Variscan Times Research Articles

Tracing the cryptic Sardic (Ordovician) metamorphism across Alpine Europe: the Krndija region in the Slavonian Mountains, Croatia

Results of a combined petrological, geochemical and geochronological study suggest that metasedimentary rock units in the Krndija region of the Slavonian Mountains, Croatia, were affected by at least three major tectonometamorphic imprints: during the Middle Ordovician (Sardic event), the early Carboniferous (Variscan event), and the Cretaceous (Alpine event). All three metamorphic phases are established by electron microprobe-based in-situ U–Th–Pb dating of monazite grains. The Sardic metamorphic event is additionally confirmed by a precise Lu–Hf garnet-whole-rock isochron age of 466.0 ± 2.3 Ma. Taken together, the data unveil a relatively large and well-preserved piece of the cryptic Sardic orogen in central Krndija, that we name the Kutjevo Zone. A Sardic subduction-related metamorphic event (ca. 540-580 ℃, 8–11 kbar) at ca. 466 Ma is manifested in the mineral paragenesis Ca-rich garnet plus rutile. A low degree of retrograde reequilibration suggests a subsequent fast exhumation. Low-Ca cores in some garnets and staurolite relics record a pre-HP metamorphic event that involves isobaric heating from 570 to 610 ℃ at ~ 7 kbar. We attribute this (so far undated) event to mid-crustal contact metamorphism caused by early Sardic magmatism. Southern parts of Krndija (the Gradište Zone) experienced an (additional?) clockwise PT evolution in Variscan times at ca. 350 Ma. Garnet formed with ilmenite during a PT increase from 580 ℃/5 kbar to 600 ℃/6 kbar and underwent later strong retrograde resorption. Slow Variscan exhumation resulted in andalusite formation at < 550 ℃/ < 3.8 kbar. Penetrative Alpine metamorphism was observed in low-grade phyllites in the north. The lithology and metamorphic history of the Kutjevo Zone is similar to what has been reported from the Sardic Strona-Ceneri Zone in the western Alps. Both areas expose metapelitic (metagreywacke) rocks with a pre-middle Ordovician formation age. These metasedimentary rocks are inter-layered with numerous small amphibolitic units as well as metagranitoids and were likely deposited along the active Gondwana margin, perhaps in a fore-arc position, prior to their subduction during the middle Ordovician. According to recent palaeogeographic reconstructions, both the Kutjevo Zone and the Strona-Ceneri Zone have once resided in an eastern sector of the northern Gondwana margin (i.e., in E-Armorica). We conclude that in the Middle Ordovician, important subduction activities took place in this E-Armorican segment of north Gondwana, which is today exposed in the Alps. The W-Armorican segment of north Gondwana (now exposed in the French, German, and Czech Variscides) had probably already mutated from a (Cadomian) subduction setting to an extensional (transtensional–transpressional) setting by the late Cambrian.

The Ordovician of Sardinia (Italy): from the ‘Sardic Phase’ to the end-Ordovician glaciation, palaeogeography and geodynamic context

Abstract This review illustrates the most important features of the Ordovician successions of the Sardinian basement. We focus on the stratigraphy and tectonic structures in the tectonic units of the External and Nappe zones of the Variscan basement. The Ordovician successions are characterized by unconformities related to tectonic events ascribed to the Sardic and Sarrabese phases. The different durations of the unconformity-related gaps in the External (17 myr) and Nappe (6 myr) zones, recent work on the trilobite fossil content, and the occurrence of a volcanic arc only in the Nappe Zone (Sarrabus and Gerrei units) highlight significant discrepancies suggesting that these domains did not share the same geodynamic setting and palaeogeographical position during the Ordovician. This implies they were amalgamated only in Variscan times. Whereas for the external and nappe zones the Ordovician features are clear, the high-grade metamorphic Inner Zone, where numerous Ordovician ortho- and para-gneiss occur, more detailed studies are needed to define a complete framework for the Ordovician evolution of Sardinia. The present revision of data for the best-preserved succession of Sardinian tectonic units suggests that at least two distinct terranes, which did not share the same Ordovician evolution, were only amalgamated during the Variscan Orogeny.

Transpressive Deformation in the Southern European Variscan Belt: New Insights From the Aiguilles Rouges Massif (Western Alps)

AbstractDespite the Mediterranean sector of the Variscan Belt being fragmented and reworked during Alpine orogenesis, evidence for the activity of a right‐lateral strike‐slip shear zone has been reported in Paleozoic fragments of the belt such as the Sardinian Variscan Basement, the Maures Massif of southern France, and in the Western Alps. To improve this correlation with new structural data, we performed a structural and microstructural analysis incorporating study of the kinematics of flow and petrochronology of a high‐strain zone in the Aiguilles Rouges Massif (External Crystalline Massifs, Western Alps). The results higlight a dextral pure‐shear dominated transpression initiated under amphibolite‐facies metamorphic conditions (~630°C, 0.4 GPa) during Variscan time. The structural evolution of the high‐strain zone is similar to the Ferriere‐Mollières shear zone in the Argentera Massif; both are transpressive shear zones active at the same time (~320 Ma) under similar metamorphic conditions. These two high‐strain zones represent well‐preserved segments of a system of ductile shear zones in the External Crystalline Massifs. The data presented in this study provide improved constraints on the extent, kinematics, and timing of the East Variscan Shear Zone in the Variscan basement of the Western Alps, with implications for refining the correlation between structures in fragments of the southern Variscan Belt. The data also better constrain a segment of a major pre‐Alpine shear zone which may have played an important role during post‐Variscan tectonics as an inherited discontinuity.

