Hercynian Belt Research Articles

New Targets of Potential Mining Interest Using Gravimetric and Satellite Data: Case Study of Hercynian Rehamna Massif, Morocco

The known mineralization in the Rehamna Massif is essentially vein type and linked to the presence of granitic bodies and pneumatolytic - hydrothermal fluid circulations. This part of western Moroccan Meseta, belongs to the Hercynian belt, was the seat of enormous mining productions mainly base metals during the 20th century. Subsequently, exploitations are ended with artisanal extractions, in relation to exploration decline. The present work aims to search for new favourable mining targets, using gravimetric and remote sensing data. Gravimetric data allowed us to delineate leucogranitic apexes of shallow depth. Data filtering and processing consist of separation between Regional and Residual fields from Bouguer anomaly, then, calculation of horizontal, vertical and tilt derivatives to get different gravity anomalies. A Satellite image is used to obtain structural lineaments of the study region through directional filters (N0°, N45°, N90°, N135°). These treatments, supported by field investigations, enabled us to discriminate potential areas, suitable for tactical mining exploration.

Acritarch findings in Early Paleozoic, Low-grade metasediments of Sardinia (Italy): A review.

Acritarchs were found in the weakly metamorphosed deposits of the Hercynian belt of Sardinia (ltaly). Silurian assemblages has been described from the Outer Zone of the Sardinian chain since 1979. From the same zone, recently, Early Tremadocian acritarchs associated with graptolites have also been found. In the Nappe Zone of Central Sardinia the palynomorphs were found in Late Cambrian to Arenigian, terrigenous formations. The preservation of acritarchs in metamorphosed and strongly deformed rocks such as the Sardinian Paleozoic metasediments is discussed. The most important geological implications of the palynological datings are also briefly examined.

Stages of Formation of the South Altai Metamorphic Belt (Central Asia)

Abstract —The Hercynian mobile belts in Central Asia include the proper Hercynian and late Hercynian (Indo-Sinian) belts, whose formation is associated with the evolution of the South and Inner Mongolian basins with oceanic crust. Within the South Altai metamorphic belt (SAMB), rock complexes compose tectonic slivers of different ranks. At the early stages, their metamorphic alteration occurred under conditions of the high-temperature subfacies of the amphibolite and, in places, granulite facies. Structurally, the band of the outcrop of these complexes is confined to the Caledonian North Asian continental margin and stretches along the southern slope of the Gobi–Mongolian–Chinese Altay Mountains from southeast to northwest (East Kazakhstan), where they occur in the Irtysh strike-slip zone. We assign these complexes to the Hercynian SAMB running for more than 1500 km. The latter comprises poly- and monometamorphic complexes. Late metamorphic granitoids of the Tseel tectonic sliver (Gobi Altay) in the southeast of the SAMB have been dated at 374 ± 2 and 360 ± 5 Ma. The previous data and these results show that the early (~390–385 Ma) low-pressure and late (375–360 Ma) high-pressure metamorphism proceeded almost along the entire belt. The interval between them was a short tectonic lull. These processes took place during the closure of a Tethyan basin of the South Mongolian Ocean (Paleo-Tethys I). The spatial position of the SAMB was controlled by the structural asymmetry of the basin, with an active continental margin at its northern edge and a passive one at the southern edge (in the present-day coordinates).

Hercynian subduction‐related processes within the metamorphic continental crust in Calabria (southern Italy)

