Late Variscan Magmatism Research Articles

Kinematics, deformation partitioning and late Variscan magmatism in the Agly massif, Eastern Pyrenees, France

In order to constrain the finite deformation pattern of the Variscan basement of the Agly massif, a detailed structural analysis over the whole Agly massif was performed. Our investigation combined geological mapping, reappraisal of published and unpublished data completed with our own structural work. Results are provided in the form of new tectonic maps and series of regional cross-sections through the Agly massif. At variance from previous studies, we identified three deformation fabrics named D1, D2 and D3. The D1 deformation is only relictual and characterized by a broadly northwest-southeast striking and eastward dipping foliation without any clear mineral and stretching lineation direction. D1 might be attributed to thickening of the Variscan crust in a possible orogenic plateau edge position. The D2 deformation is a heterogeneous non-coaxial deformation, affecting the whole massif, that produced a shallowly dipping S2 foliation, and an anastomosed network of C2 shear zones that accommodated vertical thinning and N20 directed extension. D2 is coeval with LP-HT metamorphism and plutonism at ca. 315–295 Ma. D2 corresponds to the extensional collapse of the partially molten orogenic crust in a global dextral strike-slip at the scale of the whole Variscan belt. The D2 fabrics are folded and steepened along a D3 east-west trending corridor, called Tournefort Deformation Zone (TDZ), where the Saint-Arnac and Tournefort intrusives and surrounding rocks share the same NE-SW to E-W subvertical S3 foliation. Along the D3 corridor, the asymmetrical schistosity pattern and kinematic criteria suggest a D3 dextral kinematics. The D3 deformation is a record of E-W striking dextral shearing that facilitated and localized the ascent and emplacement of the diorite and granitic sheet-shaped plutons. D3 outlasted D2 and turned compressional-dominated in response to the closure of the Ibero-Armorican arc in a transpressional regime. The progressive switch from D2 thinning to D3 transpression is attributed to the lessening of gravitational forces at an advanced stage of extensional collapse that became overcome by ongoing compressional tectonic forces at the southern edge of the Variscan orogenic plateau.

Origin and duration of late orogenic magmatism in the foreland of the Variscan belt (Lesponne — Chiroulet — Neouvielle area, French Pyrenees)

During the late stage of the Variscan orogeny, the pyrenean segment underwent intense magmatism and regional high temperature – low pressure metamorphism. In the Lesponne – Chiroulet – Neouvielle area, a granodioritic pluton was emplaced in the upper crust while dioritic to granitic magmas were emplaced in metamorphic domes. Magmatism was contemporaneous with the regional crustal partial melting recorded in the core of the domes. The area is therefore a key target in the Pyrenees to discuss potential magmatic sources as well as the age and duration of the late Variscan magmatism. Geochemical data on representative magmatic rocks highlight two distinct sources of magma: a mantle source and a metasedimentary crustal source that produced respectively metaluminous and peraluminous magmas. Geochronological results show that magmatism took place over a period of about 10 My from ca. 303 to ca. 290 Ma. During this period, the middle to lower crust was composed of partially molten metasediments intruded by mantle and crustal magmas that crystallized in a final pulse at ca. 290 Ma. Late Variscan metamorphism and magmatism recorded in the Pyrenees have to be related to a significant and rapid heating from the underlying mantle rather than to crustal processes such as the maturation of a thickened continental crust. We propose that the initiation of metamorphism and bimodal magmatism at ca. 305 Ma in the Pyrenees is the expression of the delamination of the Gondwanan lithospheric mantle at a global scale in the Variscan belt.

Geological features and geochemical characteristics of Late Devonian–Early Carboniferous K-bentonites from northwestern Turkey