Tholeiitic- and boninite-series metabasites of the Nov\xe9 M\u011bsto Unit and northern part of the Z\xe1b\u0159eh Unit (Orlica\u2013\u015anie\u017cnik Dome, Bohemian Massif): petrogenesis and tectonic significance

The Nové Město Unit and the northern part of the Zábřeh Unit comprise back-arc basin mafic rocks metamorphosed during Variscan times. In both units, nearly identical metabasites comprise variously enriched tholeiites (from N-MORB to transitional E-MORB-OIB), high-Ca, low-silica boninites and low-Ti tholeiites. The tholeiites (Ti/V: 22–58, εNd540: + 7.6 to − 4.7) represent 10–15% melting at ca. 30–60 km and temperatures of 1380–1230 °C of a depleted MORB mantle-type (DMM) wedge heterogeneously modified shortly before fusion by OIB-like melts (enriched mantle possibly of EM1–EM2 type) presumably derived from decompression melting of upwelling asthenosphere. Much less common meta-boninites (Ti/V: 6–23, εNd540: + 6.7 to − 2.9) formed by 15–25% re-melting of residual mantle (DMM after ~ 15% melt extraction) at depths of 40–65 km and temperatures of ~ 1420–1300 °C. Scarce low-Ti meta-tholeiites (Ti/V: 18–19, εNd540: + 7.1) resulted from < 10% melting of an unenriched DMM-type source or re-melting of residual mantle (after < 15% of former melting). Trace elements and Nd isotope compositions imply random fluxing of tholeiitic- and boninitic magma sources by components released from a subducted slab. The metasomatic enrichment (Th, LREE–MREE) was induced not only by sediment-derived melts but also by fluids supplied by subducted sediments or juvenile crust. The Nové Město–Zábřeh association of metabasites points to an easterly prolongation (in present-day coordinates) of the Cadomian subduction system of the Teplá–Barrandian. Boninite-type magmatism and OIB-like input into mantle beneath an extensional back-arc basin suggest an upflow of hot asthenosphere through subducted ridge (slab window) followed by a cessation of subduction zone activity.

Origin of Triassic magmatism of the Southern Alps (Italy): Constraints from geochemistry and Sr-Nd-Pb isotopic ratios

During Middle-early Late Triassic (∼243–235?Ma) a diffuse igneous activity developed in the Southern Alps (Italy). Sparse lava flow and pyroclastic succession remnants of such Southern Alps Triassic Igneous Rocks (SATIR) crop out in the Brescian Prealps, the Vicentinian Alps (Recoaro-Schio-Posina), Non Valley, Dolomites and Julian Alps. Plutonic rocks are found in two main plutonic complexes (Monzoni and Predazzo) and a small stock (Cima di Pape). Coeval igneous products can be traced eastward to Austria and Dinarides, for a total length of ∼450?km.The coeval formation of major late Anisian-late Ladinian carbonate platforms in the subsiding eastern Dolomites, and significant uplift and subaerial erosion in the western Dolomites suggest, for these areas, the occurrence of large-scale strike-slip and extensional tectonics with the development of horst-and-graben structures.This study reports the first complete review of the SATIR activity, including new mineral chemical data on 14 samples, 61 major and trace element whole rock analyses and 7 Sr-Nd-Pb isotopic ratios for volcanic and plutonic samples from Dolomites (lavas plus Monzoni and Predazzo plutonic rocks) and Vicentinian Alps lavas. Despite the variable post-magmatic modifications, the large areal distribution of the products and their wide spectrum of chemical compositions, these samples show rather common geochemical and mineralogical characteristics and define major and trace element trends that can be associated with nearly close-system magmatic evolution. Minor upper crustal contaminations can be observed in specific cases, mostly in the most differentiated products (SiO2 >70?wt%).A specific characteristic of SATIR is their calcalkaline to shoshonitic affinity, resembling the derivation from subduction-modified mantle sources, a feature at odds with the coeval strike-slip and extensional tectonics. Geochemical modelling, petrological considerations and geological constraints allow us to propose a model in which the SATIR mantle sources reflect previous subduction metasomatism (likely occurred during Variscan times). These metasomatised mantle sources were reactivated ∼90–100 Myr after the end of the subduction, when continental rifting caused a raise of the geotherms and passive upwelling of asthenospheric mantle.