AbstractLinking the deformation history of mylonitized continental rocks to the progress of devolatilization reactions that trigger reaction softening is critical for the understanding of crustal scale processes. We have analysed the field geometries and microstructures of deformed rocks within the southern Hercynian belt in Calabria, as well as modelled the pressure–temperature–deformation (P–T–d) trajectory of a main ductile shear zone that tectonically coupled the deeper crustal Mammola Paragneiss Unit with the upper crustal Stilo–Pazzano Phyllite Unit. P–T modelling of the mylonitic Mammola Paragneiss Unit was performed through calculation of phase equilibrium diagrams with the software thermocalc in the MnNCKFMASHTO model system. The prograde P–T–d trajectory is based on the zoning profiles of garnet porphyroblasts and their mineral inclusions, primarily barroisite and epidote. P–T modelling shows that peak metamorphic conditions of ~0.9 GPa and 585°C were reached during a Dn‐1 under‐thrusting event. The following exhumation during the Dn mylonitic event, and contact metamorphism during Dn+1 and Dn+2 folding events, have also been modelled because they are essential to restore the previous tectono‐metamorphic history. The exhumation trajectory was modelled down to 0.3 GPa with temperatures of 440–460°C, under fluid‐deficient conditions, as well as the final late Carboniferous contact metamorphism up to Tmax of 680–720°C. The prograde path shows clear evidence for thermal buffering during garnet growth at the expense of chlorite, with a heating‐dominated stage after chlorite breakdown. Subsequently, a rheological change associated with epidote breakdown (i.e. reaction softening) occurred, highlighted by a net steepening of the P/T trajectory towards the pressure peak. On the basis of the barroisite inclusions within garnet porphyroblasts as well as the ‘hairpin’ shape of the reconstructed P–T–d path (before contact metamorphism), we infer that the unusual low T/P gradient for the Hercynian crust exposed in the Mammola Paragneiss Unit records its involvement in the Palaeotethys–Gondwana subduction beneath Laurussia during Dn‐1 under‐thrusting. We present a new palaeotectonic interpretation along the southern Hercynian belt in Calabria during the Upper Mississippian–Lower Pennsylvanian, that is consistent with previous geochronology studies.

Magmatic and hydrothermal behavior of uranium in syntectonic leucogranites: The uranium mineralization associated with the Hercynian Guérande granite (Armorican Massif, France)

Most of the hydrothermal uranium (U) deposits from the European Hercynian belt (EHB) are spatially associated with Carboniferous peraluminous leucogranites. In the southern part of the Armorican Massif (French part of the EHB), the Guérande peraluminous leucogranite was emplaced in an extensional deformation zone at ca. 310Ma and is spatially associated with several U deposits and occurrences. The apical zone of the intrusion is structurally located below the Pen Ar Ran U deposit, a perigranitic vein-type deposit where mineralization occurs at the contact between black shales and Ordovician acid metavolcanics. In the Métairie-Neuve intragranitic deposit, uranium oxide-quartz veins crosscut the granite and a metasedimentary enclave.Airborne radiometric data and published trace element analyses on the Guérande leucogranite suggest significant uranium leaching at the apical zone of the intrusion. The primary U enrichment in the apical zone of the granite likely occurred during both fractional crystallization and the interaction with magmatic fluids. The low Th/U values (<2) measured on the Guérande leucogranite likely favored the crystallization of magmatic uranium oxides. The oxygen isotope compositions of the Guérande leucogranite (δ18Owhole rock=9.7–11.6‰ for deformed samples and δ18Owhole rock=12.2–13.6‰ for other samples) indicate that the deformed facies of the apical zone underwent sub-solidus alteration at depth with oxidizing meteoric fluids. Fluid inclusion analyses on a quartz comb from a uranium oxide-quartz vein of the Pen Ar Ran deposit show evidence of low-salinity fluids (1–6wt.% NaCl eq.), in good agreement with the contribution of meteoric fluids. Fluid trapping temperatures in the range of 250–350°C suggest an elevated geothermal gradient, probably related to regional extension and the occurrence of magmatic activity in the environment close to the deposit at the time of its formation. U-Pb dating on uranium oxides from the Pen Ar Ran and Métairie-Neuve deposits reveals three different mineralizing events. The first event at 296.6±2.6Ma (Pen Ar Ran) is sub-synchronous with hydrothermal circulations and the emplacement of late leucogranitic dykes in the Guérande leucogranite. The two last mineralizing events occur at 286.6±1.0Ma (Métairie-Neuve) and 274.6±0.9Ma (Pen Ar Ran), respectively. Backscattered uranium oxide imaging combined with major elements and REE geochemistry suggest similar conditions of mineralization during the two Pen Ar Ran mineralizing events at ca. 300Ma and ca. 275Ma, arguing for different hydrothermal circulation phases in the granite and deposits. Apatite fission track dating reveals that the Guérande granite was still at depth and above 120°C when these mineralizing events occurred, in agreement with the results obtained on fluid inclusions at Pen Ar Ran.Based on this comprehensive data set, we propose that the Guérande leucogranite is the main source for uranium in the Pen Ar Ran and Métairie-Neuve deposits. Sub-solidus alteration via surface-derived low-salinity oxidizing fluids likely promoted uranium leaching from magmatic uranium oxides within the leucogranite. The leached out uranium may then have been precipitated in the reducing environment represented by the surrounding black shales or graphitic quartzites. As similar mineralizing events occurred subsequently until ca. 275Ma, meteoric oxidizing fluids likely percolated during the time when the Guérande leucogranite was still at depth. The age of the U mineralizing events in the Guérande region (300–275Ma) is consistent with that obtained on other U deposits in the EHB and could suggest a similar mineralization condition, with long-term upper to middle crustal infiltration of meteoric fluids likely to have mobilized U from fertile peraluminous leucogranites during the Late Carboniferous to Permian crustal extension events.

