The Loki crystalline massif and adjacent territories are exposed in South Georgia within the northern marginal part of the Beiburt-Sevan terrane. It is part of the Loki-Karabakh tectonic zone. The Loki massif is a large anticlinal structure with a pre-Alpine crystalline basement exposed in the core surrounded by a Mesozoic-Cenozoic sedimentary cover. According to the complex study of the massif, it was established that it is composed of autochthonous Upper Devonian gneissose quartz-diorites, allochthonous pre-Late Paleozoic Moshevani and Sapharlo-Lok-Jandari overthrust sheets of metasediments, Precambrian Lower Gorastskali ofiolite sheet and Upper Gorastskali mélange sheet. All these rocks are cut by Late Paleozoic, Jurassic and Cretaceous intrusions. All sheets, except for ophiolite one, were metamorphosed during the Caledonian orogeny and then were overthrust during Bretonian and Early Cimmerian orogenies. The Loki massif is a part of the northern active continental margin of the Paleotethys oceanic basin, where supra-subduction regional metamorphism and granite formation occurred during Variscan orogeny. During the Late Bretonian orogeny, the obduction of Precambrian ophiolite rocks and the overthrusting of Paleozoic metamorphic sheets onto the continental margin took place. Later they were intruded by granites. Along the entire perimeter, the crystalline basement and granites are transgressively covered by Mesozoic-Cenozoic deposits. On the basis of detailed studies of terrigenous deposits in the section of the r. Gorastskali gorge on nanoplankton new age data have been obtained. Based on these data, for three lithostratigraphic units – Moshevani (conglomerates and quartz sandstones), Lokchay (mica sandstones) and Jandari (argillites) suites, the following ages were established: Norian - Rhaetian, Hettangian - Low Pliensbachian and Upper Pliensbachian - Aalenian, respectively. Triassic deposits were discovered in this area for the first time. With the accumulation of new data on the Loki massif and surrounding area, a new corrected digital geological map, lithostratigraphic column and 3D section were compiled.

How to cite: Gamkrelidze, I., Shengelia, D., Tsutsunava, T., Gavtadze, T., Chichinadze, G., Koiava, K., Beridze, G., and Javakhishvili, I.: New geological map and 3D section of the Loki crystalline massif and surrounding area (the Caucasus), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2790, https://doi.org/10.5194/egusphere-egu23-2790, 2023.

Two-mica leucogranites and/or granodiorites, often affected by various degrees of post-emplacement deformation and/or metamorphism (i.e., sheared granites, metagranites or orthogneisses), occur in several parts of the Tisza Mega-unit (Carpathian–Pannonian region), including the Apuseni Mts. (Romania), the Papuk Mt. (Croatia), and basement highs of the Pannonian Basin (Battonya–Pusztaföldvár and Algyő–Ferencszállás areas, SE Hungary). Despite the similar petrological characteristics (e.g., mineralogical composition, texture), these formations have not been compared to each other yet for correlational purposes and the scarce geochemical and almost completely lacking geochronological records also demanded further petrological investigations and datings.

Petrographically, granitoids from all the studied areas (SW Apuseni Mts., Papuk Mt. and the previously mentioned basement highs) proved to be similar, medium to coarse-grained monzogranites or granodiorites, containing quartz, plagioclase, K-feldspar, biotite, and muscovite. In some quarries or rarely in drill cores aplites and pegmatites were also found. As accessory components most commonly apatite, zircon, monazite, and xenotime, occasionally garnet were identified. As secondary phases sericite, albite, chlorite, epidote, kaolinite, and calcite appear frequently. Whole-rock geochemistry revealed that despite the various post-magmatic alterations (deformation/metamorphism/fluid effects etc.), the majority of the granitoids preserved their primary major and trace element compositions. All of them proved to be subalkaline, peraluminous, alkali-calcic or calc-alkalic with basically magnesian and S-type (rarely S/I-type) character. Major and trace element distributions, chondrite-normalized REE patterns (with slight negative Eu anomalies) and other relatively immobile trace element (HFSEs) concentrations showed significant similarities among the studied samples suggesting their common origin and local correlation possibilities within the Tisza Mega-unit. Interestingly, samples from the Papuk Mt. geochemically differ from the others as well as the aplites and pegmatites associated with the Codru granitoids (Apuseni Mts.). The former might represent a different source and igneous episode; however, the geochemical distinction of the latter (with more pronounced negative Eu anomaly and lower concentrations in REEs and HFSEs) is rather odd. Trace element-based discrimination diagrams (e.g., Yb vs. Ta, Yb+Ta vs. Rb) suggested that most of the studied rocks are volcanic-arc granites and only a few of them (basically aplites and pegmatites) are syn-collisional despite their typical S-type mineralogy (e.g., muscovite, monazite, garnet) that unequivocally referred to continental crustal sources.