Stratigraphic, magmatic and structural features of Ordovician tectonics in Sardinia (Italy): a review

The Variscan Palaeozoic successions exposed in Sardinia show clear evidences of tectonic instability during Ordovician times. In the Foreland zone (SW Sardinia), the Cambrian to lower Ordovician strata are folded giving way to an angular unconformity sealed by a Middle-Upper Ordovician syn-tectonic succession, characterized at the base by a thick conglomeratic deposit. This is indicative of a tectonic-driven uplift, followed by intensive erosion. A time-equivalent angular unconformity, sealed by Middle Ordovician volcano-sedimentary complexes, is also recorded in the External nappe zone (central–southern Sardinia). These volcanic products became rare toward NE and approaching the Inner zone, where few remnants of Ordovician magmatic rocks consist of the protoliths of small bodies of calc-alkaline orthogneisses and metabasites. Paleogeographic reconstructions generally assume that Sardinia formed a coherent crustal block since the late Cambrian; therefore, the SW–NE trend of magmatic and sedimentary features observed in the Ordovician successions has been interpreted as sequential snapshots captured along an Ordovician volcanic arc. Stratigraphic and paleontological data, sediment provenance and petro-chronology of magmatic products indicate that the Ordovician successions could have belonged to different, and possibly far paleogeographic domains. These data suggest that the actual zonation of the Palaeozoic basement of Sardinia is a puzzle resulting from extensive crustal reworking and amalgamation in Variscan times. Hence it cannot be directly referred to an Ordovician paleogeography.

Contrasting metamorphic evolution of metapelites from the Malpica-Tuy unit and the underlying so-called parautochthon at the coast of NW Spain

Two garnet-chloritoid bearing metapelites from the northern Malpica-Tuy unit (MTU) and a staurolite-andalusite bearing metapelite from the underlying parautochthon were studied to decipher their pressure–temperature (P–T) evolution, mainly based on chemically zoned minerals, such as phengite, using P–T, P–O2, and T–O2 pseudosections. In addition, monazite ages were determined with the electron microprobe in the metapelite from the parautochthon. The chloritoid schists from the MTU experienced peak P–T conditions around 21kbar at 570°C and a subsequent isothermal exhumation. Thus, two high pressure rock types can be distinguished in the MTU: orthogneiss (peak pressures: 13–16kbar) and eclogite+metapelite (peak pressures: 20–23kbar). The latter rock types were exhumed in a subduction channel environment. The staurolite-andalusite bearing schist from the parautochthon experienced only peak pressures around 8kbar at a temperature around 570°C since garnet and potassic white-mica with Si contents above 3.2 per formula unit are lacking. This rock represents sediments that were originally at the surface of the downgoing continental plate and metamorphosed in early Variscan times (around 364Ma) similar to the MTU rocks. Our mean monazite ages of 339 and 320Ma might be related to backthrusting of the MTU rocks from deeper crustal levels onto the parautochthon.

Metasomatic Processes Revealed by Trace Element and Redox Signatures of the Lithospheric Mantle Beneath the Massif Central, France

Trace element compositions of clinopyroxene and oxidation state measurements are both useful parameters for constraining metasomatic interactions within the subcontinental lithospheric mantle (SCLM). Laser ablation inductively coupled plasma mass spectrometry analyses of trace elements in clinopyroxene in combination with oxidation state data from the same samples provide a fuller picture of the metasomatic processes that occurred within the SCLM below the French Massif Central (FMC). A total of 89 spinel peridotite xenoliths from 39 localities across the FMC document variable degrees of partial melting and metasomatism at both regional and local scales. According to the degree of enrichment in light rare earth elements (LREE) and middle rare earth elements (MREE) in clinopyroxene, four main groups (A, B, C and D) are identified. Variations in other trace element concentrations are consistent with these groups. LREE enrichment correlates with a tendency to higher oxygen fugacity (fO2). These characteristics, along with their Ti/Eu–Zr/Hf systematics, are useful in assessing the different metasomatic agents that have interacted with the FMC SCLM. The behaviour of Lu and Hf additionally provides a means for distinguishing secondary clinopyroxene grains from those that have been overprinted. Most samples have been affected by cryptic metasomatism and different signatures indicate interaction with different types of metasomatic agent. Modal metasomatism is apparent by the presence of amphibole or secondary clinopyroxene. The lack of any notable trace element or redox signature related to the occurrence of amphibole suggests that it formed from small amounts of dilute H2O-rich fluids. Most samples from the northern sector of the FMC are fairly refractory and clinopyroxene exhibits a ‘tick-shaped’ REE signature (group D), consistent with metasomatism by subduction-related fluids during Variscan times. Such fluids were oxidizing (Δlog fO2 values > FMQ + 1, where FMQ is the fayalite–magnetite–quartz buffer) and responsible for LREE enrichment in clinopyroxene and low concentrations of the less mobile elements such as Ti, Eu, Zr and Hf. Clinopyroxene in several samples has very low Ti/Eu, elevated (La/Yb)N and suprachondritic Zr/Hf ratios, reflecting local carbonatite metasomatism. These samples also record oxidizing conditions, with Δlog fO2 values > FMQ + 1. Many other samples have clinopyroxenes with higher Ti/Eu and approximately chondritic Zr/Hf ratios, consistent with having undergone silicate melt metasomatism. Their chondrite-normalized REE element patterns vary from LREE-depleted to relatively flat (group A), indicating small but variable metasomatic enrichments in the more mobile elements, which can be attributed to chromatographic effects during reactive flow. The oxidation states of these samples record Δlog fO2 ∼ FMQ + 0·4 ± 0·5. Some samples have clinopyroxene with negative Zr and Hf anomalies and distinctly low Ti/Eu (<2000), but not as low as expected for carbonatite metasomatism. These may have interacted with carbonated-silicate melts, which also imposed somewhat elevated oxidation states (Δlog fO2 values up to ∼FMQ + 0·9). Some samples exhibit evidence for having been overprinted by more than one metasomatic event, in part indicated by two populations of geochemically distinct clinopyroxene. These samples, among others, also demonstrate that immobility of heavy REE (HREE) cannot be assumed when modeling the degree of melt extraction of strongly metasomatized peridotites. Addition of HREE by refertilization lowers the apparent degree of melting compared with calculations based upon Cr/(Cr + Al) in spinel. In other cases, a lowering of HREE concentrations via interaction with a low-HREE metasomatic agent produces higher apparent degrees of melting.