Extensional tectonics in the Hercynian Armorican belt (France). An overview

Abstract A synthesis of existing geological, structural and geophysical data shows that the south Armorican Hercynian belt was marked by syn-convergence crustal thinning and dextral wrenching that were in part coeval in late Carboniferous times. Our kinematic model is further supported by new structural data and 40Ar/39Ar ages on synkinematic leucogranites. Extension and strike-slip followed earlier crustal thickening and exhumation of high-pressure metamorphic units in late Devonian-early Carboniferous times. Crustal extension led to the development of core complexes cored by migmatites and crust-derived granite laccoliths. At this time, the South Armorican shear zone acted as a transfer zone separating the extending domain of South Brittany from the non-extending domain of Central Brittany submitted to dextral wrenching. The overall structural pattern and attached kinematics are compared with recent numerical models and illustrated by a 3D interpretative model that integrates geological and deep seismic reflection data (ARMOR 2 profile).

Tectonic record, magmatic history and hydrothermal alteration in the Hercynian Guérande leucogranite, Armorican Massif, France

The Guérande peraluminous leucogranite was emplaced at the end of the Carboniferous in the southern part of the Armorican Massif. At the scale of the intrusion, this granite displays structural heterogeneities with a weak deformation in the southwestern part, whereas the northwestern part is marked by the occurrence of S/C and mylonitic extensional fabrics. Quartz veins and pegmatite dykes orientations as well as lineations directions in the granite and its country rocks demonstrate both E–W and N–S stretching. Therefore, during its emplacement in an extensional tectonic regime, the syntectonic Guérande granite has probably experienced some partitioning of the deformation. The southwestern part is characterized by a muscovite–biotite assemblage, the presence of restites and migmatitic enclaves, and a low abundance of quartz veins compared to pegmatite dykes. In contrast, the northwestern part is characterized by a muscovite–tourmaline assemblage, evidence of albitization and gresenization and a larger amount of quartz veins. The southwestern part is thus interpreted as the feeding zone of the intrusion whereas the northwestern part corresponds to its apical zone. The granite samples display continuous compositional evolutions in the range of 69.8–75.3wt.% SiO2. High initial 87Sr/86Sr ratios and low εNd(T) values suggest that the peraluminous Guérande granite (A/CNK>1.1) was formed by partial melting of metasedimentary formations. Magmatic evolution was controlled primarily by fractional crystallization of K-feldspar, biotite and plagioclase (An20). The samples from the apical zone show evidence of secondary muscovitization. They are also characterized by a high content in incompatible elements such as Cs and Sn, as well as low Nb/Ta and K/Rb ratios. The apical zone of the Guérande granite underwent a pervasive hydrothermal alteration during or soon after its emplacement. U–Th–Pb dating on zircon and monazite revealed that the Guérande granite was emplaced 309.7±1.3Ma ago and that a late magmatic activity synchronous with hydrothermal circulation occurred at ca. 303Ma. These new structural, petrological and geochronological data presented for the Guérande leucogranite highlight the interplay between the emplacement in an extensional tectonic regime, magmatic differentiation and hydrothermal alteration, and provide a general background for the understanding of the processes controlling some mineralization in the western European Hercynian belt.

