The Gondwana-Laurasia boundary was subjected to a complex geodynamic evolution between Late Paleozoic and Early Mesozoic, typified by multiple magmatic cycles developed under different tectonic and thermal regimes. A variety of mantle sources was involved in these tectonic, magmatic and metamorphic events, which induced significant modifications of the continental crust.
In the last decade, detailed studies in petrology, tectonics and stratigraphy have contributed on shedding light on the articulated evolution of this area, stimulating an intense debate about the overall Permo-Triassic geodynamic framework.
A multidisciplinary session is proposed to assess and discuss the recent advancements that contribute to draw an accurate geodynamic picture of this pivotal sector of the Pangea realm in the time span between the Variscan orogeny and the Late Triassic onset of rifting in the Central Atlantic-Alpine Tethys domain. Researches from a broad range of disciplines, such as (but not limited to) petrology/geochemistry, tectonics, geochronology, stratigraphy and basin analysis, are welcome.
vPICO presentations: Wed, 28 Apr
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.
The occurrence of synsedimentary tectonics during the beginning of the Permian has been largely documented all cross the present-day region of the central Southern Alps. Evidence of active faults has been generally established based on facies variations often associated to coarse-grained deposits, a characteristic feature of the Laghi Gemelli Group, which was deposited during the Early Permian. Nevertheless, poor attention has been devoted to the reconnaissance and description of the mesoscopic fault record developed during the deposition of the Lower Permian successions, except for a few works (Berra et al., 2011) describing local synsedimentary features such as liquefaction or slumping due to seismic shaking.
Working across the Orobic Alps, we identified several key areas where the occurrence of dewatering structures testify to the activity of synsedimenary faults together with sedimentary dikes, ball and pillars, and small slumps occurring along hundreds of mesoscopic faults showing meter-scale displacement along high-angle conjugate systems as well as domino-style faults, often accompanied by growth structures. These faults mainly affect the Pizzo del Diavolo Formation, which was deposited on top of the volcaniclastic succession of the Ca’ Bianca Volcanite.
According to our structural observations, these high-angle Andersonian normal faults are often associated with low-angle normal faults, which developed along the interface between the Permian cover and the Variscan basement (Bloom & Passchier, 1997; Zanchi et al., 2019). LANF systems are responsible for significant tectonic elision of the volcaniclastic lower successions and for diffuse hydrothermal circulation, resulting in widespread tourmaline deposition along the fault surfaces.
Our analyses point to the definition of tectonic setting characterized by pure extension dominated by ENE-WSW striking normal faults all across the central Southern Alps, which were later inverted during the Alpine shortening as high-angle reverse faults (Zanchetta et al., 2015). It is important to stress that in the considered area the strikes of the Early Permian structure are at odds with the Early Jurassic normal faults which generally show a N-S strike and were reactivated as strike-slip faults, pointing to an independent tectonic extensional event occurring 80 My after the Permian extension.
Berra F. et al. (2011). Sedimentary Geology, 235, 249-263
Blom, J. C., & Passchier, C. W. (1997). Geologische Rundschau, 86, 627-636.
Zanchetta et al. (2015). Lithosphere, 7, 662-681.
Zanchi A. et al. (2019). Italian Journal of Geosciences, 138, 184-201.
How to cite: Zanchi, A., Locchi, S., and Zanchetta, S.: Early Permian syndepositional tectonics in the Orobic Basin, Southern Alps, Italy, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8056, https://doi.org/10.5194/egusphere-egu21-8056, 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.