Late Palaeozoic to Triassic formations unconformably deposited over the Ronda peridotites (Betic Cordilleras): Evidence for their Variscan time of crustal emplacement

The age of the emplacement of the Ronda Peridotites has been widely debated during recent decades, and ages ranging from the Palaeozoic to the early Miocene have been proposed, although most of the current interpretations suggest an Oligocene-Miocene age. In this article, we describe two meta-sedimentary formations (the lower one formed by detrital sediments and the upper one by marbles) that were unconformably deposited over the Ronda peridotites and now record low-grade metamorphism. The detrital formation contains layers of acidic rocks with an age of 269±9 Ma and the overlying marbles are assumed to be Triassic. The existence of these unconformable formations over the peridotites is crucial for the dating of the exhumation of the latter. The presence of peridotite clasts in the detrital formation indicates that peridotites were exposed during the Permian and other data suggest that peridotites were exhumed during the late Carboniferous. During the Alpine cycle, the peridotites operated as an element situated at the bottom of the tectonically higher Alpujarride/Sebtide unit (the Jubrique unit) and forming part of it, then being incorporated to the Alpine thrusts of this unit.

Variscan thrusting in I- and S-type granitic rocks of the Tribeč Mountains, Western Carpathians (Slovakia): evidence from mineral compositions and monazite dating

AbstractThe Tribeč granitic core (Tatric Superunit, Western Carpathians, Slovakia) is formed by Devonian/Lower Carboniferous, calc-alkaline I- and S-type granitic rocks and their altered equivalents, which provide a rare opportunity to study the Variscan magmatic, post-magmatic and tectonic evolution. The calculatedP-T-Xpath of I-type granitic rocks, based on Fe-Ti oxides, hornblende, titanite and mica-bearing equilibria, illustrates changes in redox evolution. There is a transition from magmatic stage atTca. 800–850 °C and moderate oxygen fugacity (FMQ buffer) to an oxidation event at 600 °C between HM and NNO up to the oxidation peak at 480 °C and HM buffer, to the final reduction at ca. 470 °C at ΔNN= 3.3. Thus, the post-magmatic Variscan history recorded in I-type tonalites shows at early stage pronounced oxidation and low temperature shift back to reduction. The S-type granites originated at temperature 700–750 °C at lower water activity and temperature. TheP-Tconditions of mineral reactions in altered granitoids at Variscan time (both I and S-types) correspond to greenschist facies involving formation of secondary biotite. The Tribeč granite pluton recently shows horizontal and vertical zoning: from the west side toward the east S-type granodiorites replace I-type tonalites and these medium/coarse-grained granitoids are vertically overlain by their altered equivalents in greenschist facies. Along the Tribeč mountain ridge, younger undeformed leucocratic granite dykes in age 342±4.4 Ma cut these metasomatically altered granitic rocks and thus post-date the alteration process. The overlaying sheet of the altered granites is in a low-angle superposition on undeformed granitoids and forms “a granite duplex” within Alpine Tatric Superunit, which resulted from a syn-collisional Variscan thrusting event and melt formation ~340 Ma. The process of alteration may have been responsible for shifting the oxidation trend to the observed partial reduction.

Granitoid emplacement by multiple sheeting during Variscan dextral transpression: The Saint-Laurent – La Jonquera pluton (Eastern Pyrenees)

The structural study of the Saint-Laurent – La Jonquera pluton (Eastern Pyrenees), a Variscan composite laccolithic intrusion emplaced in metasedimentary and gneissic rocks of the Roc de Frausa dome, by means of the anisotropy of magnetic susceptibility (AMS) technique has allowed the determination of the nature and orientation of its magmatic fabrics. The magmatic foliation has a predominant NE–SW strike and the mean lineation is also NE–SW trending with a shallow plunge. A strain gradient is measured so that the tonalites to granodiorites that form the basal parts of the pluton, and are intruded into amphibolite-facies metamorphic rocks, recorded the highest anisotropies, whereas the monzogranites and leucogranites, emplaced into upper crustal, low-grade metamorphic rocks, are weakly deformed. These results point to the synkinematic sequential emplacement of multiple granitoid sheets, from less to more differentiated magmatic stages, during the Late Carboniferous D2 event characterized by an E–W-trending dextral transpression. The magmatic foliation appears locally disturbed by the effects of two tectonic events. The first of them (D3) produced mylonitization of granitoids along NW–SE retrograding shear zones and open folds in the host Ediacaran metasediments of the Roc de Frausa massif, likely during late Variscan times. Interference between D2 and D3 structures was responsible for the dome geometry of the whole Roc de Frausa massif. The second and last perturbation consisted of local southward tilting of the granitoids coupled to the Mesozoic–Cenozoic cover during the Alpine.

