During the Early Permian, the post-Variscan evolution of the present-day Alpine region was characterized by crustal extension combined with strong magmatic activity at different crustal levels, which finally led to the development of intracontinental extensional basins filled with volcanoclastic sediments (e. g. the Orobic Basin, N Italy). In the central Southern Alps (cSA), the opening of these basins was controlled by complex fault system that were active at the same time of plutons intrusion and volcanic activity at the surface. Relationships among magmatism, tectonics and hydrothermal activity related to the formation of ore deposits in the Early Permian so far have been only briefly addressed. This Ph.D. research project focuses on the investigations of the above described features, trying to consider the geological processes active in the Early Permian in an integrated scenario. Several Early Permian faults of the Orobic Basin have been investigated with special emphasis on the recognition of their original features, as they have exceptionally escaped most of the Alpine deformation. In the past, the architecture of Permian basins was described as horst-and-graben structures, formed in response to wrench tectonics developed during the activity of a megashear zone that led to the Pangea B to Pangea A transformation after the collapse of the Variscan orogen. However, thanks recent studies, a different fault architecture has been suggested to had controlled the opening of the Permian basins: a combination of Low-Angle Normal faults and High-Angle Normal Faults. The identified LANFs of the Orobic Basin represent the best site to study the interplay among tectonics and magmatism, as they are characterized by cataclastic bands sealed with cm to dm thick layers of dark, aphanitic tourmalinites. These latter are proof of fluids circulation channelled along higher permeability fault zones related to opening of the Orobic Basin. Such tourmaline breccias also crop out in the Trompia Valley (BS): several authors suggested that tourmalinites from Orobic Alps, tourmalinites from Trompia Valley together with uranium mineralization of Novazza - Vedello Valley are genetically linked. They are seen as products of a large-scale high-temperature hydrothermal system coeval with the Early Permian plutonic-volcanic activity and tectonism, which was also responsible for the emplacement of various types of magmatic-hydrothermal ore deposits in the Southern Alps (Sn-W, U-Mo-Zn, Fe carbonates, sulphides, quartz). However, their genesis has never been fully characterized and the connection between tourmalinites and U ore bodies has also not been deeply investigated so far. The tourmalinized faults were first noted in various sites of the cSA during the 90’s, when the tectonic importance of LANFs was not yet recognized. In this thesis, all the occurrences of tourmalinites are characterized, looking for the cause of the regional hydrothermalism in the context of intracontinental extension during the Early Permian. Furthermore, due to the likely connection with U ore deposits, the borosilicate study is proposed as tool for better understanding the genesis of the mineralizations. New field based structural analysis are combined with mineral and whole-rock geochemistry, geochronology, microstructural studies and boron- isotopic analysis of tourmalinites from different sectors of the study area, in order to evaluate the origin of these fluids. Results coming out from this study demonstrate, together with B isotope ratios, a temporal and genetical relationship between tourmalinites and Early Permian magmatism in the cSA. Furthermore, the geochemical data on trace elements provide more clues on a direct connection between tourmalinites and the U-mineralization. All these results are finally discussed in the frame of the interplay between tectonic, magmatic and ore generation processes that interested the present day cSA area in the Early Permian

Durante il Permiano Inferiore, l’evoluzione post-Varisica dell’attuale regione Alpina è stata caratterizzata da una importante estensione crostale combinata con intensa attività magmatica, che ha favorito lo sviluppo di bacini estensionali intra-continentali con deposizione di sedimenti vulcanoclastici (e.g. Bacino Orobico). Nel settore centrale delle Alpi Meridionali, l’apertura di questi bacini era controllata da complessi sistemi di faglia, attivi contemporaneamente con l’intrusione di plutoni e l’attività vulcanica in superficie. Il seguente progetto di dottorato si focalizza sullo studio di queste caratteristiche, con l’intento di approfondire i processi geologici attivi durante il Permiano. Diverse faglie di età Permiana sono state indagate nel Bacino Orobico, con particolare enfasi sul riconoscimento dei loro tratti originali, poiché esse sono eccezionalmente sfuggite alla deformazione Alpina. Nel passato, l’architettura dei bacini Permiani era stata descritta con strutture a horst-graben, formatesi in risposta ad una tettonica transtensiva controllata da una zona di taglio che ha portato alla trasformazione da Pangea B a Pangea A. Tuttavia, studi più recenti hanno indicato un diverso setting di faglie che hanno controllato l’apertura dei bacini Permiani: si tratta di una combinazione di faglie normali a basso e alto angolo. Le faglie normali a basso angolo identificate lungo il Bacino Orobico rappresentano il miglior luogo in cui osservare l’interazione tra attività tettonica e magmatismo, poiché i loro nuclei di faglia sono caratterizzati da cataclasiti sigillate da livelli di tormaliniti. Queste ultime testimoniano la circolazione di fluidi arricchiti in boro e incanalati lungo importanti zone di faglia correlate all’apertura del Bacino Orobico. Simili brecce a tormalina affiorano anche in Val Trompia (BS): molti autori hanno suggerito che le tormaliniti delle Alpi Orobie, quelle della Val Trompia unitamente alla mineralizzazione di uranio del distretto di Novazza-Val Vedello potrebbero essere geneticamente correlate. Esse potrebbero essere correlate ad un sistema idrotermale di alta temperatura su ampia scala, coevo con l’attività tettono-magmatica del Permiano Inferiore che è responsabile anche della messa in posto di svariati depositi di minerali metalliferi nelle Alpi Meridionali. Tuttavia, la genesi delle tormaliniti non è mai stata dettagliatamente caratterizzata e la loro connessione con i giacimenti di U finora non è mai stata investigata. Le faglie tormalinizzate sono state riconosciute in molte località del settore centrale delle Alpi Meridionali quando ancora non era nota l’importanza delle faglie normali a basso angolo. In questa tesi di dottorato, vengono caratterizzate tutte le aree con i ritrovamenti di tormaliniti, ricercando la causa dell’idrotermalismo regionale nel contesto di estensione durante il Permiano Inferiore. Inoltre, per via del possibile link con i giacimenti di uranio, lo studio dei borosilicati viene proposto come strumento per meglio caratterizzare la genesi delle mineralizzazioni. Nuove osservazioni di terreno su tormaliniti sono combinate con lo studio della geochimica dei minerali e della roccia totale, insieme alla geocronologia, a studi microstrutturali e analisi isotopiche del B, con lo scopo finale di definire l’origine di questi fluidi arricchiti in boro. I risultati ottenuti da questa ricerca dimostrano che le tormaliniti e il magmatismo Permiano sono in stretta correlazione temporale e genetica. Inoltre, i dati geochimici sugli elementi in tracce forniscono maggiori indizi su una diretta connessione tra tormaliniti e la mineralizzazione a U dell’area di Novazza-Val Vedello. Tutti questi dati, per concludere, sono discussi nel contesto di interazione di processi tettonici, magmatici e di formazione di giacimenti che hanno interessato l’attuale settore centrale delle Alpi Meridionali durante il Permiano Inferiore.