The long-lived geodynamic evolution of the Permo-Triassic boundary between Laurasia and Gondwana may have created the ideal conditions for the genesis of a trans-continental Ni-Cu-PGE-(Au-Te) mineralised belt in Europe. This working hypothesis stems from the recent understanding that orogenic processes play a fundamental role in the onset of chemical and physical triggers for the transport of metals from the metasomatised mantle through to various crustal levels. An insight into our renewed framework for the polyphased genetic evolution of magmatic sulfide mineral systems is provided by a series of mineralised occurrences in the Ivrea Zone of NW Italy, which formed at multiple stages over a > 80 Ma time interval. Between 290-250 Ma, a series of hydrated and carbonated ultramafic alkaline pipes containing Ni-Cu-PGE-(Te-Au) mineralisation was emplaced in the lower continental crust. At ~200 Ma, a subsequent mineralising event occurred in association with the emplacement of the La Balma-Monte Capio (LBMC) intrusion. Modelling of the LBMC parental magma shows derivation from ~30% partial melting of an anhydrous juvenile mantle at moderate pressure (< 7 GPa). The inferred composition of the parental melt is consistent with magmatism associated with the Central Atlantic Magmatic Province (CAMP). However, its tellurium-enriched composition together with the S-C-O isotope signature of the associated magmatic sulfide mineralisation cannot be reconciled with the CAMP source. It is argued that the geochemical and isotopic signature of the LBMC intrusion reflects interaction and mixing of a primitive magma sourced from a juvenile source with localised domains enriched in carbonate and metal-rich sulfides located in the lower crust, consistent with the composition of the Permo-Triassic pipes. Evidence of this magmatic interaction informs on the first-order processes that control enhanced metallogenic fertility along the margins of lithospheric blocks. The scenario depicted here is consistent with reactivation and enrichment of a Gondwana margin Ni-Cu-PGE-(Te-Au) mineral system during the breakup of Pangea. The lessons learnt in the Ivrea Zone natural laboratory may inform on the genesis of other Permo-Triassic magmatic mineral systems in continental Europe, such as the deposits in north-west Czech Republic and southern Spain, which display significant analogies with their counterparts in the Ivrea Zone. We suggest that these systems may have a common DNA related to a metallogenic belt forming at different stages during the complex evolution and multi-phase activation of the margin between Laurasia and Gondwana. The nature and localisation of the magmatic sulfide mineral systems along this belt indicate that enhanced potential for ore formation at lithospheric margins may be due not only to favourable architecture, but also to localised enhanced metal and volatile fertility. Importantly, this hypothesis may explain why ore deposits along the margins of lithospheric blocks are not distributed homogeneously along their entire extension but generally form clusters. As mineral exploration is essentially a search space reduction exercise, this new understanding may prove to be important in predictive exploration targeting for new mineralised camps in Europe and elsewhere globally, as it provides a way to prioritise segments with enhanced fertility along extensive lithospheric block margins.
How to cite: Fiorentini, M., Holwell, D., Moroni, M., Denyszyn, S., Blanks, D., Chong, J., Ince, M., Vymazalová, A., and Hora, J.: The Ni-Cu-PGE-(Au-Te) potential of the Permo-Triassic boundary between Laurasia and Gondwana, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6758, https://doi.org/10.5194/egusphere-egu21-6758, 2021.
Mashhad granitoid complex is part of the northern slope of the Binalood Structural Zone (BSZ), Northeast of Iran, which is composed of granitoids and metamorphic rocks. This research presents new petrological and geochemical whole-rock major and trace elements analyses in order to determine the origin of granitoid rocks from Mashhad area. Field and petrographic observations indicate that these granitoid rocks have a wide range of lithological compositions and they are categorized into intermediate to felsic intrusive rocks (SiO2: 57.62-74.39 Wt.%). Qartzdiorite, tonalite, granodiorite and monzogranite are common granitoids with intrusive pegmatite and aplitic dikes and veins intruding them. Based on geochemical analyses, the granitoid rocks are calc-alkaline in nature and they are mostly peraluminous. On geochemical variation diagrams (major and minor oxides versus silica) Na2O and K2O show a positive correlation with silica while Al2O3, TiO2, CaO, Fe2O3, and MgO show a negative trend. Therefore fractional crystallization played a considerable role in the evolution of Mashhad granitoids. Based on the spider diagrams, there are enrichments in LILE and depletion in HFSE. Low degrees of melting or crustal contamination may be responsible for LILE enrichment. Elements such as Pb, Sm, Dy and Rb are enriched, while Ba, Sr, Nd, Zr, P, Ti and Yb (in monzogranites) are all depleted. LREE enrichment and HREE depletion are observed in all samples on the Chondrite-normalized REE diagram. Similar trends may be evidence for the granitoids to have the same origin. Besides, LREE enrichment relative to HREE in some samples can indicate the presence of garnet in their source rock. Negative anomalies of Eu and Yb are observed in monzogranites. Our results show that Mashhad granitoid rocks are orogenic related and tectonic discrimination diagrams mostly indicate its syn-to-post collisional tectonic setting. No negative Nb anomaly compared with MORB seems to be an indication of non-subduction zone related magma formation. According to the theory of thrust tectonics of the Binalood region, the oceanic lithosphere of the Palo-Tethys has subducted under the Turan microplate. Since the Mashhad granitoid outcrops are settled on the Iranian plate, this is far from common belief that these granitoid rocks are related to the subduction zones and the continental arcs. The western Mashhad granitoids show more mafic characteristics and are possibly crystallized from a magma with sedimentary and igneous origin. Thus, Western granitoid outcrops in Mashhad are probably hybrid type and other granitoid rocks, S and SE Mashhad are S-type. Evidences suggest that these continental collision granitoid rocks are associated with the late stages of the collision between the Iranian and the Turan microplates during the Paleo-Tethys Ocean closure which occurred in the Late Triassic.
