GD6.4

Many pre‐Mesozoic basements of the Alpine belt contain kilometre‐scaled folds with steeply inclined axial planes and fold axes. Those structures are referred to as Schlingen folds. They deform polymetamorphic gneisses, often Late‐Ordovician metagranitoids and are cross‐cut themselves by Permian intrusions. However, the structural evolution of such Schlingen is still not completely understood and their geodynamic significance for the Variscan evolution is not clear. To close this gap, this study investigates in detail a well-preserved Schlingen structure in the Gotthard nappe (Central Swiss Alps). This Schlingen fold evolved by a combination of shearing and folding under amphibolite facies conditions. Detailed digital field mapping coupled with petrological and structural investigations reveal local synkinematic migmatisation in the fold hinges parallel to axial planes. U‐Pb dating of zircons separated from associated leucosomes reveal cores that record a detrital country rock age of 450 ± 3 Ma, and rims with a range of dates from 270 to 330 Ma. The main cluster defines an age of 316 ± 4 Ma. We ascribe this Late‐Carboniferous age to peak metamorphic conditions of the late‐Variscan Schlingen phase.

The pre-Schlingen structures are subdivided into three older deformation events, which are connected to the Cenerian and post-Cenerian deformations. In addition, until now unknown, post Schlingen-, but pre-Alpine transpressional deformation have been detected and described. This superimposed deformation produced locally a low-grade foliation and minor undulation of the Schlingen structures.

The detail data of the investigated fold structures are linked with already described Schlingen folds in the wider Alpine realm, which all are concentrated in the most southern parts of the Variscides. From a geodynamic point of view and based on the new tectono-metamorphic constraints, we propose Schlingen formation preceded and concurred the crustal-scale transpressional tectonics of the East Variscan Shear Zone. This scenario separates, at least in a structural sense, the Southern Variscides from more northern parts (also Gondwana derived) inside Pangea, where Schlingen folds are absent.

How to cite: Buehler, M., Zurbriggen, R., Berger, A., Herwegh, M., and Rubatto, D.: Late Carboniferous Schlingen in the Gotthard nappe (Central Alps) and their relation to the Variscan evolution, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6547, https://doi.org/10.5194/egusphere-egu21-6547, 2021.

At the end of the Variscan orogeny, several episodes of crustal extension starting in the Early Permian occurred in central Southern Alps (cSA), affecting the Adria passive margin (Handy et al., 1999). During this period, a megashear zone with dextral kinematics led to the transition from Pangea A to Pangea B configuration (Muttoni et al., 2003). The transtensional to extensional deformation regime led to the development of intra-continental basins infilled by Upper Carboniferous to Lower Permian sedimentary successions (Cadel et al., 1996). Crustal shortening related to Alpine compression was responsible for a partial or complete inversion of favourably oriented normal faults inherited from the Permian tectonics (Blom & Passchier, 1997). Despite this, SSE-dipping Early Permian Low-Angle Normal Faults (LANFs) are well-preserved because they exceptionally escaped most of the Alpine deformations. Their surfaces are within the Lower Permian sedimentary cover, or at the interface between the sedimentary cover and the Variscan basement, passing to intra-basement shear zones.
Two major Permian LANFs (Aga-Vedello and Masoni faults) are recorded in the Pizzo del Diavolo Fm. along the northern border of the Permian Orobic Basin (N Italy). They are “non-Andersonian” normal faults whose surfaces are characterized by cataclastic bands usually sealed by centimetric to metric layers of dark grey to black aphanitic tourmalinites (Zanchi et al., 2019). Tourmalinites indicate fluids circulation channelled along high permeability fault zones and are related to magmatic-hydrothermal fluids that produced metasomatic tourmalines with different compositions at different distances from the fluid source, i.e. the crystallizing intrusive bodies. In addition to Aga-Vedello and Masoni faults, further exposures of Permian LANFs occur in other sectors of the cSA and they are always associated with the presence of tourmalinites. Several authors (De Capitani et al., 1999; Slack et al., 1996; Cadel et al., 1996) link the cSA tourmalinites with the U mineralization of Novazza - Vedello district but this correlation could not be so direct and clear, due to the low concentration of Uranium in tourmalinites coming out from our whole-rock analyses.
The main purpose of this research is to better characterize the entity and the genesis of this regional hydrothermal event and relate it to the role played by the structural setting on hydrothermal circulation in intracontinental extensional settings. Fieldwork and observations combined with microstructural and geochemical analyses of tourmalinites coming from different sectors of the cSA have been performed to reach this goal.

