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Tomek F.,Academy of Sciences of the Czech Republic | Tomek F.,Charles University | Zak J.,Charles University | Holub F.V.,Charles University | And 3 more authors.
Journal of Volcanology and Geothermal Research | Year: 2016

The Štiavnica volcano-plutonic complex is an erosional relic of Miocene caldera-stratovolcano in the Western Carpathians. The complex exposes a vertical section from the volcano basement through subvolcanic intrusions and a ring fault to volcanic edifice, comprising mostly andesitic lava flows and domes. This paper examines internal structure, magnetic fabric as derived from the anisotropy of magnetic susceptibility (AMS), and emplacement dynamics of three intra-caldera andesite domes (referred to as Domes 1-3) located near the presumed ring fault. Magnetic fabrics, carried by multi-domain titanomagnetite and titanomaghemite, are interpreted as recording various mechanisms of dome growth controlled by active caldera collapse. Dome 1 is explained as a lava coulée, fed by conduits located along the ring fault, with a long lava outflow down the sloping caldera floor. Dome 2 represents an elongated, ring fault-parallel dome wherein the lava flowed a short distance over a flat floor. Dome 3 is interpreted as a composite dome fed from multiple linear fissures opened at a high angle to the ring fault. Subsequently, the dome was intruded by ring fault-parallel dikes that may have potentially fed overlying, now largely eroded lava domes and flows. Finally, we suggest that all domes formed during collapse of the Štiavnica caldera and the various mechanisms of their growth reflect different stages of the caldera evolution from piston (Dome 2) through trap-door (Dome 1) to piecemeal (Dome 3). © 2016 Elsevier B.V. Source


Zak J.,Charles University | Verner K.,Czech Geological Survey | Verner K.,Charles University | Slama J.,University of Bergen | And 3 more authors.
Tectonics | Year: 2013

Field relationships combined with new U-Pb zircon geochronology suggest that the shallow-level Krkonoše-Jizera plutonic complex, northern Bohemian Massif, was assembled successively from bottom to top, starting with emplacement of the separately evolved S-type Tanvald granite (317.3 ± 2.1 Ma), followed by at least two voluminous batches of the I-type porphyritic Liberec (319.5 ± 2.3 Ma) and Jizera (320.1 ± 3.0 Ma and 319.3 ± 3.7 Ma) granites. The intrusive sequence was completed by uppermost, minor intrusions of the equigranular Harrachov (315.0 ± 2.7 Ma) and Krkonoše granites. The I-type granites exhibit an unusually complex pattern of superposed feldspar phenocryst and magnetic fabrics as revealed from the anisotropy of magnetic susceptibility (AMS). The outer Liberec granite preserves margin-parallel foliations and lineations, interpreted to record emplacement-related strain captured by cooling from the pluton floor and walls. In contrast, the inner Jizera, Harrachov, and Krkonoše granites were overprinted by synmagmatic strain resulting from dextral movements along regional strike-slip faults cutting the opposite ends of the plutonic complex. Late-stage felsic dikes in the Liberec and Jizera granites reorient from horizontal to vertical (lineation-perpendicular) attitude in response to changing the least principal stress direction, whereas mafic schlieren do not do so, representing only randomly oriented small-scale thermal-mechanical instabilities in the phenocryst framework. In general, this case example challenges the common approach of inferring pluton-wide magma flow from interpolated foliation, lineation, and schlieren patterns. More likely, magmatic fabrics in large plutons record complex temporal succession of superposed strains resulting from diverse processes at multiple scales. Key Points The Krkonose-Jizera plutonic complex grew successively from approximately 320 to 315 Ma Magmatic fabrics record emplacement overprinted by regional tectonic strain Fabrics indicate that granitic magmas were emplaced as phenocryst-bearing mushes ©2013. American Geophysical Union. All Rights Reserved. Source


Trubac J.,Czech Geological Survey | Trubac J.,Charles University | Trubac J.,Academy of Sciences of the Czech Republic | Zak J.,Charles University | And 3 more authors.
Journal of Structural Geology | Year: 2014