Considering another means of geotectonic discrimination (e.g., Sr/Y and La/Yb ratios) and the ascertainment of Broska et al. (2022) in case of Western Carpathians granitoids, it is feasible that the studied granites bear the geochemical signature of a slab break-off, being crust- and mantle-derived, too, while shallower level melts (aplites and pegmatites) represent purely crustal sources in the Variscan orogeny. The latter corresponds to the calculated zircon saturation temperatures, as well (granites: 740–780 °C, aplites/pegmatites: 580–600 °C).

Preliminary datings (Battonya granitoids, SE Hungary) suggested that the main zircon crystallization period occurred in the Early Carboniferous (356 Ma) that fits well into the regional geological framework of the European Variscides.

This study was financed by NRDIF (K131690).

Broska, I., Janák, M., Svojtka, M., Yi, K., Konečný, P., Kubiš, M., Kurylo, S., Hrdlička, M., Maraszewska, M. (2022). Lithos 412–413:106589

How to cite: Szemerédi, M., Kovács, Z., Dunkl, I., Lukács, R., Horvat, M., Jákri, B., and Pál-Molnár, E.: Variscan S-type granitoids in the Tisza Mega-unit (Carpathian–Pannonian region): petrology, geochronology, geotectonic implications, and correlation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1201, https://doi.org/10.5194/egusphere-egu23-1201, 2023.

The crystalline basement of the Tatra Mountains belongs to the northernmost part of the Tatric unit of the Western Carpathians and is composed of pre-Mesozoic crystalline rocks, overlain by Mesozoic and Cenozoic sedimentary cover and nappes. Metamorphic rocks are the most abundant in the Western Tatra Mts. and display an inverted metamorphic sequence with high-grade rocks in the hanging wall (Upper Unit; peak conditions: 1.6 GPa, 750-800°C; Janák et al. 1996) and lower-grade rocks in the footwall (Lower Unit; peak conditions: 0.6-0.8 GPa, 640-660°C; Janák et al. 1996) separated by mid-crustal thrust fault. Those two basement units of contrasting pressure-temperature evolution are well documented in southern part of basement (i.e. in Slovakia). However, a presence of the Lower Unit in the northern part of the Western Tatras (i.e. in Poland) is highly debated.

To tackle this problem several field campaigns were carried out targeting an inferred thrust fault allegedly separating both basement units in the north. The field studies coupled with the structural analysis revealed presence of a wide high-strain zone, but no significant lithological difference across the zone on question. A set of three metasedimentary and one metaigneous rocks were collected along the profile cross-cutting the high-strain zone (from the bottom to the top: MB21-71, -82, -03, -32), and display a gradual increase in migmatization. The provenance study of detrital zircon shows no significant differences between the samples. The metasediments MB21-71, -82, -32 define prominent peaks at ca. 570-530 Ma, whereas MB21-32 shows additional younger peak at 520-500 Ma. Additionally, minor Palaeozoic (490-460 Ma;not in MB21-71), Proterozoic (ca. 680 Ma, 1500-1400 Ma, 2000 Ma), and Archean peaks (ca. 2800-2500 Ma) are present. The samples exhibit a metamorphic signature around 360-340 Ma too. The metaigneous rock MB21-03 yields U/Pb zircon age of ca. 490-480 Ma with only few Precambrian grains.

Our preliminary results coupled with a similar study from the southern side of the Western Tatras (Kohút et al. 2022) suggest that the observed similarities in the detrital zircon populations could indicate a similar protolith, thereof all samples could represent only one basement unit . This dataset helps to reconcile the nature of the Variscan basement assembly in the Western Tatra Mts. It also shows that the local tectonostratigraphy of the Tatra Mts. needs to be re-visited and re-evaluated.

Research funded by the National Science Centre, Poland, project no. 2021/43/B/ST10/02312 and supported by the Foundation for Polish Science stipend (M. Bukała). We also acknowledge the Tatra National Park for help and permission to conduct fieldwork.

References:

Janák, M., O'Brien, J.P., Hurai, V., & Reutel, C. (1996). Metamorphic evolution and fluid composition of the garnet-clinopyroxene amphibolites from the Tatras Mountains, Western Carpathians. Lithos, 39, 57-79.

