František V. Holub

The causal link between HP-HT metamorphism and ultrapotassic magmatism in collisional orogens: case study from the Moldanubian Zone of the Bohemian Massif

In the Moldanubian Zone of the Bohemian Massif occur Variscan granulites and ultrapotassic magmatites (amphibole-bearing durbachitic suite and two-pyroxene syenitoids) in a close spatial and temporal relationship. The protolith to the most widespread felsic garnet-kyanite-mesoperthite granulites, which equilibrated at P >1.5 GPa and Tc. 1000°C, was analogous to meta-igneous rocks occurring in the Saxothuringian Zone of the NW Bohemian Massif. For the most basic ultrapotassic rocks, the high contents of Cr and Ni as well as high mg# point to derivation from an olivine-rich source (i.e. a mantle peridotite). On the other hand, elevated concentrations of U, Th, light rare earth elements (LREE) and large ion lithophile elements (LILE), pronounced depletion in Ti, Nb and Ta as well as high K 2 O/Na 2 O and Rb/Sr ratios apparently contradict the mantle origin. This dual geochemical character and crustal-like isotopic compositions require melting of anomalous lithospheric mantle sources, metasomatized and contaminated by mature crustal material. Both Moldanubian granulites and ultrapotassic rocks show mutually complementary depletions and enrichments in some trace elements (Cs, Rb, Th, U and Pb). Late Devonian-Early Carboniferous Andean-type subduction is thought to have resulted in continental collision and HP metamorphism of upper crustal, largely meta-igneous lithologies. The subduction of the mature crustal material caused direct contamination and metasomatism in the overlying lithospheric mantle wedge. Shortly after the granulite-facies metamorphic peak (at c . 340 Ma) and the slab break off, these metasomatized and contaminated mantle domains were melted by advected heat from the invading asthenosphere, generating ultrapotassic intrusions, closely related to the granulite occurrences in space and time. This tectonic, petrological and geochemical model is outlined.

Radiometric dating of granitic rocks from the Central Bohemian Plutonic Complex (Czech Republic): constraints on the chronology of thermal and tectonic events along the Moldanubian-Barrandian boundary

abstract Single-zircon dating by step-wise evaporation has established that successive granitic intrusions were emplaced in the Central Bohemian Plutonic Complex (CBPC) during a short time span of about 10 Ma. In agreement with field data, the Požary trondhjemite, emplaced early at 351 ±11 Ma and subcontemporaneously with the Sazava granodiorite dated at 349 ±12 Ma, was followed by the Blatna granodiorite at 346 ±10 Ma. The magnesium-potassium-rich units (durbachites) indicate younger ages both for the Certovo Břemeno melagranite at 343 ±6 Ma (within the CPBC) and for durbachite from the Třebic Massif (south-east of the CPBC) at 340 ±8 Ma. These data provide evidence that the sequence of intrusion and the age of the emplacement of the CBPC are comparable with those of other western Variscan batholiths (i.e. the Vosges or the French Massif Central) in similar structural environment.

Anticlockwise and clockwise rotations of the Eastern Variscides accommodated by dextral lithospheric wrenching: palaeomagnetic and structural evidence

New palaeomagnetic data from Lower Carboniferous granitoids of the orogenic root of the eastern Variscan belt (Moldanubian domain) show a polyphase palaeomagnetic record. Comparison of the data with existing palaeomagnetic measurements from Lower Palaeozoic sequences of the Bohemian Massif demonstrates a Carboniferous remagnetization of these latter units. All the data presented suggest that the Saxothuringian basement, the Moldanubian orogenic root system as well as the eastern Neoproterozoic Brunovistulian basement were already assembled during the Early Carboniferous. The whole Variscan belt subsequently rotated in a clockwise direction during Mid–Late Carboniferous times. Structural and geochronological data indicate that this rotation was accompanied by large-scale dextral wrenching along NW–SE-trending lithospheric faults. In a first stage, the blocks limited by wrench-faults rotated anticlockwise in bookshelf manner. The data presented rule out the existing model of oroclinal bending of the Rhenohercynian zone at the eastern termination of the Variscan belt.

