Central Bohemian Plutonic Complex Research Articles

Plane-confined magnetic lineations in mingled mafic and felsic magmas, the Sázava pluton, Bohemian Massif

Multiple magnetic lineations were revealed using the anisotropy of magnetic susceptibility (AMS) and anisotropy of magnetic remanence (AMR) methods in a domain of mafic–felsic magma mingling at the northwestern margin of the Sázava pluton, Central Bohemian Plutonic Complex, Bohemian Massif. Gabbrodioritic enclave swarms and sheets are steeply oriented and exhibit magnetic lineations plunging at steep to moderate angles (mostly 40–70°) whereas in their tonalitic host lineations plunge from vertical to horizontal angles, but mostly less than 40°. The magnetic lineations in both rock types spread along a ~ NNE-SSW steep plane that could be simplified as representing an “average” margin-parallel magmatic foliation in the pluton concordant with an “average” regional cleavage in the wall-rock. The plane-confined lineations are interpreted as having recorded the heterogeneous superposition of two processes: (1) vertical stretching during emplacement and magma mingling which left behind the steep lineations; and (2) regional tectonic stretching, which progressively rotated the mineral grains in rheologically weaker domains (chiefly in the host tonalite) to form the sub-horizontal lineation. The average foliation bearing the multiple lineations is interpreted as a composite foliation that recorded both margin-perpendicular shortening during emplacement, overprinted by coaxial regional tectonic shortening. This example reaffirms that (1) magmatic fabrics in crystallizing magmas can record accumulated strain resulting from both emplacement and regional tectonic deformation; and that (2) separating magmatic (and also magnetic) fabrics related exclusively to the internal chamber processes from fabrics caused by regional tectonic deformation is problematic or even impossible in cases where composite fabrics are recognized.

Strain coupling between upper mantle and lower crust: natural example from the Běstvina granulite body, Bohemian Massif

AbstractStrain patterns within mantle rocks and surrounding coarse‐grained felsic granulites from the Kutná Hora Crystalline Complex in the Variscan Bohemian Massif have been studied in order to assess their strain coupling. The studied rock association occurs within low‐strain domains surrounded by fine‐grained granulite and migmatite. The Doubrava peridotite contains closely spaced and steeply dipping layers of garnet clinopyroxenite, which are parallel to the NE–SW‐striking, high‐temperature foliation in nearby granulites, while the Úhrov peridotite lacks such layering. The Spačice eclogite is not associated with peridotite and shows upright folds of alternating coarse‐ and fine‐grained varieties bearing NE–SW‐striking axial planes. Electron back‐scattered diffraction measurements revealed full strain coupling between clinopyroxenites and coarse‐grained granulites in the S1 fabric that is superposed on the S0 fabric preserved in peridotites. The B‐type olivine lattice preferred orientation (LPO) characterizes the S0 fabric in peridotites and its reworking is strongly controlled by the presence of macroscopic clinopyroxenite layering. The S1 in clinopyroxenites and coarse‐grained granulites is associated with the LS‐type clinopyroxene LPO and prism <c> slip in quartz respectively. While the S1 fabric in these rock types is accompanied invariably by a sub‐vertical stretching lineation, the S1 fabric developed in reworked Úhrov peridotite is associated with strongly planar axial (010) type of olivine LPO. The peridotites with the S0 fabric are interpreted to be relicts of a fore‐arc mantle wedge hydrated to a various extent above the Saxothuringian subduction zone. The prograde metamorphism recorded in peridotites and eclogites occurred presumably during mantle wedge flow and was reaching UHP conditions. Strain coupling in the S1 fabric between clinopyroxenites and granulites at Doubrava and upright folding of eclogites at Spačice document a link between tectonic and magmatic processes during orogenic thickening, coeval with intrusions of the arc‐related calcalkaline suites of the Central Bohemian Plutonic Complex (c. 360–345 Ma). Juxtaposition of peridotites and granulites could be explained by a rheological heterogeneity connected to the development of clinopyroxenite layering in the upper mantle and a previously published model of a lithospheric‐scale transpressional arc system. It invokes vertical shearing along NE–SW trending, sub‐vertical foliations in the upper mantle that could have led to an emplacement of mantle bodies into the granulitized, orogenic root in the sub‐arc region. Clearly, such a transpressional arc system could represent an important pathway for an emplacement of deep‐seated rocks in the orogenic lower crust.