Kohút M., Linnemann U., Hofmann M., Gärtner A., Zieger J. (2022) Provenance and detrital zircon study of the Tatric Unit basement (Western Carpathians, Slovakia). International Journal of Earth Sciences 111:2149-2168.

How to cite: de Doliwa Zieliński, L., Bukała, M., Bazarnik, J., Mikołajczak, M., Kośmińska, K., and Majka, J.: On the track of thrust faults within the Variscan basement of the Western Tatras: structural and geochronological approach, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6470, https://doi.org/10.5194/egusphere-egu23-6470, 2023.

The Halle Volcanic Complex is composed of rhyolites interpreted as intrusive-extrusive complexes that pierced host sedimentary cover during their vertical growth. Zircon ages from several units vary from 291.7 ± 1.8 Ma to 301 ± 3 Ma suggesting the prolonged evolution of this subvolcanic-volcanic system. In this study, we sampled the Landsberg (301 ± 3 Ma) and the Petersberg (292 ± 3 Ma) laccoliths to better identify the magmatic processes involved in silicic magma formation and their duration.  Altogether seven depths have been analyzed from these two laccoliths including electron microprobe analyses of zircon and apatite and U-Pb SHRIMP dating of zircon. At the first sight, zircon is chemically similar within and between laccoliths. Additionally, SHRIMP ages are scattered over 30 Ma for each sample in Landsberg. These ages overlap with two Concordia ages obtained for the uppermost horizon (289.7±2.8 Ma) and the lowermost horizon (297.1±1.7 Ma) in the Petersberg laccolith. The ages suggest that the volcanic system was active for at least 10 Ma and similar age range is recorded in both laccoliths. The scatter of ages seems to indicate the formation of the laccoliths over a prolonged period of time with periodic reactivation of the magma chamber, but the lead loss cannot be excluded. Also, prolonged formation may indicate either younger pulses reactivating previously formed parts of the magma chamber or multiple unrelated  magma injections amalgamated separately within the system.

The processes involved in the prolonged evolution of the magmatic system in Halle are evident from petrographic analyses of thin sections, where zircon can be imagined in association with other phases. Both zircon and apatite occur almost exclusively within complex glomerocrysts, an assemblage of major phases (variably altered biotite, feldspar, pyroxene). Such glomerocrysts were described in the literature and interpreted as remnants of crystal mush, probably re-mobilized at the final stage (heating episode) before laccoliths emplacement. The glomerocrysts in Petersberg and Landsberg laccoliths are similar leftovers of previous magmatic episodes, but they are special in that they contain abundant zircon and apatite. Such a picture is consistent with the evolution of magma in a long-lived magmatic system that underwent at least one reactivation. The major implication is that in some systems large proportion of zircon may represent the early stages of magma evolution, this context may be missed without detailed textural observations of zircon occurrence and associations.

Acknowledgements: Christoph Breitkreuz is thanked for his constant help with our rhyolitic research. The research has been funded by the NCN research project to AP no. UMO-2017/25/B/ST10/00180

How to cite: Pietranik, A., Słodczyk, E., and Przybyło, A.: Re-heating of rhyolitic leftovers in the Halle Volcanic Complex: an insight from zircon ages and composition., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5532, https://doi.org/10.5194/egusphere-egu23-5532, 2023.

The Devonian to early Carboniferous geodynamic evolution of the Bohemian Massif is largely controlled by a “diffuse cryptic suture zone” (Schulmann et al., 2014), which crops out between the Saxothuringian Zone (forming the lower plate) in the north and the Teplá-Barrandian and Moldanubian Zones (collectively the upper plate) in the south. Oceanic passing to continental subduction within this suture zone has been linked to at least three discrete episodes of high-pressure to ultra-high-pressure metamorphism; the formation and obduction of Devonian age ophiolites; the up to 50 myr building of a continental magmatic arc; and potentially, the large-scale relamination of continental crust beneath the upper plate and its exhumation into the upper plate in the form of trans-lithsopheric diapirs.

Taken together, this would appear to require long-lasting subduction of a vast oceanic domain likely including old and dense oceanic lithosphere. Yet, significant separation between the lower and upper plates during the Early Paleozoic is not consistent with litho-stratigraphic, paleontological, or paleomagnetic data, which indicate a shared peri-Gondwanan shelf derivation of these units. Nonetheless, within the high-grade rocks of the suture zone itself, an exotic assemblage of Cambrian age volcanic-arc related rocks are identified. These rocks have been variably metamorphosed up to eclogite- and granulite-facies conditions during an early phase of the Variscan Orogen, but, also include lower-grade segments that experienced only lower amphibolite- or greenschist-facies conditions. A compilation of whole-rock geochemical, isotopic and zircon U-Pb and Lu-Hf data from this Cambrian arc assemblage is presented to argue for the exotic nature of this terrane including its possible derivation from the Baltica paleo-continent and for an association with old oceanic lithosphere (Stenian-Tonian age) likely captured from the circum-Rodinia Mirovoi Ocean.