Copper, Zn and Pb soil geochemistry data from the NE domain of the Iberian Pyrite Belt in Portugal: implications for mineral exploration

The area-concentration multifractal modelling of a soil geochemical survey performed in the Alfarrobeira–Chança region (covering the Pulo do Lobo Terrane, PLT, and the Iberian Pyrite Belt, IPB) shows that the upper limit of Cu, Zn and Pb background concentrations in soils over PLT metasediments are c . 20, 55 and 20–30 ppm, respectively. In soils derived from the Volcano-Sedimentary Complex of IPB, the Cu, Zn and Pb regional threshold values are within the 25–45, 40–60 and 20–90 ppm intervals, respectively. The lower limit of Cu, Zn and Pb soil anomalies overlying metasediments are within the 30–50, 90–115 and 45–60 ppm intervals, respectively; over soils with abundant volcanic-derived components those lower limits are scattered in the 30–70, 70–90 and 33–100 ppm intervals. Proximity to mineralized rocks is indicated by Cu, Pb and Zn soil contents above c . 100, 120–150 and 250 ppm, respectively. Most of the anomalies display strong anisotropy. Separation of Cu, Zn and Pb soil anomalies reflects mostly the existence of distinct metal sources related to original compositional differences or to chemical changes developed during Variscan times. Metavolcanics represent the prevailing sources of Cu and, to a lesser extent, Pb; the prevalent sources of Zn are metasediments of the Phyllite-Quartzite (IPB) and Chança (PLT) Groups.

Geotechnical properties in relation to grain-size and mineral composition: case study landslide in the Rječina Valley (Croatia)

In this study two small, but genetically very interesting iron carbonate deposits Macje Jame and Vranjski Potok, south west of Busovaca (MBSM), located in the pre-Devonian metamorphic complex were investigated in detail. The analyses of the main chemical components, trace elements, REE, isotope composition of C, O, S, 87Sr/86Sr ratio of rhyolite, plot of REE normalized to CI chondrites and a microscopic study of thin and polished sections were performed. Three mineralization phases were identified: the oldest is the main phase (90-95 wt %) with Fe (Ca, Mg, Mn) carbonates (siderite, ankerite, Fe-dolomite) as the predominant minerals whereas magnetite, hematite, albite, allanite, pyrite I, quartz I represent subordinate minerals. The second phase is the pneumatolytic-kathathermal phase (5-10 wt %) characterized by pyrrhotite relicts, pyrite II, quartz II, arsenopyrite and some supposed accessory minerals such as cassiterite, wolframite, gersdorffite, columbite. The youngest is the katha-mezo-epithermal-hydatogene phase (<1 wt %) which gave quartz III-chalcedony as gangue minerals and marmatite, chalcopyrite, boulangerite, gel-pyrite-marcasite, sphalerite, enargite, tetrahedrite, galena, cosalite, bismuth and sternbergite as ore minerals. Almost an identical paragenesis has been found in the oldest ore deposit of the Gemericum Paleozoic metamorphic complex (RADVANEC and BARTALSKY, 1987).The first and second mineralization phase of both ore deposits formed in the Carboniferous from metamorphogeno-hydrothermal fluids generated from S-granitoid magmatic rocks and their protoliths. This claim is strongly supported by very similar REE and their interior disposition between these deposits and metarhyolites as well as by the obtained values of strontium ratios. The third, youngest phase is the product of a very weak overprint of the Late Variscan (290-260 Ma and in the Upper Permian-Lower Triassic (260-240 Ma) time) hydrothermal activity.

U–Pb detrital zircon analysis of the lower allochthon of NW Iberia: age constraints, provenance and links with the Variscan mobile belt and Gondwanan cratons

Detrital U–Pb laser ablation inductively coupled plasma mass spectrometry zircon ages from six siliciclastic samples from the lower allochthon of NW Iberia are analysed to constrain their maximum sedimentation age and provenance, and to evaluate the connections to the adjacent tectonostratigraphic domains. Deposited in the external sections of the Gondwana platform, their maximum depositional age is latest Neoproterozoic ( c . 560 Ma). Comparison of the age populations of the lower allochthon with those of the rest of the allochthonous and autochthonous units of NW Iberia suggests that the terranes located in the footwall of the Variscan suture should not be considered as exotic elements, but as contiguous pieces of the same continental margin transported onto the adjacent Gondwana mainland in Variscan times. The data are in agreement with the regional trend defined by the drop in Early Neoproterozoic and Mesoproterozoic zircon content upward in the tectonic pile, which had been previously proposed as a marker of proximity to the eastern part of the West African Craton. Based on the age spectra, the palaeoposition for the time of sedimentation is placed in northern Africa, between the West African and Saharan cratons. Particular attention is paid to the occurrence of an Early Neoproterozoic input, probably derived from the Pan-African Hoggar suture. Supplementary materials: Isotope ratios and ages for the selected analyses are available at www.geolsoc.org.uk/SUP18548 .