AbstractNewly discovered K-bentonite beds, interstratified with limestones/dolomitic limestones of the Upper Devonian–Lower Carboniferous Yılanlı Formation, are exposed in the northwestern Black Sea region of Turkey, around Zonguldak and Bartın. K-bentonite samples collected from four different locations: the Gavurpınarı and Yılanlı Burnu quarries from the Bartın area, the Çimşir Çukurları quarry from the Şapça area, and the Güdüllü and Gökgöl highway tunnel section near Zonguldak city were investigated using optical microscopy, X-ray diffraction and inductively coupled plasma mass spectrometry in order to reveal their mineralogical and geochemical characteristics and understand their origin and evolution. The K-bentonites occur at different levels in the Yılanlı Formation as 2–40 cm-thick, greenish to yellowish beds cropping out several hundred metres along strike. Preliminary biostratigraphic data suggest that the protoliths of the Bartın (Gavurpınarı and Yılanlı Burnu) and Güdüllü K-bentonites were deposited at around the boundary between the Frasnian and Famennian, whereas those in the Şapça and Gökgöl sections are slightly younger (Devonian–Carboniferous boundary interval). The lithofacies types of the host carbonate rocks suggest an ‘epeiric’ shallow carbonate platform environment. Illite and mixed-layer illite-smectite were the major clay minerals in the K-bentonites. The K-bentonites from the Bartın area display a high degree of illitization and consist mainly of illite indicating high-grade diagenesis, whereas illite-smectite-rich samples from the Şapca and Gökgöl tunnel locations reflect relatively lower diagenetic conditions. According to their geochemical compositions, two groups of K-bentonites were distinguished, one with alkali basalt (Bartın area and Güdüllü locations) and one with trachyte affinities (Gökgöl tunnel and Şapça locations). Geochemical fingerprinting of K-bentonites by trace and rare earth element (REE) data suggest that tephras with alkali basalt composition have been derived by a source formed in a ‘continental back-arc’ setting, whereas the source of K-bentonites with trachytic precursors is related to ‘continental within-plate rifting’. An evaluation of the global Late Devonian and Devonian–Carboniferous volcanism suggests that the bentonite precursors may be related to late-Variscan magmatism in Laurussia.

New LA-ICP-MS U-Pb zircon dating for Strandja granitoids (SE Bulgaria): evidence for two-stage late Variscan magmatism in the internal Balkanides

The Strandja Massif (Sakar-Strandja Zone) forms an important link between the Balkan Zone (external Balkanides) of Bulgaria, which is commonly correlated with the Variscan orogen in Central Europe, and the Western Pontides of Turkey. The Bulgarian part of the massif is composed of a metamorphic basement (various granite gneisses, paragneisses, and schists) traditionally interpreted as having Precambrian age, Triassic-Jurassic metasedimentary cover, and Upper Cretaceous volcanosedimentary sequences. The basement is intruded by large granitic plutons of Variscan age that are widespread mostly across Turkish territory. New LA-ICP-MS data support the suggestion of Variscan granitoid magmatism in the studied area but do not confirm the presence of Precambrian rocks. Furthermore, two stages of magmatism are determined in relation to the Variscan metamorphism and deformation. The first one (301.9 ± 1.1 Ma) is represented by strongly deformed metagranites and thus is interpreted as syntectonic, while the second one is relatively younger (293.5 ± 1.7 Ma) and postmetamorphic.

Late Variscan magmatism in the Nevado-Filábride Complex: U-Pb geochronologic evidence for the pre-Mesozoic nature of the deepest Betic complex (SE Spain)

Orthogneisses crop out in different lithostratigraphic positions throughout the Nevado-Filábride metamorphic Complex (Betic Cordillera, southern Spain). Their abundance offers the opportunity to establish its origin and to date the ages of acid magmatism and of early deposition ages of metasedimentary host rocks of the orthogneisses, in this complex metamorphosed and deformed during the Alpine Orogeny. Petrography and bulk-rock chemistry indicate that the orthoderivates samples are peraluminous tourmaline leucogranites and strongly peraluminous biotite gneisses. Compositional similarity in different outcrops points to common genetic relationships. Although the studied gneisses are affected by metamorphic and deformational processes their compositional variations may be explained by differing degrees of crustal melting, source heterogeneity or minor feldspar fractionation. Variation in REE contents is attributed to accessory minerals fractionation. The geochemical data show that the protoliths originated by low degree of partial melting of biotite-rich, muscovite-plagioclase-bearing metasediments. U-Pb SHRIMP dating of magmatic zircons from six samples yielded ages between ~282±5 and 295±3Ma. Their ages and chemical composition indicate that these intrusive bodies strongly resemble the peraluminous, post-tectonic, leucogranites widespread in the Variscan Orogen, particularly those produced by crustal partial melting of the Central Iberian Zone. Zircons in host pelitic metasediments provide older ages ranging from ~336 to 2.050Ma. They testify to the recycling of Early Carboniferous and older granites, which are found in the Central Iberian zones of the Iberian Massif. The new geochronological data combined with detailed field observations allow drafting a new chronostratigraphy of the Nevado-Filábride Complex. Traditionally proposed Triassic ages for the metasedimentary upper series of the complex are not consistent with the ages reported here, which suggests that the entire complex consists of Paleozoic or older rocks. This conclusion has profound implications on the validity of the tectonic subdivisions proposed so far. Metamorphic zircon rims in two gneisses and a pelitic metasediment yield Miocene (17.3±0.4Ma) ages in line with what previously proposed for the high-pressure metamorphic peak in the Nevado-Filábride Complex. The new radiometric data moreover confirm independent metamorphic and tectonic evolutions for the Nevado-Filábride and the Alpujárride complexes of the Betic Cordillera.