Thus, it is proposed that the geodynamic evolution of the Bohemian Massif cannot be reconciled with a single-phase of oceanic passing to continental subduction. Instead, a three stage evolution is proposed involving: (1) initial subduction of an old oceanic crust and extinct Cambrian age arc terrane derived from the Baltica paleo-continent beneath the peri-Gondwanan margin; (2) transcurrent displacement of a strip of peri-Gondwanan crust behind the initial subduction zone; (3) a second phase of oceanic passing to continental subduction of this displaced peri-Gondwanan crust beneath the initial subduction zone.

Schulmann, K., Lexa, O., Janoušek, V., Lardeaux, J.M. and Edel, J.B., 2014. Anatomy of a diffuse cryptic suture zone: an example from the Bohemian Massif, European Variscides. Geology 42, 275–278.

How to cite: Collett, S.: Reconstructing Devonian-Carboniferous subduction in the Northern Bohemian Massif, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5263, https://doi.org/10.5194/egusphere-egu23-5263, 2023.

The pre-Alpine evolution of the Central Alpine basement is dominated by magmatic and metamorphic events that occurred during an Ordovician orogenic cycle (ca. 480-440 Ma) and the Variscan orogenic cycle (ca. 350-300 Ma). A detailed zircon U-Pb data and Hf-isotope study of a large set of magmatic and meta-magmatic rocks revealed four magmatic pulses (Ruiz et al. 2022): at 340-350 Ma (calc-alkaline diorite and tonalite from the Surselva Group), 330-335 Ma (shoshonitic diorites, monzonites, granites and syenites of the Rötifirn Group), 307-310 Ma (calc-alkaline diorites, ranging from cumulate-like hornblende gabbros to hornblende-diorites and hornblende- or biotite quartz- monzonite, granodiorites and metaluminous weakly peraluminous I-type granites of the Fruttstock Group), and 297-300 Ma (late-orogenic, calc-alkaline I-type granites of the Haslital Group). High precision U-Pb dates from meta-magmatic rocks indicate a minor, but variable impact of Alpine metamorphism on the U-Pb dates (Gaynor et al. 2022, Ruiz et al. 2022). However, given the poly-cyclic metamorphic record of the country rocks, the relative contributions of the Alpine, Variscan and an earlier Ordovician orogenic cycle are difficult to quantify. More specifically, the physical conditions of the Variscan metamorphic overprint are only weakly constrained, and available radio-isotopic ages are not reliable. However, monazite, rutile, titanite and zircon ages of 329-317 Ma in high-grade metapelites and calcsilicate gneiss indicate a major high grade Variscan metamorphism along the northern rim of the massif (Schaltegger et al., 2003). In addition, frequently found U-Pb dates between 478 and 445 Ma on gabbros, metapelitic to metapsammitic gneisses in the northern part of the Aar massif (Schaltegger et al., 2003) show relics of an older metamorphism in these polycyclic basement units

In order to understand better the temperature-time evolution of this poly-cyclic basement, we will apply detailed U-Pb geochronology on different minerals together with mineralogical, chemical and textural characterization. Combining the mineralogical data with microstructures and petrological data should give better insights in the link of metamorphism and magmatism for the Variscan orogenic cycle. These data will allow placing the Aar massif evolution in a wider framework of the European Variscan orogen. Moreover, they will reveal the existence of one or several pulses of earlier, Ordovician-age high-grade metamorphism, anatexis and magmatism.

References: Gaynor S.P., Ruiz M., & Schaltegger U. (2022) Chem. Geol., 603, 120913; Ruiz M., Schaltegger U., Gaynor S.P., Chiaradia M., Abrecht J., Gisler C., Giovanoli F. & Wiederkehr M. (2022) Swiss J. Geosci., 115, 20; Schaltegger. U., Abrecht J. & Corfu F. (2003) Schweiz. Mineral. Petrogr. Mitt. 83, 183-195

How to cite: Schaltegger, U., Abrecht, J., Berger, A., Spikings, R., and Wiederkehr, M.: Re-assessing the magmatic and metamorphic evolution of the Aar Massif, Central Alpine basement, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10890, https://doi.org/10.5194/egusphere-egu23-10890, 2023.