Blom, J. C., & Passchier, C. W. (1997). Geologische Rundschau, 86, 627-636.
Cadel, G., et al. (1996). Memorie di Scienze Geologiche, 48, 1-53.
De Capitani, L., et al. (1999). Periodico di Mineralogia, 68, 185-212.
Handy, M., R., et al. (1999). Tectonics 18, 1154-1177.
Muttoni, G., et al. (2003). Earth Planet Science Letters, 215, 379–394.
Slack, J., F., et al. (1996). Schweiz. Mineral. Petrogr. Mitt., 76, 193-207.
Zanchi A. et al. (2019). Italian Journal of Geosciences, 138, 184-201

How to cite: Locchi, S., Zanchetta, S., Moroni, M., and Zanchi, A.: Interaction between low-angle normal faults and hydrothermal circulation during Early Permian extensional tectonic in the central Southern Alps, N Italy, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3891, https://doi.org/10.5194/egusphere-egu21-3891, 2021.

Abstract：The Zongwulong tectonic belt (ZTB) is located between the northern Qaidam tectonic belt and the south Qilian orogenic belt and contains Late Paleozoic and Early- Middle Triassic strata. Structural features and geochronology of Zongwulong ductile shear zone have key implications for the tectonic property of the ZTB. This study integrated field structure, microscopic structure and ⁴⁰Ar/³⁹Ar laser probe analysis. The shear zone strikes ~NEE-SWW and dips at a high angle, with a NWW-SEE trending and WE stretching lineation, indicating the shear zone as a thrust- slip shear ductile shear. The asymmetric folds, rotating porphyroclast,structural lens and crenulation cleavage can be seen in the field. Mica fish, S − C fabrics, σ type quartz porphyroclastic and quartz wire drawing structure can also be observed under microscope, indicating that the strike- slip- related ductile deformation and mylonitization occurred under low- grade greenschist facies conditions at temperatures of 300° C − 400° C.  The highly deformed
mylonite schist yielded ⁴⁰Ar/³⁹Ar ages (245.8±1.7)Ma and (238.5±2.6)Ma for muscovite and biotite, respectively, indicating that the shear deformation occurred during the Early- Mid Triassic. Combined with comprehensive analysis of regional geology and petrology, the authors hold that the age of ductile shear deformation represents the time of Triassic orogeny in the ZTB. The oroginic activity was probably related to the oblique collision between the South Qilian block and the Oulongbuluke block after the closure of the northermost Paleo-Tethys ocean.

How to cite: Gao, W. and Wang, Z.: 40Ar/ 39Ar laser dating of Zongwulong ductile shear zone in northeastern Tibetan Plateau ：Constrains on the closure time of the northmost Paleo-Tethys ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6843, https://doi.org/10.5194/egusphere-egu21-6843, 2021.

The Ivrea – Verbano Zone (IVZ) is a virtually complete lower-to-middle continental crustal section exposed in the Western Italian Alps in result of exhumation processes during the Alpine orogenic cycle. To the northwest, the IVZ is juxtaposed to the basement of the Austro-Alpine Domain by the lnsubric Line; to the southeast, it is separated from the middle-to-upper crustal levels of the Strona – Ceneri Zone by the Pogallo and the Cossato-Mergozzo-Brissago (CMB) lines. The IVZ crustal section is constituted by two main units: the Kinzigite Formation, amphibolite- to granulite-facies sedimentary and igneous metamorphic rocks, and the Mafic Complex, a thick, composite gabbroid-to-dioritic intrusion.

Additionally, the lower crustal rocks of IVZ embed a series of kilometre-scale peridotite bodies; Baldissero, Balmuccia and Finero are the most relevant. These peridotites are thought to represent remnants of the oldest portion of subcontinental lithospheric mantle (SCLM) beneath Europe. Geochemical and isotopic studies indicate that peridotitic bodies experienced an Upper Devonian partial melting event followed by protracted enrichments while resident in the mantle. Field and structural relationships coupled with radiometric dating suggest that the emplacement of the mantle peridotite bodies at crustal levels has occurred since the end of the Variscan orogeny, prior to the intrusion of the Mafic Complex.