Petrology and monazite dating of the Fe-rich gneisses from Kokava (Veporic Unit, Western Carpathians, Slovakia): Devonian sediments supplied from Gondwanan-sources metamorphosed in the Variscan times

*Corresponding author The Fe-rich gneisses from vicinity of Kokava nad Rimavicou (Central Slovakia) were studied by electron microprobe to resolve the origin of the iron ore bodies. Whole-rock chemistry of the ironstones shows the lack of CaO, MgO and partly elevated P 2 O 5 contents. This, together with the mineral assemblage of almandine + magnetite + grunerite + ilmenite + quartz + apatite + allanite + zircon + monazite suggests an origin by metamorphism of sedimentary protolith and disproves the older idea about the formation as a skarn. The Fe–Ti oxides thermometry suggests temperatures of 360–420 °C, and the average oxygen fugacity of ∆log fO 2 normalized to the FMαQ of +0.13. The reconstructed oxygen fugacity for temperatures 500–600 °C (amphibolite facies) yields an interval of c. +1 to +3 ∆log fO 2 (FMαQ). The presence of detrital zircons, and monazites dated in this study, reveals a participation of lithologies of the pan-African orogen. The source rocks of the Devonian ironstones sedimented probably as oolitic chamosite in lagoons. The dated detrital monazite cores (612–400 Ma) show an affinity to source rocks formed in northern peri-Gondwana due to rifting and opening of the Rheic Ocean and separation of Avalonia and Armorica microcontinents. The majority of monazite data from the rims of detrital grains and from unzoned metamorphic grains bear an evidence of Meso-Variscan metamorphism with ages clustering between 360 and 320 Ma (342 ± 4Ma – weighted mean ± 2σ). The younger population with elevated Th contents (18.0–20.5 wt. %) giving the ages of 310–240 Ma (279 ± 2 Ma – weighted mean) is connected with the collapse of Variscan orogen and/or the onset of the early Alpine continental rifting. The Alpine ages are very rare, but a few monazites and one Th–U–Si inclusion in quartz yielded Cretaceous ages (115–85 Ma).

Amphibolites from the Szklarska Poręba hornfels belt, West Sudetes, SW Poland: magma genesis and implications for the break-up of Gondwana

Amphibolites from the Szklarska Poreba hornfels belt (northern part of the Karkonosze-Izera Massif) represent rocks of alkali-basalt composition metamorphosed during Variscan times. Despite the intense thermal influence of the Karkonosze granite superimposed on the effects of regional amphibolite-facies metamorphism, the geochemical affinities of the Szklarska Poreba amphibolites are well preserved. They are similar to alkaline OIB basalts derived from an enriched (undepleted) sub-lithospheric source in the garnet stability field at depths ca 80–120 km. Trace-element characteristics and geochemical modelling indicate that the source was not modified by metasomatism in a supra-subduction zone or by alkali (silicate, carbonatitic) infiltration. Subsequent intra-crustal fractional crystallization involved olivine and clinopyroxene, and subordinate spinel and, presumably, plagioclase. The chemical composition of the rocks is most similar to that of modern magmas generated in an extensional setting (intra-continental rift). Neither geochemical characteristics nor estimated mantle temperatures only slightly higher than those of ambient mantle convincingly attest to the involvement of deep-mantle plume activity. Instead, decompression melting of passively upwelling asthenosphere beneath opening fractures in fragmented lithosphere is invoked. The origin of the amphibolite protolith was presumably associated with the Early Palaeozoic rifting of northern Gondwana, well documented throughout the Karkonosze-Izera massif. Locally rifting must have ceased earlier (immature rift) as reflected by mafic dykes exposed in the northern part of the massif, i.e., in the Szklarska Poreba hornfelses, and by the Izera gneisses and the Stara Kamienica metapelites. A passive rift system controlled by lithosphere extension provides a plausible explanation for the origin of mafic rocks in the Karkonosze-Izera Massif and sheds light on possible mechanisms involved in the break-up of Gondwana.

U–Th–Pb and Lu–Hf isotopic constraints on the evolution of sub-continental lithospheric mantle, French Massif Central