Late-Hercynian intrusion-related gold deposits: An integrated model on the Tighza polymetallic district, central Morocco

Gold have been recently recognized in the Tighza (formerly Jebel Aouam) district, in the Hercynian belt of central Morocco. This district has long been known for its W mineralization, as well as major Pb–Ag–Zn, and minor Sb–Ba deposits, all geographically associated with late-Hercynian calc-alkaline magmatism. Gold mineralization in the district is mainly hosted by thick W–Au quartz veins located around the “Mine granite” small granitic plug. Within the veins, gold grade is highest (up to 70g/t) close to the granite but rapidly decreases going outward from the granite, defining a perigranitic zoning. Anomalous gold grades have also been measured in hydrothermal skarn layers close to two other granitic plugs (Kaolin granite and Mispickel granite), associated with disseminated As–Fe sulfides. The paragenetic sequence for the W–Au quartz veins shows three stages: (1) an early oxidized stage with wolframite-scheelite associated with early quartz (Q1), (2) an intermediate Bi–As–Te–Mo–Au sulfide stage with loellingite, bismuth minerals and native gold with a later quartz (Q2), restricted to a narrow distance from the granite, and (3) a late lower temperature As–Cu–Zn–(Pb) stage with abundant massive pyrrhotite, arsenopyrite and sphalerite, locally forming independent veins (“pyrrhotite vein”). Both Q1 hyaline and Q2 saccharoidal gold-bearing quartz display aqua-carbonic fluids with minor H2S and Cu and an homogeneous composition (81mole% H2O, 18mole% CO2 and about 1mole% NaCl). The trapping pressure is estimated to 1.5–2kbar with temperature ranging from 300 to 350°C. Q1 inclusions have exploded indicating an uplift of the Tighza block, that lead to saccharoidal Q2 quartz deposition with multiphase NaCl-saturated fluid inclusions. 40Ar/39Ar dating demonstrates that the “Mine granite”, tungsten skarnoid, scheelite-molybdenite veins, and very likely gold-bearing veins are coeval, emplaced at 286±1Ma. Multiple and widespread metal sources are indicated by radiogenic isotope studies. Nd and Sr isotope compositions of scheelite and granites suggest the participation of a juvenile component while lead isotopes demonstrate a major participation of the basement.Both gold mineralization and zoning suggest that the system developed at the end of the magmatic activity, accompanying a major transition in magmatic fluid composition. The morphology of the gold-bearing mineralization is dependent of the permeability and the reactivity of host-rocks: focus circulation of fluids through pre-existing tectonic corridors, reactivated by late-Hercynian intrusions favor the formation of large W-type gold veins, while infiltration of fluid within reactive stratigraphic layers gives rise to skarn mineralization. A 40Ar/39Ar date (W1 north vein: 291.8±0.3Ma) indicates that hydrothermal circulation predates gold and tungsten deposition in open fractures as well as Mine granite emplacement.The W–Au mineralization preceded the onset of a large convective hydrothermal cell around the intrusion that led to the formation of the Pb–Ag–Zn mined veins. The Tighza polymetallic district displays numerous similarities with the R-IRG model that was defined in the American Cordillera, such as thermal and zonation patterns, carbonic hydrothermal fluids and chronology of intrusion and related deposits, but also provides new insight to the R-IRG model such as wide Au-quartz veins instead of sheeted Au-veins, oxidation state of the magma, and Sr–Nd isotopic data. These results establish a major magmatic contribution and discard a direct genetic relationship between gold mineralization and major neighboring Pb–Ag–Zn veins. A large number of classic Pb–Zn district of the Western Hercynides belong to the same clan.

Geochronological study of zircons from continental crust rocks in the Frido Unit (southern Apennines)

Zircon crystals have been separated from gneisses and metagranitoids of the Pollino area (southern Apennines) in order to unravel the origin of these crustal slices within the ophiolite-bearing Frido Unit. The morphology of the zircon has been investigated by SEM, and the internal structure was revealed by cathodoluminescence. Data obtained by U/Pb dating have been used to deduce the age and significance of the different crystallization stages of zircon, connected to the evolutionary stages of the continental crust (Late Paleozoic–Early Mesozoic). Zircons in gneisses are characterized by inherited cores of magmatic origin, bordered by metamorphic rims. Inherited zircons generally show Paleoproterozoic to Ordovician ages, indicating the provenance of the sedimentary protolith from different sources. The exclusive presence of Late Neoproterozoic zircon cores in leucocratic gneisses may suggest a different magmatic source possibly connected to Pan-African events. Late Carboniferous–Early Permian ages are found mainly in zircon rims of metamorphic origin. These are similar to the emplacement ages of protolith of the metagranites in the middle crust portion. Late Carboniferous–Early Permian metamorphism and magmatism testify the extensional collapse of the Hercynian belt, recorded in European, particularly, in the Corsica–Sardinia block and in Calabria. Late Permian–Triassic ages have been detected in zircon rims from gneisses and metagranitoids. These younger ages appear related to deformation and emplacement of albite–quartz veins in both lithologies, and are related to an extensional episode predating the Middle Triassic to Middle Jurassic rifting in the Tethyan domain, followed by Middle to Late Jurassic spreading.