Paired uraninite and molybdenite dating of the Königshain granite: implications for the onset of late-Variscan magmatism in the Lausitz Block

We present geochronological data for late-Variscan magmatism in the Lausitz Block of the Saxo-Thuringian Zone, Germany. The Th–U–total Pb age of uraninite and the Re–Os age of molybdenite from the composite biotite–monzogranite pluton of Königshain overlap at the 2σ confidence limit: 328.6 ± 1.9 Ma (uraninite), and 327.0 ± 1.3 Ma and 327.6 ± 1.3 Ma (molybdenite), indicating that crystallization of magmatic uraninite and deposition of molybdenite were nearly contemporaneous. These data imply that magmatic processes in this part of the Variscan orogen already started in latest Visean time, about 10 Ma earlier than previously assumed (315–320 Ma). The new ages correspond to ages for plutonic rocks in the Elbe Zone immediately west of the Lausitz (around 335–325 Ma) and the bulk of late-Variscan igneous rocks in the Saxo-Thuringian Zone (335–320 Ma).

SHRIMP zircon study of a micromonzodiorite dyke in the Karkonosze Granite, Sudetes (SW Poland): age constraints for late Variscan magmatism in Central Europe

AbstractThe large Variscan Karkonosze Granite in the West Sudetes, representative of the vast Variscan granite plutonism in Central Europe and located adjacent to regional tectonic suture and strike-slip-zones, has been difficult to date precisely; a range of published data varies between c. 304 and 328 Ma. However, the granite is cut by locally numerous lamprophyre and other dykes. Dating of the dyke rocks, emplaced shortly after the granite intrusion and cooled more rapidly, provides a promising tool for the verification of published SHRIMP results on the granite itself. SHRIMP zircon geochronology of a studied micromonzodiorite dyke indicates substantial admixture of inherited zircons of c. 2.0, 1.4 Ga (207Pb–206Pb minimum ages), and c. 570 (and 500?) Ma. The average concordia age of the main magmatic population of the zircons in the dyke is 313 ± 3 Ma (2σ); however, the true magmatic age might be older, around 318 Ma. This would constrain the age of the hypabyssal magmatism in the Karkonosze Massif and the minimum age of the host Karkonosze Granite. Thus, the Karkonosze Granite is confirmed as representative of an early phase of Variscan granite plutonic activity in the central-European Variscides.

The Salsigne Au-As-Bi-Ag-Cu Deposit, France

The Salsigne mesothermal Au-As-Bi-Cu-Ag deposit is located in the Montagne Noire, a part of the southern external zones of the French Variscan orogen. This part of the orogen consists of a pile of thrust nappes stacked on the autochthonous Axial zone of the Montagne Noire. The Salsigne deposit is located in and close to the lowest unit, the Fournes nappe. The Fournes nappe is less than 500 m thick and intercalated between more competent terranes, with the large Minervois nappe above and the autochthonous Axial zone below. It is intensely deformed, resulting from several stages of folding, shearing, and brittle faulting. The deposit is hosted in siliciclastic and carbonate rock and consists of many orebodies including crosscutting veins, strata-bound stockworks in sandstone, replacement orebodies in carbonate rocks, and orebodies hosted in thrust faults, particularly in the major thrust which separates the nappes from the autochthonous terranes. A succession of nine hydrothermal events, commonly separated by tectonic events, is recognized. The first four stages, controlled by ductile to ductile-brittle structures synchronous with the emplacement of the nappes, were responsible for stockwork, massive, and disseminated mineralization. They include (1) arsenopyrite-pyrrhotite-gold associated with biotite, (2) quartz-muscovite alteration, (3) massive sulfides (arsenopyrite-pyrrhotite-pyrite) and gold associated with chlorite, and (4) feldspathization. The subsequent stages, controlled by brittle tectonics that frequently reactivated earlier structures, include barren silicification and quartz veins followed by four stages of sulfide precipitation separated by fracturing. Two main types of fluids, characterized through the associated mineral associations, were responsible for mineralization in alternating successive pulses. Type I fluids, responsible for most of the gold and for the development of biotite (450°C) in host rocks of very low metamorphic grade, were reduced, CO 2 -rich, and alkaline, with high H 2 S contents but low f S 2 . There is evidence of large-scale, regional circulation of similar fluids, in particular in rocks that lie several kilometers below the deposit. Type II fluids that produced the massive sulfides associated with chlorite (and several later stages) and mainly redistributed gold, were acid with comparatively high f O 2 and f s 2 (about 2 log units higher than type I fluids), possibly in equilibrium with the country rocks below the deposit. The hydrothermal system was initiated, possibly in relationship to late Variscan magmatism, when the tectonic regime changed from ductile-brittle to brittle conditions and from compressional to extensional regime. Fluids were channeled by a shear mesh created at the base of the nappe pile and in the upper part of the autochthonous terranes by the nappe emplacement. The Salsigne deposit is located along this major (>100 km) shear zone at its intersection with transverse brittle structures and sheared fold structures.