We have carried out a Pb double-spike and Lu–Hf isotope study of clinopyroxenes from spinel-facies mantle xenoliths entrained in Cenozoic intraplate continental volcanism of the French Massif Central (FMC). U–Th–Pb and Lu–Hf isotope systematics verify the existence of different lithospheric domains beneath the northern and southern FMC. Northern FMC clinopyroxenes have extreme Lu/Hf ratios and ultra-radiogenic Hf ( ε Hf = +39.6 to +2586) that reflect ∼15–25% partial melting in Variscan times (depleted mantle model ages ∼360 Ma). Zr, Hf and Th abundances in these clinopyroxenes are low and unaffected by hydrous/carbonatitic metasomatism that overprinted LILE and light REE abundances and caused decoupling of Lu/Hf–Sm/Nd ratios and Nd–Hf isotopes ( ε Nd = +2.1 to +91.2). Pb isotopes of northern FMC clinopyroxenes are radiogenic ( 206Pb/ 204Pb > 19), and typically more so than the host intraplate volcanic rocks. 238U/ 204Pb ratios range from 17 to 68, and most samples have distinctively low 232Th/ 238U (<1) and 232Th/ 204Pb (3–22). Clinopyroxenes from southern FMC lherzolites are generally marked by overall incompatible trace element enrichment including Zr, Hf and Th abundances, and have Pb isotopes that are similar to or less radiogenic than the host volcanic rocks. Hf isotope ratios are less radiogenic ( ε Hf = +5.4 to +41.5) than northern FMC mantle and have been overprinted by silicate-melt-dominated metasomatism that affected this part of FMC mantle. Major element and Lu concentrations of clinopyroxenes from southern FMC harzburgites are broadly similar to northern FMC clinopyroxenes and suggest they experienced similar degrees of melt extraction as northern FMC mantle. 238U/ 204Pb (53–111) and 232Th/ 204Pb ratios (157–355) of enriched clinopyroxenes from the southern FMC are extreme and significantly higher than the intraplate volcanic rocks. In summary, mantle peridotites from different parts of the FMC record depletion at ∼360 Ma during Variscan subduction, followed by differing styles of enrichment. Northern FMC mantle was overprinted by a fluid/carbonatitic metasomatic agent that carried elements like U, Pb, Sr and light REE. In contrast, much of the southern FMC mantle was metasomatised by a small-degree partial silicate melt resulting in enrichment of all incompatible trace elements. The extreme mantle 238U/ 204Pb (northern and southern FMC), 232Th/ 238U (northern FMC) and 232Th/ 204Pb ratios (southern FMC), coupled with unremarkable present-day Pb isotope ratios, constrain the timing of enrichment. Mantle metasomatism is a young feature related to melting of the upwelling mantle responsible for Cenozoic FMC volcanism, rather than subduction-related metasomatism intimately associated with mantle depletion during the Variscan orogeny. The varying metasomatic styles relate to pre-existing variations in the thickness of the continental lithospheric lid, which controlled the extent to which upwelling mantle could ascend and melt. In the northern FMC, a thicker and more refractory lithospheric lid (⩾80 km) only allowed incipient degrees of melting resulting in fluid/carbonatitic metasomatism of the overlying sub-continental lithospheric mantle. The thinner lithospheric lid of the southern FMC (⩽70 km) allowed larger degrees of melting and resulted in silicate-melt-dominated metasomatism, and also focused the location of the volcanic fields of the FMC above this region.

4: Transpressional tectonics, lower crust decoupling and intrusion of deep mafic sills: A model for the unusual metallogenesis of SW Iberia

SW Iberia is interpreted as an accretionary magmatic belt resulting from the collision between the South Portuguese Zone and the autochthonous Iberian terrane in Variscan times (350 to 330 Ma). In the South Portuguese Zone, pull-apart basins were filled with a thick sequence of siliciclastic sediments and bimodal volcanic rocks that host the giant massive sulphides of the Iberian Pyrite Belt. Massive sulphides precipitated in highly efficient geochemical traps where metal-rich but sulphur-depleted fluids of dominant basinal derivation mixed with sulphide-rich modified seawater. Massive sulphides formed either in porous/reactive volcanic rocks by sub-seafloor replacement, or in dark shale by replacement of mud or by exhalation within confined basins with high biogenic activity. Crustal thinning and magma intrusion were responsible for thermal maturation and dehydration of sedimentary rocks, while magmatic fluids probably had a minor influence on the observed geochemical signatures. The Ossa Morena Zone was a coeval calc-alkaline magmatic arc. It was the site for unusual mineralization, particularly magmatic Ni–(Cu) and hydrothermal Fe-oxide–Cu–Au ores (IOCG). Most magmatism and mineralization took place at local extensional zones along first-order strike-slip faults and thrusts. The source of magmas and IOCG and Ni–(Cu) deposits probably lay in a large mafic–ultramafic layered complex intruded along a detachment at the boundary between the upper and lower crust. Here, juvenile melts extensively interacted with low-grade metamorphic rocks, inducing widespread anatexis, magma contamination and further exsolution of hydrothermal fluids. Hypersaline fluids ( δ 18O fluid > 5.4‰ to 12‰) were focused upward into thrusts and faults, leading to early magnetite mineralization associated with a high-temperature (> 500 °C) albite–actinolite–salite alteration and subsequent copper–gold-bearing vein mineralization at somewhat lower temperatures. Assimilation of sediments by magmas led in turn to the formation of immiscible sulphide and silicate melts that accumulated in the footwall of the layered igneous complex. Further injection of both basic and sulphide-rich magmas into the upper crust led to the formation of Ni–(Cu)-rich breccia pipes. Younger (330 to 280 Ma?) peraluminous granitoids probably reflect the slow ascent of relatively dry and viscous magmas formed by contact anatexis. These granitoids have W–(Sn)- and Pb–Zn-related mineralization that also shows geochemical evidence of major mantle–crust interaction. Late epithermal Hg–(Cu–Sb) and Pb–Zn–(Ag) mineralization was driven by convective hydrothermal cells resulting from the high geothermal gradients that were set up in the zone by intrusion of the layered igneous complex. In all cases, most of the sulphur seems to have been derived from leaching of the host sedimentary rocks ( δ 34S = 7‰ to 20‰) with only limited mixing with sulphur of magmatic derivation. The metallogenic characteristics of the two terranes are quite different. In the Ossa Morena Zone, juvenile magmatism played a major role as the source of metals, and controlled the styles of mineralization. In the South Portuguese Zone, magmas only acted as heat sources but seem to have had no major influence as sources of metals and fluids, which are dominated by crustal signatures. Most of the magmatic and tectonic features related to the Variscan subduction and collision seem to be masked by those resulting from transpressional deformation and deep mafic intrusion, which led to the development of a metallogenic belt with little resemblance to other accretionary magmatic arcs.