A plate-kinematic model for the assembly of the Bohemian Massif constrained by structural relationships around granitoid plutons

Abstract This paper summarizes the current knowledge on the nature, kinematics and timing of movement along major tectonic boundaries in the Bohemian Massif and demonstrates how the Variscan plutonism and deformation evolved in space and time. Four main episodes are recognized: (1) Late Devonian–early Carboniferous subduction and continental underthrusting of the Saxothuringian Unit beneath the Teplá–Barrandian Unit resulted in the orogen-perpendicular shortening and growth of an inboard magmatic arc during c. 354–346 Ma; (2) the subduction-driven shortening was replaced by collapse of the Teplá–Barrandian upper crust, exhumation of the high-grade (Moldanubian) core of the orogen at c. 346–337 Ma and by dextral strike-slip along orogen-perpendicular NW–SE shear zones; (3) following closure of a Rhenohercynian Ocean basin, the Brunia microplate was underthrust beneath the eastern flank of the Saxothuringian/Teplá–Barrandian/Moldanubian ‘assemblage’; this process commenced at c. 346 Ma in the NE and ceased at c. 335 Ma in the SW; and (4) late readjustments within the amalgamated Bohemian Massif included crustal exhumation and mainly S-type granite plutonism along the edge of the Brunia indentor at c. 330–327 Ma, and peripheral tectonothermal activity driven by strike-slip faulting and possibly mantle delamination around the consolidated Bohemian Massifs interior until late Carboniferous–earliest Permian times.

Mineral fabrics in high-level intrusions recording crustal strain and volcano–tectonic interactions: the Shellenbarger pluton, Sierra Nevada, California

The Minarets caldera is a volcano–plutonic complex in the Sierra Nevada, California, that exemplifies complex interactions between volcanism and tectonic deformation in continental-margin arcs. Caldera evolution commenced with emplacement of pre-collapse rhyolitic ash-flow tuff, followed by collapse and deposition of volcanic breccia and rhyodacitic ash-flow tuff. Subsequently, the volcanic rocks were deformed along the regional Bench Canyon shear zone. The caldera centre was then intruded by the resurgent c. 100 Ma steep-sided Shellenbarger granite pluton, which steepened the shear zone foliation. The pluton was overprinted by syn- to post-magmatic ∼NNE–SSW horizontal shortening; the same shortening was documented in several other Late Cretaceous syntectonic plutons in the Sierra Nevada and interpreted to record dextral transpression during convergence of the Farallon and North American plates. To explain the unusual tectonic fabric in the shallow-level Shellenbarger pluton, we develop a general model for strain partitioning in syntectonic magma bodies emplaced at various crustal levels. We propose that shallow intrusions, isolated within stiff crust, may tend to accommodate minor pure shear strain whereas simple shear dominates along weak faults and shear zones. By contrast, a rheological reversal is crossed deeper in the crust and magma bodies become the weakest, simple shear-dominated parts of the system. Supplementary material: Analytical methods and anisotropy of magnetic susceptibility and U–Th–Pb isotopic data are available at https://doi.org/10.6084/m9.figshare.c.3582749

Simultaneous batholith emplacement, terrane/continent collision, and oroclinal bending in the Blue Mountains Province, North American Cordillera

The North American Cordillera is a classic example of accretionary orogen, consisting of multiple oceanic terranes attached to the western margin of Laurentia during the Mesozoic times. Although the Cordillera is linear for most parts, terrane boundaries are at a high angle to the overall structural grain in several segments of the orogen, which has been a matter of longstanding controversy as to how and when these orogenic curvatures formed. This paper discusses mechanisms, kinematics, and timing of initiation of one of these major curvatures, the Blue Mountains Province in northeastern Oregon. Here magmatic fabric patterns and anisotropy of magnetic susceptibility in the Wallowa batholith record three phases of progressive deformation of the host Wallowa terrane during Early Cretaceous. First is terrane-oblique ~NE-SW shortening, interpreted as recording attachment of the amalgamated oceanic and fringing terranes to the continental margin during dextral convergence at ~140 Ma. Deformation subsequently switched to pure shear-dominated ~NNE-SSW shortening associated with crustal thickening, caused by continued impingement of the amalgamated Blue Mountains superterrane into a presumed westward concave reentrant in the continental margin at ~135–128 Ma. Upon impingement (at ~126 Ma), the northern portion of the superterrane became “locked,” leading to reorientation of the principal shortening direction to ~NNW-SSE while its still deformable southern portion rotated clockwise about a vertical axis. We thus propose oblique bending as the main mechanism of the orocline formation whereby horizontal compressive forces resulting from plate convergence acted at an angle to the terrane boundaries.