The northeastern part of Bohemian Massif is composed of various lithotectonic domains interpreted as microplates, sedimentary basins, and fragments of ancient oceanic lithosphere, which have been amalgamated during the Late Devonian multistage collision. Fragments of the Variscan Central-Sudetic Ophiolite (CSO) preserve information on the nature of the mantle at the onset of Variscan orogeny. They are mainly composed of ultramafic-mafic rocks (UMR) dated at 400 Ma, but these are not the only components. Other lithologies include (a) carbonate veins crosscutting the UMR, (b) dolomite-rich domains associated with clinopyroxenites, and (c) silicic dyke of diorite composition also crosscutting the UMR. The origin of these lithologies may be contemporaneous with UMR or later (Variscan or Cenozoic) and obtaining the ages is the first step to understanding which events they record. Zircons from the diorite yield a concordia age of 378.0 ± 5.0 Ma (SHRIMP) consistent with the diorite representing an early Variscan magmatic episode. The obtained age of the intrusion suggests an affinity with a located nearby outcrop of ultrapotassic syenites (from 378.2 ± 2.4 to 354.7 ± 4.3 Ma). A striking relationship between the two rocks is evident; if certain elements are strongly enriched in one rock they are equally impoverished in the other. Such unusual chemical fractionation can be achieved during the formation of alkaline and carbonatite melts. Also, dolomite domains recently found in clinopyroxenites or puzzling anhydrite inclusions in Ca-amphiboles may support this hypothesis suggesting an enriched mantle as a common source of dioritic, syenitic, and dolomitic lithologies. On the other hand, carbonate veins record another episode. Recently, the U-Pb radiometric dating of calcite sampled from one of the CSO massifs yielded an isochrone age of 15.4 ± 19.7 Ma that generally suits Paleogene and Neogene tropical weathering events, moreover, some parts of CSO contain abundant carbonates mineralization accompanied by plenty of quartz zonal clusters. The co-occurrence of these phases may suggest hydrothermal origin and becomes a foothold for further studies on the carbonation of obducted oceanic lithosphere.
Altogether, it is important to bear in mind CSO’s 400 Ma-long evolution. It seems that Central-Sudetic Ophiolite and associated younger lithologies still have more to tell us about the orogenic and post-orogenic history of the northeastern Bohemian Massif.

Funding: The research is funded by NCN grant PRELUDIUM no. UMO-2022/45/N/ST10/00879 awarded to Błażej Cieślik.

How to cite: Cieślik, B., Pietranik, A., and Kierczak, J.: Variscan and post-Variscan processes in the Central-Sudetic Ophiolite: records from carbonate and silicate rocks., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5732, https://doi.org/10.5194/egusphere-egu23-5732, 2023.

The Iberian Massif presents a geometry characterized by two oroclines: the conspicuous and tight Ibero-armorican arc to the north and the older and less pronounced Central Iberian Arc (CIA) to the south. The latter is clearly depicted by low amplitude, short wavelength magnetic anomalies in its external part and long wavelength, higher amplitude magnetic anomalies in its internal part. The origin of these is under study as they seem to be indicators of the deep evolution of this orogen in the Iberian Peninsula.

Three magnetic anomalies stand out in the internal part of the CIA. To the north, the Eastern Galicia Magnetic Anomaly overlaps the Lugo Gneiss Dome, a structure delineated by extensional detachments. Here, the exhumation at high temperatures during late Variscan gravitational collapse triggered the formation of magnetite in metasediments, migmatites and S-type syn-orogenic granites, thus stablishing a clear relationship between tectonics and magnetization. To the west, the Porto-Viseu-Guarda Magnetic Anomaly has still an unclear origin, but it also overlaps an area characterized by extensional tectonics, gneiss domes development and granite intrusion. However, the most magnetic outcropping rocks are late Variscan I-type granites (Lavadores granite) and the Mindelo Migmatitic Complex. None of them shows a relationship between magnetization and extension. Contrarily, magnetic minerals seem to be related to the composition of the resisters in migmatites and to the formation of the Lavadores granite itself. Finally, the Spanish Central System Magnetic Anomaly, at the core of the CIA, also overlaps the exhumed products of late Variscan extension (granites and migmatites). The magnetic anomaly associated to this part of the arc has been studied in the Castellanos Antiform, a discrete extensional dome located in its northern part, where the interaction between high degree metasediments, extension, and migmatization can be revised. New high resolution magnetic and gravity data indicate that the magnetic anomaly coincides with a high Bouguer gravity anomaly, and overlaps an outcrop of granitoids with tonalitic xenoliths and gabbros. The relationship between gravity and magnetic anomalies, together with the lack of outcropping magnetic granitoids and/or migmatites in the Central System, and the high amount of heterogeneous xenoliths, including basic rocks, suggest that in central Iberia, late Variscan extension might have involved deeper levels of the crust and maybe the mantle. Considering the location of this area, in the core of the CIA, and the simultaneity between late Variscan extension and the CIA formation, we suggest that the development of the latter might have played an important role in the supply of mantle material.