Plumbotectonic Evolution of the Ossa Morena Zone, Iberian Peninsula:Tracing the Influence of Mantle-Crust Interaction in Ore-Forming Processes

The Ossa Morena zone in the Variscan belt of the Iberian Peninsula is characterized by a complex geotec- tonic evolution. It hosts a Neoproterozoic-Paleozoic sequence that records the existence of two orogenic events (Cadomian and Variscan) separated by rift and stable platform stages (Middle Cambrian to Late Devonian). Volcanogenic and sediment-hosted massive sulfides, skarns, magmatic Ni-(Cu) deposits, synmetamorphic and intragranitic and perigranitic veins, and replacement bodies are related to different magmatic-hydrothermal systems ranging in age from 650 Ma to about 250 Ma. The lead isotope signatures from 26 different mineral deposits show variable ratios of 206Pb/204Pb (17.49–18.44), 207Pb/204Pb (15.47–15.67) and 208Pb/204Pb (37.37–38.78) that have low average μ values (9.7) but high ω values (38.3). As a whole, the lead isotope data define an irregular cluster that plots around the Stacey and Kramers (1975) growth curve but clearly marks the existence of two main ore-forming events during Cadomian and Variscan times. The most radiogenic samples are from some minor Zn-Pb stratiform (sedimentary exhalative) mineralization. They are interpreted as having a dominantly crustal derivation (μ >10.1) in which most of the lead was leached from a pre-Cadomian continental basement with a long crustal history. This basement has strong geochemical affinities with the oldest rocks of the Iberian terrane. However, most of the ore samples investigated plot below the Stacey and Kramers (1975) reference curve, indicating polyphase but variable mixing of lead from upper crustal and mantle lead reservoirs at the terrane scale. Clear mixing lines between the two end-member reser- voirs are evident only in the oldest volcanic rock- and sedimentary rock-hosted massive sulfide deposits related to the Cadomian (late Neoproterozoic-Early Cambrian) magmatic arc; in the other deposit types lead isotopes have more restricted values. The crustal lead is interpreted as having been derived from ancient crust (2.1–2.5 Ga) and the derived shales (Montemolin Formation) that predated the Cadomian orogeny. The mantle-derived lead may have been derived from three different sources: Neoproterozoic tholeiitic orthoamphibolite, syn- Cadomian calc-alkaline andesite, or calc-alkaline metaluminous plutonic rocks intruded during the Variscan orogeny. The first two sources probably controlled the lead isotope signatures of the Cadomian massive sulfide deposits and the Variscan synmetamorphic veins. The input of primitive lead is especially significant during Variscan times (350–300 Ma) and is interpreted as having been related to the existence of a large primitive reservoir that controlled the Variscan magmatic, metallogenic, and hydrothermal evolution of the Ossa Morena zone. This primitive lead is recognized in most of the magmatic-hydrothermal deposits, including the typically crust-derived W- and Pb-bearing perigranitic hydrothermal systems. Likewise, associated plutonic rocks have more primitive lead isotope signatures than their counterparts in other Iberian terranes. The lead isotope evolution of the region is consistent with recent geophysical data that suggest that a deep, large mafic sill was intruded below the Ossa Morena zone during Variscan times. This sill was a likely key factor in the different Pb isotope evolution of the Ossa Morena zone compared with the nearby Iberian terranes and the entire Western Mediterranean province, which are char- acterized by lengthy crustal histories with very little input of primitive lead. The extensive mantle-crust inter- action probably explains the wide variety and the unusual metallogenic features of the Ossa Morena zone.

Mantle‐like Sr–Nd isotope composition of Fe–K subalkaline granites: the Peneda–Gerês Variscan massif (NW Iberian Peninsula)

AbstractThe Peneda–Gerês massif is one of the most representative NW Iberian late‐ to post‐orogenic Variscan granitic plutons. It resulted essentially from the subsynchronous emplacement, at 290–296 Ma, of two granitic magmas of Fe–K subalkaline affinity, with primitive isotopic composition: Sri = 0.703–0.707 and εNdi=−1.5 to −2.4. An origin by mantle input followed by mantle–crust interactions is proposed, implying the contribution of a less enriched mantle component than that involved in the genesis of synorogenic hybrid granitoids of Mg–K subalkaline affinity. A less voluminous aluminopotassic and isotopically more evolved magma (Sri=0.708–0.709 and εNdi=−3.5 to −3.9) with little or no mantle input was also generated, suggesting the involvement of lower crust materials. Therefore, this study suggests an input of juvenile magmas in late Variscan times, the mantle‐like isotope signature of Fe–K granitic magmatism being clearly related to a geodynamic setting of extensional processes, large‐scale uplift and thinning.