure, an extrusion of the hot lower crustal rocks associated with vertical magma transfer and a juxtaposition of the hot magmatite–migmatite core with supracrustal rocks. This study provides information about tectono‐thermal history and time‐scales of horizontal flow and vertical mass and heat transfer in the Altai orogen. It is shown that, similar to collisional orogens, doming of partially molten rocks assisted by syn‐orogenic magmatism can be responsible for the exhumation of orogenic lower crust in accretionary orogenic systems.     

Intermediate granulite produced by transformation of eclogite at a felsic granulite contact,in Blanský les,Bohemian Massif

Layers or bodies of intermediate granulite on scales from a centimetre to a hundred metres occur commonly within the felsic granulite massifs of the Bohemian Massif. Their origin is enigmatic in that they commonly have complex microstructures that are difficult to interpret, and therefore even the sequence of crystallization of minerals is uncertain. At Kle?, in the Blanský les massif, there is a revealing outcrop in a low‐strain zone in which it is clear that intermediate granulite can form by the interaction of felsic granulite with eclogite. The eclogite, retains garnet from its eclogite heritage, the grains at least partially isolated from the matrix by a plagioclase corona. The original omphacite‐dominated matrix of the eclogite now consists of recrystallized diopsidic clinopyroxene, orthopyroxene and plagioclase, with minor brown amphibole and quartz. The modification of the eclogite is dominated by the addition of just K₂O and H₂O, rather than all the elements that would be involved if the process was one of pervasive melt infiltrations. This suggests that the main process involved is diffusion, with the source being the felsic granulite, or local partial melt of the granulite. The diffusion occurred at ~950 °C and 12 kbar, with the main observed effects being (i) the un‐isolation and preferential destruction of the interior part of some of the garnet grains by large idiomorphic ternary feldspar; (ii) textural modification of the matrix primarily involving the recrystallization of clinopyroxene into large poikiloblasts containing inclusions of ternary plagioclase; and (iii) conversion of low‐K plagioclase in the matrix into ternary feldspar by incorporation of the diffused‐in K₂O. The phase equilibria in the intermediate granulite are consistent with the chemical potential relationships that would be superimposed on the original eclogite by the felsic granulite at 950 °C and 12 kbar.     

Metamorphic record of burial and exhumation of orogenic lower and middle crust: a new tectonothermal model for the Drosendorf window (Bohemian Massif, Austria)

Summary A continuous, but attenuated section through orogenic lower and middle crust overthrust by a second lower-crustal complex was distinguished at the eastern margin of the Bohemian Massif. This indicates the existence of two lower-crustal “autochthonous” extrusions into middle crust that is not compatible with the model of “allochthonous” lower crustal klippen remaining after flat thrusting of the Gf?hl nappe over large distances. The base of the lower crust is represented by a granulite body exhumed from c. 15 kbar and 800 °C. A hangingwall complex of layered amphibolites gradually passes into amphibolite bearing paragneisses (the Monotonous unit) and micaschists intercalated with marbles at the top (the Varied unit). The metamorphic grade and anatexis decreases upwards and the micaschists preserve a prograde path to c. 8 kbar and 700 °C. This sequence is overthrust by a second lower crustal strongly migmatitized complex, referred to as the Raabs complex, which is marked by an eclogite-bearing belt at the base. The garnet zoning of eclogite indicates burial from 10 kbar to min. 15 kbar. In all units, relics of a steep metamorphic fabric were identified, reworked by folding and a moderately west-dipping foliation. The conditions of 7–10 kbar and approximately 750 °C for the flat foliation were obtained in all units indicating that exhumation of the lower crust into a middle crustal level occurred earlier, probably during the development of steep fabrics. The intense flat fabric is interpreted as a result of thrusting of the whole assembly over the middle crustal Brunian indentor.     

Rare eclogite‐mafic granulite in felsic granulite in Blanský les: precursor of intermediate granulite in the Bohemian Massif?

Mafic granulite, generated from eclogite, occurs in felsic granulite at Kle?, Blanský 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).     

Re-evaluation of polyphase kinematic and 40Ar/39Ar cooling history of Moldanubian hot nappe at the eastern margin of the Bohemian Massif

International Journal of Earth Sciences - A structural and geochronological 40Ar/39Ar study was performed in kilometre-scale middle and lower crustal lens-shaped domains dominated by a preserved...     

Decay mechanism of indoor porous opuka stone: a case study from the main altar located in the St. Vitus Cathedral,Prague (Czech Republic)

A carving of the indoor main altar of St. Vitus Cathedral in Prague (Czech Republic) is made of the opuka stone—a clayey–calcareous silicite—which now exhibits the development of decay phenomena such as the formation of salt-laden case-hardened subsurface layer (approx. 150 mm thick), with detachment of the case-hardened layer manifested by blistering and/or flaking. Formation of this gypsum-rich layer is linked to the reaction of components (SO₂) from polluted air (both outdoor and indoor) and from the rock itself (calcium ion from calcite). Development of brittle damage in the subsurface layer and underlying stone is interpreted based on the results from previous environmental monitoring in the Cathedral’s interior, which indicated highly fluctuating temperature and humidity, resulting in a hygrothermal stress in the material described by the “double-layer sandwich” model. The sensitivity of the studied stone to the above-mentioned processes is evidenced by its microstructural properties, specifically parameters of the pore spaces which indicate an extremely high susceptibility to damage by the actions of freezing water and/or salt crystallisation.