The Balmuccia Massif is dominated by fresh spinel lherzolites recording moderate degrees of melt extraction, subordinated harzburgites, reactive dunites and diffuse cross-cutting websteritic dykes. The melt extraction and melt-fluid/rock-reactions preserved in the Balmuccia peridotite, together with the lack of substantial low-temperature alteration, enable to track the evolution of the SCLM prior to its uplift and emplacement in crust. Therefore, reconstructing the density structure of the Balmuccia body could have major implications on the comprehension of the geodynamic evolution of the oldest portions of the European lithospheric mantle.

In this study, we modelled the density structure of the spinel lherzolite from the Balmuccia Massif, starting from the chemical composition and modal abundance of its main phase constituents. It is well known that the bulk density is function of modes, compositions and elastic properties of constituent minerals and can be explored from the perspective of their Equations of State (EoS) (see also Faccincani et al., 2021, abstract to session GD7.3 for a more holistic view of the density structure of the lithospheric mantle). By assuming that the EoS for a polyphase aggregate (e.g., a rock) may be calculated as weighted mean of the EoS of the constituting minerals (in our case olivine, orthopyroxene, clinopyroxene, spinel and garnet at increasing depths), we investigated the density structure of a virtual 1-D vertical profile of the lithospheric mantle below the IVZ at pre-Variscan ages.

How to cite: Faccincani, L., Casetta, F., Faccini, B., Mazzucchelli, M., Nestola, F., and Coltorti, M.: Density model of the Permo – Triassic lithospheric mantle of the Ivrea Verbano Complex, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10596, https://doi.org/10.5194/egusphere-egu21-10596, 2021.

Zircon is a common accessory mineral in evolved magmatic rocks and its investigation can provide unevaluable geochronological and geochemical information. The lower continental crust forming the Ivrea-Verbano Zone (IVZ, Southern Alps) locally shows the discordant intrusion of swarms of felsic dykes, which petrology was poorly constrained. Corundum-rich (Crn up to 55 vol.%) felsic dykes were sampled in two different outcrops along the Sabbiola valley (central IVZ). Besides corundum, they consist mainly of sodic plagioclase (An=5-10 %), biotite-siderophyllite, ±K-feldspar and ±hercynite. These dykes intrude granulites and Permian mafic intrusives, showing either pegmatite-like or porphyroclastic textures and contain abundant zircon. Trace elements concentration, as well as the isotopic U-Pb and Lu-Hf compositions of zircons have been determined by LA-ICP-(MC)MS to unravel emplacement ages and nature of parental melts. U-Pb weighted average ages point to Norian emplacement (ca. 224 Ma). Zircons are characterized by very high concentrations in REE, Th, U, Nb and Ta. REE patterns show marked negative Eu anomaly. These data, in association with the enrichments of Na in plagioclases and of Fe in micas and oxides, suggest that the parent melts were extremely evolved differentiates. Porphyroclastic texture developed in the frame of magmatic processes due to volatiles overpressure. Strongly positive Hf_(t) values (+13 on average) suggest a derivation of the parental melts from depleted to mildly enriched mantle sources. This observation and the corundum saturation (evidence for low silica activity) point to limited crustal contamination, which was favored by the high eutectic temperature of the host rocks. It is proposed that studied dykes segregated from peraluminous melts produced by exsolution processes affecting volatile-rich differentiates during alkaline magmatism (Bonazzi et al., 2020).

Triassic magmatic activity is largely documented throughout the Southern Alps, being related to different tectono-magmatic cycles. Nevertheless, before this study, the evidence of Triassic magmatism in IVZ was restricted only in its northernmost tip (Finero area, e.g. Zanetti et al., 2013; Schaltegger et al., 2015). This work provides robust constraints about the transition of the geochemical affinity of Southern Alps magmatism from orogenic-like to anorogenic during Norian, linked to a regional uprising of the asthenosphere and changes of tectonic regime.

References

Bonazzi, M.; Langone, A.; Tumiati, S.; Dellarole, E.; Mazzucchelli, M.; Giovanardi, T.; Zanetti, A. Mantle-Derived Corundum-Bearing Felsic Dykes May Survive Only within the Lower (Refractory/Inert) Crust: Evidence from Zircon Geochemistry and Geochronology (Ivrea–Verbano Zone, Southern Alps, Italy). Geosciences 2020, 10, 281.