How to cite: Vahdati, B. and Mazaheri, S. A.: The petrological and geochemical study of granitoid rocks from Mashhad area, Iran: Evidence for the Late Triassic Collisional Belt Northeast of Iran, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6494, https://doi.org/10.5194/egusphere-egu21-6494, 2021.
Early Permian Post-Variscan magmatism is widespread throughout the Alps and consists mainly of felsic to mafic plutonic and volcanic bodies emplaced between ca. 285 and 275 Ma. This study focuses on the acidic to intermediate intrusions in the areas of Trento and Bolzano/Bozen (North-eastern Italy) like the Cima d’Asta gabbrodiorite/granite, the Pergine granodiorite, the Monte Sabion and the Bressanone (Brixen) granites. New U-Pb zircon data along with ages for the Ivigna (Ifinger) and Monte Croce (Kreuzberg) granites and the Bressanone (Brixen) gabbro constrain the age of the Permian intrusions and Hf isotopic data highlight the interaction between mantle-derived melts and crustal rocks during ascent of the former through the crust. Moreover, the studied intrusions represent the shallow crustal plumbing system of the coeval widespread volcanics of the Athesian Volcanic Group and the mega-caldera of the Bolzano/Bozen supervolcano. This acid intrusive-extrusive magmatism, which identified an elliptic structure of more than 4200 square kilometers, represents the biggest magmatic event outcropping in the Southern Alps and likely influenced the ecosystems of the Athesian Volcanic District and of the dolomitic area l.s. during the Permian.
How to cite: Boscaini, A., Davies, J. H. F. L., Sassi, R., Mazzoli, C., Callegaro, S., De Min, A., and Marzoli, A.: The igneous plumbing system of the Early Permian Bolzano/Bozen supervolcano (North-eastern Italy), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2685, https://doi.org/10.5194/egusphere-egu21-2685, 2021.
Zoned crystals play a fundamental role in modern volcanology as a key to unravel the geometry and the dynamics of plumbing systems. Ancient volcano-plutonic complexes, nowadays exposed at the surface, can sometimes preserve textural-chemical record of such dynamics inside their constituting mineral phases. This is the case of the Cima Pape Middle Triassic complex (Dolomites, Southern Alps), which is composed by a 50 to 300 metres thick gabbroic to monzodioritic sill overlaid by basaltic to trachyandesitic volcanites with high Porphyricity Index (P.I. 43-48 %).
Volcanites contain a large number of concentric-zoned clinopyroxenes, while intrusive rocks are mostly made up of homogeneous and unzoned crystals. In volcanites, the typical clinopyroxene zoning pattern consists of one or more high-Mg# and high Cr2O3 bands (Mg# 84-91; Cr2O3 up to 1.2 wt%) with variable thickness, formed between cores and rims with relatively lower Mg# and Cr contents (Mg# 70-77; Cr2O3 <0.1 wt%). Chondrite-normalized incompatible element patterns of low-Mg# portions show Nb, Ta, Sr, Zr and Ti negative anomalies and Th-U positive peaks, while high-Mg# bands have a generally more depleted patterns maintaining similar profile. REE patterns in both high-Mg# and low-Mg# domains have a convex-upward shape and La/YbN from 1.3 to 2.1. Thermobarometric calculations reveal that the high-Mg# bands were in equilibrium with a more primitive, hotter and more H2O depleted melt (Mg# = 65-70; T = 1130-1150°C; H2O = 2.1-2.6 wt%) than cores and rims, which likely formed in a colder, H2O-rich evolved melt (Mg# = 43-45; T = 1035-1075°C; H2O = 2.6-3.8 wt%). According to our model, a first crystallization stage in a high crystallinity (P.I. almost 50%) “mush-type” system led to the formation of low-Mg# clinopyroxenes (Mg# 70-77) at P of 2-4 kbar. The ascent of one or multiple pulses of primitive, hot, and H2O-poor basaltic magmas (Casetta et al., 2020) in the shallower portions of the plumbing system led to the formation of the high-Mg# bands. Later on, re-equilibration of clinopyroxene with the post-mixing melt system resulted in the formation of the low-Mg# rims. Cima Pape products have many textural-chemical similarities with those reported at the active Stromboli volcano, suggesting that they were formed through similar dynamics at comparable T-P conditions (Petrone et al., 2018; Di Stefano et al., 2020). The peculiarity of clinopyroxene texture in Cima Pape rocks allowed us to study the processes occurred in the plumbing system beneath an ancient volcano and offered the opportunity to test the approaches/models currently adopted for active systems.