Geodynamic settings and formation conditions of crystalline complexes in the south Altai and south Gobi metamorphic belts

The Hercynian mobile belts in Central Asia comprise the Hercynian proper and the Late Hercynian (Indosinian) belts separated by the South Gobi microcontinent, the origin of which is related to the evolution of the South Mongolian and Inner Mongolian basins with the oceanic crust. Crystalline complexes within these belts occur as tectonic sheets of a variety of sizes. At the early stages, the metamorphic grade of these complexes reached conditions of high-temperature subfacies of amphibolite and locally developed granulite facies. In tectonic terms, the Hercynian belt of metamorphic rocks is situated at the margin of the North Asian Caledonian continent and extends from the southeast to the northwest along the southern slope of the Gobi, Mongolian, and Chinese Altai to East Kazakhstan, where metamorphic rocks are localized in the Irtysh Shear Zone. All these rocks are combined into the South Altai metamorphic belt of more than 1500 km in extent. Another belt of isolated outcrops of crystalline rocks conventionally combined into the Indosinian South Gobi metamorphic belt is traced along the junction of the Hercynides with the South Gobi microcontinent. The high-grade metamorphic rocks within both belts are not fragments of an ensialic Caledonian or older basement. These rocks were formed 390–360 and 230–220 Ma ago as a result of the closure of the Tethian South Mongolian and Inner Mongolian oceanic basins (Paleotethys I and Paleotethys II). The spatial position of the South Altai and South Gobi metamorphic belts is caused by the asymmetric structure of the Tethian basins, where active continental margins are expressed most distinctly along their northern parts, while passive margins extend along the southern parts (in present-day coordinates).

Dike magmatism in the Sila Grande (Calabria, southern Italy): Evidence of Pennsylvanian–Early Permian exhumation

A dike network transecting a basement of intrusive and metamorphic rocks related to the Hercynian orogeny is exposed in the Sila Grande (southern Italy). Dike magmatism, similarly to other regions of the western Mediterranean, such as Sardinia, Corsica, and Catalonia, is of calc-alkaline to alkali-calcic affinity. Zircon U-Pb geochronology indicates that dike magmatism took place between 295 ± 1 to 277 ± 1 Ma, after the main late Hercynian emplacement of granitoids (306 ± 1 Ma). Barometry indicates that the basement underwent exhumation of 8 ± 3 km before dike injection. The dike network has a geometrical arrangement consistent with a transtensional stress regime that resulted in ductile thinning of the lower crust during the late stage of the Hercynian orogeny and concurrent fracturing of the upper crust that made possible magma ascent through dikes. The proposed tectonic evolution is related to dismemberment of the southern Hercynian belt in the central Mediterranean area as a result of dextral transtension of Gondwana in relation to Laurasia during the Pennsylvanian–Early Permian.

Deep reflection seismic imaging of the internal zone of the South Armorican Hercynian belt (western France) (ARMOR 2/Géofrance 3D Program)

We present results and interpretation of a 72 km long deep seismic reflection profile acquired across the internal zone of the Hercynian belt of South Brittany. The profile is of excellent quality, most of the crust being highly reflective. The “ARMOR 2 South” profile, is correlated with the “ARMOR 2 North” profile that was published in 2003. Correlation of the main subsurface reflections with surface geological and structural data provides important information about the crustal structure that resulted from thickening during Late Devonian and regional-scale extension during Late Carboniferous. In particular, seismics image shows a very high reflectivity zone, lying flat over more than 40 km at about 10–12 km depth. This zone is interpreted as a major zone of ductile crustal thinning.