Funding:  grant PID2020-117332GB-C21 and projects SA084P20,  MCIN/AEI/10.13039/501100011033 and TED2021-130440B-I00

How to cite: Ayarza, P., Martínez Catalán, J. R., Gómez Barreiro, J., Palomeras Torres, I., Sánchez Sánchez, Y., and Rivero Montero, M.: Heterogeneous origin of the magnetic anomalies of the Central-Iberian Arc. Constraints on the source of the Central System Magnetic Anomaly, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6391, https://doi.org/10.5194/egusphere-egu23-6391, 2023.

The Iberian Massif presents a complex Late Paleozoic evolution, with intense compressional tectonics followed by gravitational collapse of the thickened crust and orocline development. In NW Iberia, extensional detachments and associated shear zones developed during high temperature-low pressure metamorphism in relation to partial melting in gneiss domes. These structures also feature a conspicuous relationship with magnetic anomalies that define a curvature, delineating the geometry of the internal part of the Central Iberian Arc. Regardless of the geometry of these anomalies and their relationship to extensional tectonics, their source probably differs from northern to central and western Iberia. While in northern Iberia extensional tectonics triggered oxidation and development of magnetite in migmatites and S-type granites, in central Iberia basic rocks associated with I-type granites seem to be the carriers of the magnetization. This study aims to describe the western branch of the Central Iberian Arc magnetic anomaly: the Porto-Viseu-Guarda Magnetic Anomaly (PVGMA) and its metallogenetic potential previously related with magnetite-type granites.

Polyphase deformation within the Porto-Viseu metamorphic belt later affected by the Douro-Beira shear zone and Porto-Tomar fault presents syn-tectonic staurolite and sillimanite-bearing schists and migmatites (Mindelo Migmatite Complex), great abundance of syn and late S-type two mica-granites, and a post-orogenic porphyritic biotite I-type granite with uncommon high values of magnetic susceptibility (Lavadores granite). These rocks crop out at the northwestern tip of the PVGMA and are thought to be related to it. We sampled migmatites, calc-silicate resisters embedded on them and Lavadores granite for its mineralogical and magnetic characterization.

Anisotropy of the magnetic susceptibility sometimes show stable N-S to N90°E, 0°-20° E to NE plunge magnetic lineations and a WNW-ESE magnetic foliation subparallel to the shearing in the area. In migmatites, thin sections feature the expected high temperature metamorphism manifested by sillimanite and ptygmatic folding. Here, rock magnetism studies show Curie temperatures (Tc) around 300°C and low coercivities indicative of titanomagnetite or some sort of multidomain pyrrhotite. Low to moderate magnetic susceptibilities contrast with very high magnetic remanences leading to Königsberger ratios (Qn) of up to 22 in resisters and 10 in the migmatites. Contrarily, the Lavadores granite has high magnetic susceptibilities and moderate Qn (0.1-2). These rocks feature higher Tc=550° and low coercivities indicative of magnetite. Paleomagnetic results show heterogenous directions for both lithologies implying complicated thermal evolutions and possibly late tilting. Despite their proximity, no relationship seems to exist between the Lavadores granite and the Mindelo Migmatite complex protolith. Contrarily to what it is found in northern Iberia, no relationship has been found between extensional features and magnetic mineralization, so if these rocks are the source of the PVGMA, it is most probably related to the characteristics of the protoliths.

Despite the PVGMA lies on top of the Sn belt across Portugal, geochemical results do not support Lavadores as a potential Sn metallogenetic granite, further indicating the lack of relationship between the formation of magnetite and that of Sn mineralizations.

Acknowledgements: Project SA084P20 (regional CYL government); Grants PID2020-117332GB-C21 funded by MCIN/AEI/10.13039/501100011033 and TED2021-130440B-I00; Projects UIDB/04683/2020 and UIDP/04683/2020 (Portugal).