Forearc deformation and strain partitioning during growth of a continental magmatic arc: The northwestern margin of the Central Bohemian Plutonic Complex, Bohemian Massif

The Late Devonian subduction followed by the Early Carboniferous continental collision of the Saxothuringian and overriding upper-crustal Teplá–Barrandian units led to the growth of a large magmatic arc (the ∼ 354–337 Ma Central Bohemian Plutonic Complex) in the central part of the Bohemian Massif. Far-field tectonic forces resulting from the collision produced ∼WNW–ESE to ∼NW–SE regional shortening across the forearc upper crust above the subduction zone; the shortening was accommodated by predominantly top-to-the-ESE tectonic transport along the southeastern flank of the Teplá–Barrandian unit. Approaching the magmatic arc margin, the regional structural pattern changes and exhibits significant across- and along-strike variations interpreted as a result of strain partitioning, where the Saxothuringian/Teplá–Barrandian convergence interacted in different ways with the intruding magma pulses. Around the voluminous, northeasterly ∼ 354 Ma Sázava pluton the principal shortening was at high angle to the forearc-facing intrusive contact and the host rocks were significantly thermally softened. The regional top-to-the-ESE tectonic transport converted here into arc-parallel ductile flow within the structural aureole around and above the pluton. In contrast, a narrow to nonexistent ductile strain aureole is preserved in the host rocks around discordant sheet-like plutons (the southwesterly pre-354 (?) Ma Marginal granite and the Milín granodiorite of unknown radiometric age). Our AMS study of the Marginal granite and Milín granodiorite, and mapping of mesoscopic magmatic foliations and lineations in another neighboring sheet-like pluton (the ∼ 346 Ma Kozárovice granodiorite), reveals sigmoidal map-scale fabric patterns consistent with dextral transpression. We thus suggest that the thin sheet-like plutons were oriented obliquely to the principal shortening and were rheologically weaker than the host rocks prior to final crystallization, producing dextral transpression recorded by the internal fabrics of these plutons. Our study shows that the far-field plate kinematics during pluton emplacement is not the only factor that controls strain partitioning in continental magmatic arcs. The two contrasting styles of pluton emplacement documented here indicate that the pluton shape, orientation of intrusive contacts with respect to the background plate convergence vector, and heat budget of intrusions (magma volume, composition, and proportion of hot mantle component) may also govern strain partitioning.

Transition from island-arc to passive setting on the continental margin of Gondwana: U–Pb zircon dating of Neoproterozoic metaconglomerates from the SE margin of the Teplá–Barrandian Unit, Bohemian Massif