Comparison of seasonal variability in European domestic radon measurements

Abstract. Analysis of published data characterising seasonal variability of domestic radon concentrations in Europe and elsewhere shows significant variability between different countries and between regions where regional data is available. Comparison is facilitated by application of the Gini Coefficient methodology to reported seasonal variation data. Overall, radon-rich sedimentary strata, particularly high-porosity limestones, exhibit high seasonal variation, while radon-rich igneous lithologies demonstrate relatively constant, but somewhat higher, radon concentrations. High-variability regions include the Pennines and South Downs in England, Languedoc and Brittany in France, and especially Switzerland. Low-variability high-radon regions include the granite-rich Cornwall/Devon peninsula in England, and Auvergne and Ardennes in France, all components of the Devonian-Carboniferous Hercynian belt.

Age constraints on Late Paleozoic evolution of continental crust from electron microprobe dating of monazite in the Peloritani Mountains (southern Italy): another example of resetting of monazite ages in high-grade rocks

In situ monazite microprobe dating has been performed, for the first time, on trondhjemite and amphibolite facies metasediments from the Peloritani Mountains in order to obtain information about the age of metamorphism and intrusive magmatism within this still poorly known sector of the Hercynian Belt. All samples show single-stage monazite growth of Hercynian age. One migmatite and one biotitic paragneiss yielded monazite ages of 311 ± 4 and 298 ± 6 Ma, respectively. These ages fit with previous age determinations in similar rocks from southern Calabria, indicating a thermal metamorphic peak at about 300 Ma, at the same time as widespread granitoid magmatism. The older of the two ages might represent a slightly earlier event, possibly associated with the emplacement of an adjacent trondhjemite pluton, previously dated by SHRIMP at 314 Ma. No evidence for pre-Hercynian events and only a little indication for some monazite crystallization starting from ca. 360 Ma were obtained from monazite dating of the metasediments, suggesting either a single-stage metamorphic evolution or a significant resetting of the monazite isotope system during the main Hercynian event (ca. 300 Ma). Rare monazite from a trondhjemite sample yields evidence for a late-Hercynian age of about 275 Ma. This age is interpreted as representing a post-magmatic stage of metasomatic monazite crystallization, which significantly postdates the emplacement of the original magmatic body.

Kinematic records of subduction and exhumation in the Ile de Groix blueschists (Hercynian belt; Western France)

Deciphering between deformations related to subduction and exhumation in HP metamorphic rocks represents a challenge for the understanding of the dynamic processes involved and more particularly, the exhumation mechanisms. An analysis of shear criteria in the Ile de Groix blueschists in the Hercynian Belt of southern Brittany (Western France) is carried out in terms of relative chronology and in relation to lithology (mainly micaschists and glaucophane-bearing metabasites). Structural mapping shows that foliations are flat-lying, bearing stretching lineations oriented in a N150°E ± 30° direction. Associated shear criteria are dominantly top-to-northwest and locally, in the southeast part of the island, both top-to-northwest and -southeast. Both shearing deformations lead to intense finite strains as demonstrated by the occurrence of sheath folds and the general orientation of fold axes parallel to the stretching lineation. Detailed mapping in this area showing opposite senses of shear, demonstrates that only top-to-southeast shear is observed in rocks containing well-shaped lawsonite pseudomorphs. In rocks affected by top-to-northwest shear, the lawsonite pseudomorph shape is destroyed by intense stretching. The top-to-southeast shear is coeval with prograde HP metamorphism and precedes a top-to-northwest shear coeval with a retrograde metamorphic path. In the frame of the hercynian belt of southern Brittany, the southeast directed shear is attributed to thrusting related to a northward dipping subduction and the northwest directed shear to exhumation controlled by a northward dipping extensional detachment.

Phylogenetic Analysis Informed by Geological History Supports Multiple, Sequential Invasions of the Mediterranean Basin by the Angiosperm Family Araceae