How to cite: Pérez Cáceres, I., DeFelipe, I., Ayarza, P., Gómez Barreiro, J., Sant’Ovaia, H., Cruz, C., Ribeiro, M. D. A., Villalaín, J. J., Durán Oreja, M., and Martínez Catalán, J. R.: Variscan tectonic evolution, magnetic anomalies and metallogenetic potential in the western Central Iberian Zone (Iberian Massif), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9797, https://doi.org/10.5194/egusphere-egu23-9797, 2023.

The late Variscan gravitational collapse and coeval magmatism are getting the attention of the community due to their role in the generation of strategic mineral resources. In this regard, the GOLDFINGER project’s main scope is to study how the Variscan orogenic architecture controls the generation of strategic ore deposits (i.e. Sn, W, Nb, Ta, Sc, Au, Sb). With this goal, a 3D model of a gneissic dome with several mineral deposits will be constructed based on high-resolution geophysics (Seismic/Gravity/Magnetism), and regional geology. The study area encompasses the Martinamor gneiss dome which represents a Late-Variscan syn-collisional extensional system with a well-preserved architecture. This gneiss dome structure presents low topography, relatively flat structural geometry in-depth, and contrasting lithotypes regarding seismic, gravity, and magnetic properties. As part of the project, in spring 2022 the area was covered by 30 low-period seismic recorders with 2Hz sensors in a regular grid. The 35x40 km grid consisted of 60 nodes, separated by approximately 4.5 km. To achieve the final node number, the stations were deployed twice, first in a regular grid with nodes each 6 km, and then the grid was moved 3 km to the west and to the south for a second deployment. The seismic stations were continuously recording in the field for up to 40 days in each deployment. We are using a state-of-the-art technique to retrieve high-resolution seismic images of the Martinamor gneiss dome using seismic interferometry applied to seismic background noise (SBN). The preliminary results show that SBN interferometry allows us to 1) detect and track discontinuities that can be related to the structures that control the ore deposits, and 2) identify the location of deep intrusions that are inferred as sources of metallogenic fluids. In this contribution, we present the GOLDFINGER geophysical experiment and the preliminary results.

Funding: grant PID2020-117332GB-C21 funded by MCIN/ AEI /10.13039/501100011033; EIT-Raw Materials project 17024 (SIT4ME: Seismic Imaging Techniques for Mineral Exploration); SA085P20 from the JCYL government, and TED2021-130440B-I00 by MCIN. IP is funded by MCIU and USal (BEAGAL18/00090).

How to cite: Palomeras, I., Gomez-Barreiro, J., Ayarza, P., Martínez-Catalán, J. R., Martí, D., Ruiz, M., Barrios, S., Dos Santos, K., Sanchez-Sanchez, Y., Elez, J., Yenes, M., DeFelipe, I., Pérez-Cáceres, I., Crespo, E., and Castiñeiras, P.: The GOLDFINGER Project: Imaging a Late-Variscan gneissic dome. Preliminary results., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15192, https://doi.org/10.5194/egusphere-egu23-15192, 2023.

The allochthonous complexes of the Galicia-Trás-os-Montes Zone (GTMZ) of the NW Iberian Massif consist of an ensemble of peri-Gondwanan terranes and ophiolitic units stacked during the Variscan orogeny. The Middle Allochthon also known as the ophiolitic complex represents the variscan suture of one or more peri-Gondwanan oceans, and includes Cambro-Ordovician to Lower Devonian units. In the allochthonous Morais Complex (Trás-os-Montes, Portugal), the ophiolitic complex comprises four structural units, which from bottom to top are Macedo de Cavaleiros, Pombais, Izeda-Remondes and Morais-Talhinhas.

The two first units are quite similar to each other and consist of greenschists and metapelites, with metabasites dominating in Pombais and metapelites in Macedo de Cavaleiros. No age data are available for these two units. Their structural position is comparable to that of the Cambro-Ordovician Vila de Cruces Unit in the Órdenes Complex, but also to that of the Lower Devonian Moeche Unit in the Cabo Ortegal Complex, both in Galicia.

The Izeda-Remondes and Morais-Talhinhas units mostly consist of fine grained amphibolites associated with deformed gabbros, mafic cumulates and serpentinized ultramafics. The Izeda-Remondes Unit is structurally the lower and the older of the two, dated by Pin et al. (2006) around 447 ± 24 Ma (Sm-Nd whole rock isochron). The upper ophiolitic Morais-Talhinhas Unit was also dated by Pin et al. (2006) giving U-Pb ages of 405 ± 1 Ma and 396 ± 1 Ma.