Sedimentological, petrological and geochemical data combined with SHRIMP and LA ICP-MS U–Pb ages of pebbles of granitic rocks and detrital zircons from Neoproterozoic Vletice metaconglomerates of the SE margin of the Teplá–Barrandian Unit were used to characterize the regional geotectonic setting at the Proterozoic/Paleozoic boundary. The metaconglomerates form several-metres thick intercalations in metasediments of the Svrchnice Formation (roof pendants of Central Bohemian Plutonic Complex), which is considered to have been deposited in Neoproterozoic/Cambrian times. A sedimentological study indicates that the metaconglomerates are products of debris flows deposited in submarine canyons or proximal parts of submarine fans. Geochemistry of metaconglomerate matrix and petrography of pebbles revealed an obvious difference from the western and central part of the Teplá–Barrandian Unit Proterozoic where pebbles eroded from uplifted parts of the basin and island arc sources are most common. The abundance of quartz, quartzite, granite and crystalline limestone in pebbles together with the more mature composition of host metagreywackes indicate the presence of relatively evolved continental crust in the source area for the Vletice metaconglomerates, and shallowing of the Neoproterozoic basin to the SE (in the present geographic co-ordinate system). The Neoproterozoic to lowermost Cambrian age of the Svrchnice Formation, suggested by its comparison with similar rocks in the central part of the Teplá–Barrandian Unit, was confirmed by U–Pb dating. Ages of detrital zircons from the metaconglomerate matrix cluster around two values: Archaean to Paleoproterozoic U/Pb ages between ca. 2580 and 1780 Ma and Neoproterozoic to early Cambrian ages between 660 and 520 Ma with a significant peaks at 600 and 544 Ma suggesting local volcanic activity extending shortly beyond the Proterozoic/Paleozoic boundary. Zircons separated from two trondhjemite pebbles from the metaconglomerate yielded a mean U–Pb age of 610 ± 17 Ma arguing for active arc magmatism during the latest Neoproterozoic. The overall age distribution of detrital zircons and the absence of Grenvillian zircon ages favour a location of the Teplá–Barrandian Unit close to the West African craton at the Neoproterozoic/Paleozoic boundary. The rapid onset of a transtensional geotectonic regime and maturing of sediments is evidenced by the overlying Cambrian shallow-water clastics alternating with predominantly felsic calc-alkaline to alkaline volcanics. During the Cambrian oblique plate convergence, the active continental margin was transformed into belts of horsts and narrow pull-apart or small rift basins. After the Ordovician break-off of Armorican Terrane Assemblage blocks from Gondwana, the extensional regime in the Teplá–Barrandian Unit continued until the beginning of Variscan orogeny in the Middle Devonian.

Magmatic stoping as an important emplacement mechanism of Variscan plutons: evidence from roof pendants in the Central Bohemian Plutonic Complex (Bohemian Massif)

The presence of numerous roof pendants, stoped blocks and discordant intrusive contacts suggests that magmatic stoping was a widespread, large-scale process during the final construction of the Central Bohemian Plutonic Complex, Bohemian Massif. The measured total length of the discordant contacts that cut off the regional cleavage and were presumably formed by stoping corresponds to about half of all contacts with the upper-crustal host rocks. In addition, at least some of the straight, cleavage-parallel intrusive contacts may also have recorded complex intrusive histories ending with piecemeal stoping of thin cleavage-bounded host rock blocks into the magma chamber. Based on the above, we argue that the fast strain rates required for emplacement of large plutons of the Central Bohemian Plutonic Complex into brittle upper crustal host rocks over relatively short-time span could not have been accommodated entirely by slow ductile flow or slip along faults. Instead, the emplacement was largely accommodated by much faster thermal cracking and extensive stoping independent of regional tectonic deformation. Finally, we emphasize that magmatic stoping may significantly modify the preserved structural patterns around plutons, may operate as an important mechanism of final construction of upper-crustal plutons and thus may contribute to vertical recycling and downward transport of crustal material within the magma plumbing systems in the crust.

Multiple magmatic fabrics in the Sázava pluton (Bohemian Massif, Czech Republic): a result of superposition of wrench-dominated regional transpression on final emplacement