Despite the remarkable species richness of the Mediterranean flora and its well-known geological history, few studies have investigated its temporal and spatial origins. Most importantly, the relative contribution of geological processes and long-distance dispersal to the composition of contemporary Mediterranean biotas remains largely unknown. We used phylogenetic analyses of sequences from six chloroplast DNA markers, Bayesian dating methods, and ancestral area reconstructions, in combination with paleogeographic, paleoclimatic, and ecological evidence, to elucidate the time frame and biogeographic events associated with the diversification of Araceae in the Mediterranean Basin. We focused on the origin of four species, Ambrosina bassii, Biarum dispar, Helicodiceros muscivorus, Arum pictum, subendemic or endemic to Corsica, Sardinia, and the Balearic Archipelago. The results support two main invasions of the Mediterranean Basin by the Araceae, one from an area connecting North America and Eurasia in the Late Cretaceous and one from the Anatolian microplate in western Asia during the Late Eocene, thus confirming the proposed heterogeneous origins of the Mediterranean flora. The subendemic Ambrosina bassii and Biarum dispar likely diverged sympatrically from their widespread Mediterranean sister clades in the Early-Middle Eocene and Early-Middle Miocene, respectively. Combined evidence corroborates a relictual origin for the endemic Helicodiceros muscivorus and Arum pictum, the former apparently representing the first documented case of vicariance driven by the initial splitting of the Hercynian belt in the Early Oligocene. A recurrent theme emerging from our analyses is that land connections and interruptions, caused by repeated cycles of marine transgressions-regressions between the Tethys and Paratethys, favored geodispersalist expansion of biotic ranges from western Asia into the western Mediterranean Basin and subsequent allopatric speciation at different points in time from the Late Eocene to the Late Oligocene.

Fluid history during deep burial and exhumation of oil-bearing volcanics, Hercynian Belt of southern Brittany, France

In southern Brittany (France), the Porphyroid nappe, a Variscan tectonic unit made of volcanic pyroclastic deposits that was involved in a low-temperature/high-pressure prograde metamorphic path followed by exhumation and extension, displays evidence for very large and distributed strains achieved through dissolution/crystallization processes. Such distributed strains at low temperature are rather unusual in felsic lithologies. Recent studies have documented that these rocks deformed within a system closed to fluids. This paper presents results of detailed analyses of fluid inclusions and stable isotopes within the deformed metavolcanics and their undeformed counterparts in order to document the fluid history of the tectonic unit. Results show that its initial thermal evolution, likely occurring in a context of basin inversion, involved important open-system alteration, with K-metasomatism (adularization), oil impregnation and hydration. During the subsequent tectonic evolution, the fluids maintained a constant composition in the C-H-O-N system within the Porphyroid nappe and its para-autochthon, being water-rich and containing hydrocarbon components likely related to oil pyrolysis/oxidation. Atoll-like fluid inclusion morphology and rare undisturbed fluid inclusions with steep isochore slopes are consistent (\> 750 MPa) with pressures estimated for the peak of metamorphism using mineral solid-solution thermodynamics. Fluid inclusions indicate a sub-isothermal retrograde path (between 420 and 300°C) followed by a high thermal gradient at the end of the retrograde path. Within the underlying para-autochthonous unit, only the late, high-temperature gradient is recorded by fluid inclusions. The sub-isothermal low-grade path suggests fast exhumation of the unit, without thermal relaxation. The high geotherm observed at the end of the P-T path is consistent with the post-thickening extensional collapse and associated intrusion of granites documented in the area. Hydration and heterogeneous oil/water distribution achieved in the early metasomatic evolution of the porous pyroclastic deposits are responsible for subsequent fluid confining in the Porphyroid nappe and for its strongly distributed strain pattern.

Metamorphic evolution of the Briançonnais units along the ECORS-CROP profile (Western Alps): new data on metasedimentary rocks

Brianconnais units are squeezed between two Mesozoic eclogitic belts (Piemont-Ligurian ocean and Valaisan ocean) along the ECORS-CROP seismic line in the Italian-French Western Alps (France, Italy). The metamorphic evolution of this area plays a key role for understanding the evolution of the Western Alps and is discussed on the basis of detailed petrographic investigations carried out on weathered sediments issued from the erosion of the Hercynian belt, especially on lower Permian to Mesozoic sediments. In the Zone Houillere, as well in the Permo-Triassic cover of the Brianconnais basement, the index metamorphic mineral assemblage is mainly composed of white micas with varying chemical composition, chloritoid and garnet. This same assemblage occurs within different lithologies (metaarkose, metapelite, metasandstone). Consequently, equilibrium phase diagrams were computed for different whole rock compositions using DOMINO software. The results of the P-T investigations clearly show that each unit underwent a different sequence of metamorphic reactions. An increase in metamorphic grade from greenschist facies conditions in the Northwest (Zone Houillere) to the transition between blueschist and eclogite facies conditions in the Southeast (Internal Brianconnais) is observed. A major discontinuity in metamorphic grade is located at the contact between Zone Houillere and Ruitor unit, as documented by a pressure gap of ~ 7 kbar. In general, the observed metamorphic field gradient is inverted and is interpreted to represent different depths of burial during subduction, which correlates with the paleogeographic position of the different units.