This contribution brings new geochronological and geochemical data from the Middle Allochthon providing new understanding of the history of the suture of the Morais allochthonous complex. Zircons have been collected for LA-MC-ICP-MS U–Pb analyses in two felsic intrusions in the Izeda-Remondes Unit giving concordant ages ranging from 422 ± 4 Ma to 432 ± 4 Ma. These ages together with new and previous whole rock geochemical data obtained from basic and felsic igneous samples from the ophiolitic complex are interpreted to date and reflect the formation of igneous protoliths in an oceanic ridge setting forming part of the Rheic oceanic realm, during Silurian to Devonian. The mantle source for the basic rocks of all four units is similar to that of N-MORB with some influence from a subduction zone.

REFERENCE:

Pin, C., Paquette, J. L., Ábalos, B., Santos, F. J., & Gil Ibarguchi, J. I. (2006). Composite origin of an early Variscan transported suture: Ophiolitic units of the Morais Nappe Complex (north Portugal). Tectonics, 25(5).

Acknowledgements: Spanish Ministry of Science and Innovation, project PID2020-117332GB-C21.

How to cite: Malecki, J., Collett, S., Martínez Catalán, J. R., Gómez Barreiro, J., and Schulmann, K.: New age and geochemistry data from the Middle Allochthon ophiolitic units of the Morais Complex (Portugal)., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8080, https://doi.org/10.5194/egusphere-egu23-8080, 2023.

The Waldbach Complex is an amphibolite-grade basement unit within the Lower Austroalpine nappes of Eastern Alps, which differs from other Alpine basement units and which represents a magmatic arc-related tectonic setting. For the first time, twenty samples of magmatic and metasedimentary rocks were studied by the LA-ICP U-Pb zircon dating method supplemented by a geochemical survey. Two units are distinguished: (1) The western structurally Lower Waldbach Unit includes phyllonitic micaschist, paragneiss, and quartzites associated with various orthogneisses including augengneisses. Metasedimentary rocks contain mainly Late Ediacaran (550 Ma) detrital zircon populations. Older zircons are rare and include populations at 2.6 Ga (Late Archean) and 700 Ma (Cryogenian). Youngest ages are at ca. 510 Ma constraining the maximum depositional age. Six granitic orthogneisses from distinct lenses were studied and yield ages between 463.4 ± 3.7 Ma and 492.9 ± 3.1 Ma. Abundant inherited Neoproterozoic zircons suggest their S-type origin by remelting of Neoproterozoic crust. A further, granitic, biotite-gneiss intruded at 340 Ma (Early Carboniferous). All data together suggest a late Cambrian metasedimentary succession subsequently intruded by late Cambrian to Middle Ordovician porphyric granites. (2) The Upper Waldbach Unit  is dominated by various types of amphibolites, hornblende-gneisses, coarse-grained garnet-micaschists and sulphidic micaschists. Associated stratiform massive sulphides are exposed as up to two meter thick synsedimentary layers together with black, carbon-rich micaschists. A hornblende-gneiss is interpreted as a tuff and contains a pronounced population at 455 Ma. Coarse-grained garnet-micaschists include zircon populations with ages at 455 Ma and 505 Ma, and youngest ages at 430 and 410 Ma, respectively. Amphibolites vary in their U-Pb zircon ages between 450 and 340 Ma. Both amphibolites and metasedimentary rocks contain zircons with low Th/U ratios between 330 and 315 Ma supported by a chemical  monazite age  at 304.4 ± 7.8 Ma constraining together the age of amphibolite facies metamorphism of the Upper Waldbach Unit. We interpret the Upper Waldbach Unit as a Late Ordovician to Devonian arc system, which was deposited in an anoxic depositional environment with extensive hydrothermal activity leading to stratiform massive sulphides.

Paleogeographically, the Waldbach Complex was located close to Austroalpine-Penninic interface within the Alpine basement and can be likely traced to Carpathians. Tectonically, it is interpreted as the Late Ordovician to Devonian arc system formed during subduction of oceanic lithosphere as also constrained by Devonian eclogites in adjacent Western Carpathians and Devonian blueschists in Southern Carpathians. Consequently, elements of subduction-related settings allow trace a hitherto unknown subduction zone within the Alpine-Carpathian basement, which is potentially part of the Protogonos arc was recently proposed by A. M. Celal Sengör.

How to cite: Neubauer, F., Liu, Y., Chang, R., Hauzenberger, C., Yuan, S., Yu, S., Genser, J., and Guan, Q.: The Waldbach Complex of Eastern Alps: An early Paleozoic arc system and its significance for Variscan geodynamics, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16923, https://doi.org/10.5194/egusphere-egu23-16923, 2023.