The Sázava pluton (∼354 Ma) of the Central Bohemian Plutonic Complex was emplaced syntectonically into the upper crustal Neo-Proterozoic and Lower Paleozoic host rock along the SE margin of the Teplá-Barrandian Zone, Bohemian Massif (Czech Republic). The host rock is characterized by transpressional fabric dominated by steep SE–NW dipping foliations and subhorizontal NE–SW stretching lineations associated with flattening to a plane strain ellipsoid. The strain intensity, degree of oblateness and temperature conditions of deformation increase in the host rock towards the pluton margin. In the pluton, quartz microfabric indicates progressive cooling of the syntectonically emplaced magma. The AMS method revealed multiple magmatic fabrics and lateral variations in the shapes of the AMS ellipsoid and the magnetic fabric intensity in the pluton. The central part of the pluton yields prolate shapes of the AMS ellipsoid and a low degree of anisotropy with preserved steep magnetic lineations, whereas, along the pluton margins, oblate AMS ellipsoids are associated with a high degree of anisotropy. We interpret the multiple fabrics and the partially decoupled fabric pattern between the host rock and the pluton as being a result of superposition of regional tectonic strain during highly oblique transpression on emplacement-related intrusive strain, where outer solidified and more rigid edges of the pluton enabled preservation of older steep fabrics in its interior.

Tectonic evolution of a continental magmatic arc from transpression in the upper crust to exhumation of mid-crustal orogenic root recorded by episodically emplaced plutons: the Central Bohemian Plutonic Complex (Bohemian Massif)

The Central Bohemian Plutonic Complex (CBPC) consists of episodically emplaced plutons, the internal fabrics of which recorded tectonic evolution of a continental magmatic arc. The ~354–350 Ma calc-alkaline plutons were emplaced by multiple processes into the upper-crustal Tepla-Barrandian Unit, and their magmatic fabrics recorded increments of regional transpression. Multiple fabrics of the younger, ~346 Ma Blatna pluton recorded both regional transpression and the onset of exhumation of mid-crustal orogenic root (Moldanubian Unit). Continuous exhumation-related deformation during pluton cooling resulted in the development of a wide zone of sub-solidus deformation along the SE margin of the CBPC. Finally, syn-exhumation tabular durbachitic pluton of ultrapotassic composition was emplaced atop the intrusive sequence at ~343–340 Ma, and the ultrapotassic Tabor pluton intruded after exhumation of the orogenic root (~337 Ma). We suggest that the emplacement of plutons during regional transpression in the upper crust produced thermally softened domain which then accommodated the exhumation of the mid-crustal orogenic root, and that the complex nature of the Tepla-Barrandian/Moldanubian boundary is a result of regional transpression in the upper crust, the enhancement of regional deformation in overlapping structural aureoles, the subsequent exhumation of the orogenic root domain, and post-emplacement brittle faulting.

Geology and genesis of Variscan porphyry-style gold mineralization, Petr�?kova hora deposit, Bohemian Massif, Czech Republic