Casetta, F., et al., 2020. The Variscan subduction inheritance in the Southern Alps Sub-Continental Lithospheric Mantle: Clues from the Middle Triassic shoshonitic magmatism of the Dolomites (NE Italy). Lithos, 105856.
Di Stefano, F., et al., 2020. Mush cannibalism and disruption recorded by clinopyroxene phenocrysts at Stromboli volcano: New insights from recent 2003–2017 activity. Lithos, 360–361.
Petrone, C. M., et al., 2018. Rapid mixing and short storage timescale in the magma dynamics of a steady-state volcano. Earth and Planetary Science Letters, 492, 206–221.
How to cite: Nardini, N., Casetta, F., Giacomoni, P. P., and Coltorti, M.: Magma ascent, ponding and mixing in a Middle Triassic plumbing system: clues from clinopyroxene chemical-textural features in the Cima Pape volcano-plutonic complex (Southern Alps, Italy), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9924, https://doi.org/10.5194/egusphere-egu21-9924, 2021.
The Mid-Triassic emplacement of shoshonitic magmas at the NE margin of the Adria plate in concomitance with extensional/transtensional tectonics is one of the most intriguing and peculiar aspects typifying the geodynamic evolution of the Western Tethyan realm. Although often hypothesized, the link between this magmatic event and the metasomatised Southern Alps Sub-Continental Lithospheric Mantle (SCLM) has never been constrained.
Geochemical and petrological analyses of lavas, dykes and ultramafic cumulates belonging to the shoshonitic magmatism of the Dolomites, coupled with pre-existing data on peridotite massifs (i.e. Finero, Balmuccia, Baldissero), were used to reconstruct the evolution of the Southern Alps SCLM between Carboniferous and Triassic. According to our model, a metasomatised amphibole + phlogopite-bearing spinel lherzolite, similar to the Finero phlogopite peridotite and likely generated by interaction between a depleted mantle and slab-derived components during the Variscan subduction, was able to produce magmas with orogenic-like affinity during Mid-Triassic. In this context, partial melting degrees of ca. 5-7% were required for producing primitive SiO2-saturated to -undersaturated melts with shoshonitic affinity (87Sr/86Sri = 0.7032-0.7058; 143Nd/144Ndi = 0.51219-0.51235; Mg #~ 70; ~1.1 wt% H2O). As testified by the H2O content in mineral phases from the Finero phlogopite peridotite (Tommasi et al., 2017), the modelled Mid-Triassic fertile lithospheric mantle could have been able to preserve a significant enrichment and volatile content (600-800 ppm H2O) for more than 50 Ma, i.e. since the Variscan subduction-related metasomatism. During the Mid-Triassic partial melting event, the modelled Finero-like mantle exhausted the subduction-related signature inherited during the Variscan subduction. Around 20 Ma later, the same lithosphere portion was affected by an asthenospheric upwelling event related to the Late Triassic-Early Jurassic opening of the Alpine Tethys (Casetta et al., 2019).
Casetta, F., Ickert, R.B., Mark, D.F., Bonadiman, C., Giacomoni, P.P., Ntaflos, T., Coltorti, M., 2019. The alkaline lamprophyres of the Dolomitic Area (Southern Alps, Italy): markers of the Late Triassic change from orogenic-like to anorogenic magmatism. Journal of Petrology 60(6), 1263-1298.
Tommasi, A., Langone, A., Padrón-Navarta, J.A., Zanetti, A., Vauchez, A., 2017. Hydrous melts weaken the mantle, crystallization of pargasite and phlogopite does not: Insights from a petrostructural study of the Finero peridotites, Southern Alps. Earth and Planetary Science Letters 477, 59-72.
How to cite: Casetta, F., Coltorti, M., Ickert, R. B., Mark, D. F., Giacomoni, P. P., Bonadiman, C., Ntaflos, T., and Zanetti, A.: From the Finero phlogopite peridotite to the shoshonitic magmatism of the Dolomites: unveiling the evolution of the Sub-Continental Lithospheric Mantle beneath the Southern Alps (Northern Italy), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10137, https://doi.org/10.5194/egusphere-egu21-10137, 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.
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 εHft (+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.
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