Late-Variscan magmatism revisited: new implications from Pb-evaporation zircon ages on the emplacement of redwitzites and granites in NE Bavaria

Pb-evaporation zircon analyses coupled with a detailed cathodoluminescence (CL) study on the complete series of granitoids from the northern Oberpfalz, NE Bavaria, provide new evidence for the commencement and timing of late-Variscan magmatism. All granitoids analysed in this study were dated before by Rb–Sr and/or K–Ar methods. Investigated samples comprise medium-grained, I-type dioritic rocks (redwitzites), I/S-type granites (Leuchtenberg, Marktredwitz (G1), Zainhammer) and S-type granites (Falkenberg, Liebenstein, Mitterteich, Friedenfels, Steinwald, Flossenburg, Barnau). The zircon evaporation technique reveals three groups of 207Pb/206Pb ages which are interpreted to represent magmatic crystallisation: (1) ages of 324–321 Ma are found in all analysed redwitzites and in almost all I/S-type granites; (2) the granites of Falkenberg and Liebenstein yield ages of ~315 Ma; (3) ages between 312 and 310 Ma are recorded in the Mitterteich, Friedenfels, Steinwald and Flossenburg granites. Titanite crystals from different redwitzite bodies yield conventional U–Pb ages of 325–322 Ma, identical to the Pb-evaporation zircon data of these rocks. The S-type granites of groups 2 and 3 contain zircons with relict cores but only a small number of them yield older ages, indicating that some of the cores must have lost their radiogenic Pb. From the geochronological data, we infer that metamorphic conditions of the Variscan crust produced different granite types at different times. The data support a model involving an early period of mantle-related magmatism which postdates the final convergence stage of the Variscan orogen. This magmatic activity was at the same time as the thermal peak of regional metamorphism and is followed by a late period of crustal anatexis, which is probably related to post-collisional extension of the thickened Variscan crust.

Reply to the comment by F. Tornos et al.

We welcome the discussion of our paper by Tornos et al. The epithermal character of the Hiendelaencina veins might have been ‘an assumption’ in the early to mid 1980s, however, this early idea has been reaffirmed after many years of research involving fieldwork and mineralogical, sulphur isotopes, and fluid inclusions studies. The same applies to the ‘alleged’ extensional frame, a tectonic episode now well documented not only in central Spain (Spanish Central System: Doblas 1987; Doblas et al. 1988; Doblas 1991) but in France (French Central Massif: Menard and Molnar 1988; Malavieille et al. 1990; Munoz et al. 1992). The deposits are hosted by metamorphic rocks and the nearest volcanic outcrops to Hiendelaencina are those of Atienza (andesites; some 12 km northward). This is the reason why the relationships between the Atienza volcanics and the Hiendelaencina veins were initially regarded as ‘obscure’. These Stephanian-Permian volcanic outcrops are only local evidence of the late Variscan magmatism, which in the case of Hiendelaencina remained concealed. It is evident that the geologic environments of Hiendelaencina and Atienza are very different (see Discussion, p. 88 of the paper). As a direct consequence of this, the local structural conditions led to contrasted expressions of the late Variscan magmatism i.e. subaereal at Atienza and hypabyssal at Hiendelaencina.