Schaltegger, U.; Ulianov, A.; Muntener, O.; Ovtcharova, M.; Peytcheva, I.; Vonlanthen, P.; Vennemann, T.; Antognini, M.; Girlanda, F. Megacrystic zircon with planar fractures in miaskite-type nepheline pegmatites formed at high pressures in the lower crust (Ivrea Zone, southern Alps, Switzerland). Am. Miner. 2014, 100, 83–94.

Zanetti, A.; Mazzucchelli, M.; Sinigoi, S.; Giovanardi, T.; Peressini, G.; Fanning, C.M. SHRIMP U-Pb Zircon Triassic Intrusion Age of the Finero Mafic Complex (Ivrea-Verbano Zone, Western Alps) and its Geodynamic Implications. J. Pet. 2013, 54, 2235–2265.

How to cite: Bonazzi, M., Langone, A., Tumiati, S., Dellarole, E., Mazzucchelli, M., Giovanardi, T., and Zanetti, A.: Corundum-rich dykes constraining Triassic alkaline magmatism in the Ivrea-Verbano Zone (Southern Alps): a zircon approach, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3502, https://doi.org/10.5194/egusphere-egu21-3502, 2021.

The Ivrea-Verbano Zone (IVZ, westernmost sector of the Southern Alps) represents a unique opportunity to investigate the Paleozoic to Mesozoic geodynamic evolution of the Gondwana and Laurasia boundary from the perspective of the lower continental crust. Only recently, the petrochemical record of Triassic-Jurassic magmatism has been recognized. It mainly affected the northernmost tip, the Finero Complex, where the continental crust was tectonically thinned before opening of Alpine Tethys. However, the Mesozoic magmatism in the Finero Complex is still poorly-constrained. Firstly, its extent is largely unknown, because the mantle and crustal intrusives were already enriched by Paleozoic processes. Secondly, Mesozoic melts migration started when the Finero Complex was still placed at P-T conditions typical of a continental crust-mantle transition (1 GPa): this has promoted the reopening of the geochronological clocks in both Paleozoic and Mesozoic rocks, which usually provides wide time intervals. Lastly, the finding of Mesozoic magmatism as composite veins/pods and metasomatised layers has not allowed an exhaustive reconstruction of the primitive melts geochemistry. To place further constraints on such issue, a new dyke swarm cropping out in the Finero Phlogopite Peridotite mantle unit has been investigated. Dykes usually cut at high angle the mantle foliation and are up to 60 cm thick. They are composed by coarse-grained hornblendite to anorthosite, both phlogopite/biotite-bearing. Many dykes are composite, showing variable proportions of hornblendite and anorthosite. In places, the dyke swam was affected by volatiles overpressure as late magmatic stage, which produced plastic flow and development of a porphyroclastic structure by deformation of the early cumulates, with widespread segregation of a fine-grained mica matrix.

Dykes mainly consist of pargasite, phlogopite/biotite, albite (An 8-10), in association with apatite, monazite, ilmenite, zircon, Nb-rich oxides, carbonates. Enrichments in Fe (amphibole and biotite) and Na (plagioclase) suggest segregation from evolved melts, strongly enriched in H₂O, P, C. The large LILE and LREE contents in amphiboles, sometimes associated to high Nb, Ta, Zr and Hf concentrations, as well as the mineral assemblage, support an alkaline affinity of the melts. The strongly positive εHf_t (+10) of zircons and the isotopic Sr composition of amphiboles (0.7042) point to a derivation of the melts from mildly enriched sources, possibly located at the crust-mantle interface.

Zircons from anorthosite layers are mostly anhedral fragments. They show homogenous internal structure or sector zoning. Concordant ²⁰⁶Pb/²³⁸U zircon ages vary from 221 ± 9 Ma to 192 ± 8 Ma. The results of this study confirm that mantle input to the Southern Alps magmatism was of alkaline affinity from Norian to Sinemurian. A widespread fluids circulation induced by such magmatism at high P-T conditions was likely the main cause of the diffuse geochronological reset towards Mesozoic ages of the northern IVZ.

How to cite: Ogunyele, A. C., Giovanardi, T., Bonazzi, M., Mazzucchelli, M., and Zanetti, A.: Geochemistry and geochronology of alkaline dykes from the Finero Phlogopite Peridotite (Ivrea-Verbano Zone): insights into the Triassic-Jurassic tectono-magmatic events of the Southern Alps, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10286, https://doi.org/10.5194/egusphere-egu21-10286, 2021.