A large number of Variscan mesothermal gold deposits are located in the central part of the Bohemian Massif, close to the Central Bohemian Plutonic Complex. The Petrackova hora deposit has many features that distinguish it from other deposits in the region and suggest its mineralization is closely related to the late magmatic processes associated with the Petrackova hora granodiorite. The gold ores occur as sheeted arrays of quartz veins and veinlets hosted by the small Petrackova hora granodiorite stock. Gold is found mainly as free grains of >900 fineness, and is accompanied by abundant pyrrhotite and chalcopyrite, and accessory pyrite, arsenopyrite, loellingite, and molybdenite. Molybdenite from the Petrackova hora deposit has been dated by the Re–Os method at 344.4±2.8 Ma. Hydrothermal alteration in the Petrackova hora deposit exhibits a distinct temporal paragenesis. Selectively pervasive, early K-alteration and silicification are the oldest hydrothermal phases. These were followed by early quartz veins (Q1 to Q4) that contain most of the gold mineralization. Late quartz veins (Q5) and fracture-controlled silicification are gold-poor or barren. Barren calcite veins are the youngest hydrothermal product. Extensive low-temperature, meteoric-water dominated alteration, as is typical of classic porphyry deposits, is absent. However, the lower δ18O whole rock values for Petrackova hora granodiorite and aplite (+2.4 to +5.1‰ SMOW) compared to other intrusions in the region reflect either interaction with isotopically light external fluids or magma assimilation of small volumes of hydrothermally altered country rock. The δ18O isotopic compositions for quartz, scheelite and hornblende (7.7 to 13.4‰ SMOW) and the δ34S compositions for sulfide minerals (–1 to +3.5‰ CDT) from early, gold-rich quartz veins indicate formation at high temperatures (590 to 400 °C) from fluids with a magmatic isotopic signature (δ18OFLUID of 5.7 to 7.2‰). Fluids related to late quartz veins (Q5) suggest the presence of a significant component of non-magmatic water (δ18OFLUID: +2.5 to +4.0‰). The δ34S values of post-Q5 sulfide minerals (–4.5 to –3.5‰) reflect at least partial derivation of late-stage sulfur from a source external to the intrusions. Aqueous, aqueous–carbonic and nitrogen-bearing fluid inclusions were identified in hydrothermal and igneous quartz, with the aqueous inclusions being the most common. In hydrothermal vein quartz, the salinity of primary aqueous inclusions falls into ranges 6 to 23 and 33 to 41 equiv. wt% NaCl; in igneous quartz, populations in salinity were observed between 5 to 16, 35 to 40 and 62 to 70 equiv. wt% NaCl. The salt component of these fluids is best, and minimally, approximated by the NaCl–KCl–CaCl2 system. Low- and high-salinity aqueous–carbonic inclusions are accessory in many of the analyzed samples. Three large successive pulses of fluids are recognized. Each pulse begins with a high-salinity (>30 equiv. wt% NaCl) magmatic fluid and evolves toward a lower salinity (~5 equiv. wt% NaCl) fluid. Data suggest that external (meteoric?) water(s) were significant for only the third fluid pulse, which formed the late Q5 quartz veins and the calcite veins. Polyphase fluid inclusions hosted by igneous quartz of the Petrackova hora granodiorite indicate minimum trapping conditions of about 3 kbar and 550 °C. The gold-rich Q1 to Q4 veins may have formed along a quasi-isobaric cooling path at 2.5 to 1.5 kbar and 590 to 400 °C. This was followed by uplift, and formation of late Q5 quartz veins (0.5 to 1.5 kbar; ~300 °C) and post-ore calcite veins (<0.5 kbar; 100 to 140 °C). The characteristics of the Petrackova hora deposit suggest that it may represent a position intermediate between intrusion-related gold systems (e.g., Fort Knox deposit, Alaska) and gold-rich, copper-poor porphyry deposits (e.g., Maricunga Belt in Chile). As such, the Petrackova hora deposit might be an example of the reduced gold sub-type of porphyry deposit.

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

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 Požáry trondhjemite, emplaced early at 351 ±11 Ma and subcontemporaneously with the Sázava granodiorite dated at 349 ±12 Ma, was followed by the Blatná granodiorite at 346 ±10 Ma. The magnesium-potassium-rich units (durbachites) indicate younger ages both for the Čertovo Břemeno melagranite at 343 ±6 Ma (within the CPBC) and for durbachite from the Třebíč 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. Dans le complexe plutonique de Bohême centrale (CPBC), la datation par évaporation du Pb sur monozircon a permis de mettre en évidence le faible laps de temps durant lequel se mettent en place successivement, et en accord avec les données de terrain, le groupe d'intrusions calco-alcalin daté à 351 ±11 Ma (trondhjémite de Požáry) et 349 ±12 Ma (granodiorite de Sázava), puis le groupe calco-alcalin potassique daté à 346 ±10 Ma (granodiorite de Blatná) et enfin le groupe ultrapotassique à 343 ±6 Ma (granite mélanocrate de Čertovo Břemeno) et 340 ±8 Ma (durbachite de Třebíč au sud-est du CPBC). Ces données indiquent que la chronologie et l'âge de mise en place des intrusions du CPBC sont comparables à celles d'autres batholites varisques, comme ceux des Vosges ou du Massif central français.