The Melechov pluton, Bohemian Massif, is interpreted as a mid-crustal nested granitic diapir with an apical part exposed at the present-day erosion level. The diapir head exhibits a concentric structure defined by lithologic zoning and by the anisotropy of magnetic susceptibility (AMS). In concert with theoretical models, outward-dipping margin-parallel magnetic foliations are associated with oblate shapes of the susceptibility ellipsoids and higher degree of anisotropy, passing inward into weaker triaxial to prolate fabric. By contrast, magnetic fabric in an inner granite unit is in places oriented at a high angle to internal contacts and is interpreted as recording an internal diapir circulation. We use inverse modeling to calculate strain variations across the diapir from the AMS data. The magnetic fabric parameters and calculated strains are in agreement with strain distribution in heads of model Newtonian diapirs traveling a distance of two body radii and suggest granitic magma ascent as a crystal-poor suspension followed by crystallization of fabric markers and their response to strain near the final emplacement level. The intrusive fabric thus formed late but, though generally weak, was still capable of recording incremental strain gradient in the granite diapir. © 2014 Elsevier Ltd. Source


Zak J.,Charles University | Verner K.,Czech Geological Survey | Verner K.,Charles University | Holub F.V.,Charles University | And 3 more authors.
Journal of Structural Geology | Year: 2012

The ∼354-336Ma Central Bohemian Plutonic Complex is a Variscan magmatic arc that developed in the central Bohemian Massif in response to subduction of the Saxothuringian lithosphere beneath the Teplá-Barrandian microplate. Magmatic to solid state fabrics in the most voluminous portion of this arc (the∼346Ma Blatná pluton) record two superposed orogenic events: dextral transpression associated with arc-parallel stretching and arc-perpendicular shortening, and normal shearing associated with exhumation of the high-grade core of the orogen (Moldanubian unit). This kinematic switch is an important landmark in the evolution of this segment of the Variscan belt for it marks the cessation of subduction-related compressive forces in the upper crust giving way to gravity-driven normal movements of the Teplá-Barrandian hanging wall block relative to the high-grade Moldanubian footwall. We use thermal modeling to demonstrate that the emplacement of huge volumes of arc magmas and their slow cooling produced a thermally softened domain in the upper crust and that the magmatic arc granitoids may have played a major role in initiating the orogenic collapse in the Bohemian Massif through lubrication and reactivation of a pre-existing lithospheric boundary and decreasing the overall strength of the rigid orogenic lid. © 2012 Elsevier Ltd. Source


Zak J.,Charles University | Zak J.,Czech Geological Survey | Verner K.,Czech Geological Survey | Verner K.,Charles University | And 4 more authors.
Lithos | Year: 2011

This paper presents new structural, anisotropy of magnetic susceptibility (AMS), petrological, and geochronological data to examine the link between LP-HT metamorphism and S-type granite formation in the Moldanubian unit, Bohemian Massif. We first describe the intrusive relationships of an S-type granite to its host cordierite-bearing migmatites, superbly exposed in the Rácov locality, northeastern Moldanubian batholith. The knife-sharp contacts and rectangular stoped blocks establish that the migmatites cooled and were exhumed above the brittle-ductile transition prior to the granite emplacement. The U-Pb monazite geochronology combined with P-T estimations constrain the age and depth of migmatization at ~329Ma and ~21km (T≈730 °C). The migmatite complex was then exhumed at a rate of 6-7mmy -1 to a depth of <9km where it was intruded by the granite at ~327Ma. These data indicate that the hot fertile metapelitic middle crust in this part of the Moldanubian unit, newly defined as the Pelhřimov complex, underwent rapid isothermal decompression at ~329-327Ma, giving rise to biotite melting and generation of large volumes of S-type granite magma. We propose that the rapid ~. 329-327. Ma exhumation of the Pelhřimov complex may have been partly assisted by the crustal-scale Přibyslav mylonite zone, which delineates the underlying western edge of the Brunia microplate underthrust beneath the eastern half of the Moldanubian unit during the early Carboniferous. The front edge of Brunia thus acted as a rigid backstop at depth, localizing the exhumation of the Pelhřimov complex and separating the hot fertile middle crust to the west from the already cooled overthrust complexes to the east. The magnetic fabric of the granite around the migmatite blocks further reveals that the Pelhřimov complex was shortened vertically and extended in the ~. WNW-ESE direction during and after its exhumation, implying that the SSE-directed underthrusting of Brunia along the eastern margin of the Bohemian Massif was replaced by vertical thinning and ~. WNW-ESE stretching of the Moldanubian crust.As a general conclusion, we suggest that even for extremely rapid crustal exhumation, S-type granite magmas can be formed at greater depths by isothermal decompression of the metapelitic host, and then ascend almost instantaneously to the already exhumed (and cooled) shallow parts of the same metamorphic core complex. This model may explain the short time spans for the extensive migmatization and associated S-type granite formation, crustal exhumation, and granite emplacement, as well as the presence of "cold", discordant granite-migmatite contacts in once "hot" migmatite terrains. © 2010 Elsevier B.V. Source

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