Composition and conditions of formation of andalusite-kyanite-sillimanite pegmatoid segregations in metamorphic rocks of the Tsel block (Mongolian Altay)

Pegmatoid segregations containing three polymorphous Al 2SiO 5 modifications have been revealed in metamorphic rocks of the Tsel block localized in the Hercynian belt on the southern flank of Mongolian Altay. Petrographic study showed a successive substitution of polymorphs in the sequence: andalusite–kyanite–fibrolite–sillimanite. Estimated parameters of the host-rock metamorphism indicate that the mineral assemblage of pegmatoid veins formed at two successive stages of metamorphism: andalusite-sillimanite and kyanite-sillimanite. It is suggested that the transformation of Al 2SiO 5 from one to another polymorphous modification occurs by the ion exchange mechanism with the participation of muscovite.

Draa Sfar, Morocco: A Visean (331 Ma) pyrrhotite-rich, polymetallic volcanogenic massive sulphide deposit in a Hercynian sediment-dominant terrane

Draa Sfar is a Visean, stratabound, volcanogenic massive sulphide ore deposit hosted by a Hercynian carbonaceous, black shale-rich succession of the Jebilet terrane, Morocco. The ore deposit contains 10 Mt grading 5.3 wt.% Zn, 2 wt.% Pb, and 0.3 wt.% Cu within two main massive sulphides orebodies, Tazakourt (Zn-rich) and Sidi M'Barek (Zn–Cu rich). Pyrrhotite is by far the dominant sulphide (70 to 95% of total sulphides), sphalerite is fairly abundant, chalcopyrite and galena are accessory, pyrite, arsenopyrite and bismuth minerals are rare. Pyrrhotite is monoclinic and mineralogical criteria indicate that it is of primary origin and not formed during metamorphism. Its composition is very homogeneous, close to Fe 7S 8, and its absolute magnetic susceptibility is 2.10 − 3 SI/g. Ar–Ar dating of hydrothermal sericites from a coherent rhyolite flow or dome within the immediate deposit footwall indicates an age of 331.7 ± 7.9 Ma for the Draa Sfar deposit and rhyolite volcanism. The Draa Sfar deposit has undergone a low-grade regional metamorphic event that caused pervasive recrystallization, followed by a ductile–brittle deformation event that has locally imparted a mylonitic texture to the sulphides and, in part, is responsible for the elongated and sheet-like morphology of the sulphide orebodies. Lead isotope data fall into two compositional end-members. The least radiogenic end-member, ( 206Pb/ 204Pb = 18.28), is characteristic of the Tazakourt orebody, whereas the more radiogenic end-member ( 206Pb/ 204Pb ∼ 18.80) is associated with the Sidi M'Barek orebody, giving a mixing trend between the two end-members. Lead isotope compositions at Draa Sfar testify to a significant continental crust source for the base metals, but are different than those of the Hajar and South Iberian Pyrite Belt VMS deposits. The abundance of pyrrhotite versus pyrite in the orebodies is attributed to low fO 2 conditions and neither a high temperature nor a low aH 2S (below 10 − 3 ) is required. The highly anoxic conditions required to stabilize pyrrhotite over pyrite are consistent with formation of the deposit within a restricted, sediment-starved, anoxic basin characterized by the deposition of carbonaceous, pelagic sediments along the flank of a rhyolitic flow-dome complex that was buried by pelitic sediments. Deposition of sulphides likely occurred at and below the seafloor within anoxic and carbonaceous muds. Draa Sfar and other Moroccan volcanogenic massive sulphide deposits occur in an epicontinental volcanic domain within the outer zone of the Hercynian belt and formed within a sedimentary environment that has a high pelagic component. In spite of the diachronous emplacement between the IPB deposits (late Devonian to Visean) and Moroccan deposits (Dinantian), all were formed around 340 ± 10 Ma following a major phase of the Devonian compression.