(2023). The Post-Variscan evolution of the central Southern Alps: insights from synchronous fault activity, hydrothermalism and magmatism in the Orobic and Collio Basins. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2023).

The Post-Variscan evolution of the central Southern Alps: insights from synchronous fault activity, hydrothermalism and magmatism in the Orobic and Collio Basins

LOCCHI, SOFIA
2023

Abstract

During the Early Permian, the post-Variscan evolution of the present-day Alpine region was characterized by crustal extension combined with strong magmatic activity at different crustal levels, which finally led to the development of intracontinental extensional basins filled with volcanoclastic sediments (e. g. the Orobic Basin, N Italy). In the central Southern Alps (cSA), the opening of these basins was controlled by complex fault system that were active at the same time of plutons intrusion and volcanic activity at the surface. Relationships among magmatism, tectonics and hydrothermal activity related to the formation of ore deposits in the Early Permian so far have been only briefly addressed. This Ph.D. research project focuses on the investigations of the above described features, trying to consider the geological processes active in the Early Permian in an integrated scenario. Several Early Permian faults of the Orobic Basin have been investigated with special emphasis on the recognition of their original features, as they have exceptionally escaped most of the Alpine deformation. In the past, the architecture of Permian basins was described as horst-and-graben structures, formed in response to wrench tectonics developed during the activity of a megashear zone that led to the Pangea B to Pangea A transformation after the collapse of the Variscan orogen. However, thanks recent studies, a different fault architecture has been suggested to had controlled the opening of the Permian basins: a combination of Low-Angle Normal faults and High-Angle Normal Faults. The identified LANFs of the Orobic Basin represent the best site to study the interplay among tectonics and magmatism, as they are characterized by cataclastic bands sealed with cm to dm thick layers of dark, aphanitic tourmalinites. These latter are proof of fluids circulation channelled along higher permeability fault zones related to opening of the Orobic Basin. Such tourmaline breccias also crop out in the Trompia Valley (BS): several authors suggested that tourmalinites from Orobic Alps, tourmalinites from Trompia Valley together with uranium mineralization of Novazza - Vedello Valley are genetically linked. They are seen as products of a large-scale high-temperature hydrothermal system coeval with the Early Permian plutonic-volcanic activity and tectonism, which was also responsible for the emplacement of various types of magmatic-hydrothermal ore deposits in the Southern Alps (Sn-W, U-Mo-Zn, Fe carbonates, sulphides, quartz). However, their genesis has never been fully characterized and the connection between tourmalinites and U ore bodies has also not been deeply investigated so far. The tourmalinized faults were first noted in various sites of the cSA during the 90’s, when the tectonic importance of LANFs was not yet recognized. In this thesis, all the occurrences of tourmalinites are characterized, looking for the cause of the regional hydrothermalism in the context of intracontinental extension during the Early Permian. Furthermore, due to the likely connection with U ore deposits, the borosilicate study is proposed as tool for better understanding the genesis of the mineralizations. New field based structural analysis are combined with mineral and whole-rock geochemistry, geochronology, microstructural studies and boron- isotopic analysis of tourmalinites from different sectors of the study area, in order to evaluate the origin of these fluids. Results coming out from this study demonstrate, together with B isotope ratios, a temporal and genetical relationship between tourmalinites and Early Permian magmatism in the cSA. Furthermore, the geochemical data on trace elements provide more clues on a direct connection between tourmalinites and the U-mineralization. All these results are finally discussed in the frame of the interplay between tectonic, magmatic and ore generation processes that interested the present day cSA area in the Early Permian
ZANCHETTA, STEFANO
ZANCHI, ANDREA MARCO
Tettonica Permiana; Faglie Normali; Idrotermalismo; Interazione; Giacimento di uranio
Permian tectonics; Normal faults; Hydrothermalism; Process Interplay; U-ore deposit
GEO/03 - GEOLOGIA STRUTTURALE
Italian
15-feb-2023
SCIENZE CHIMICHE, GEOLOGICHE E AMBIENTALI
35
2021/2022
open
(2023). The Post-Variscan evolution of the central Southern Alps: insights from synchronous fault activity, hydrothermalism and magmatism in the Orobic and Collio Basins. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2023).
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Descrizione: The post-Variscan evolution of the central Southern Alps: insights from synchronous fault activity, hydrothermalism and magmatism in the Orobic and Collio Basins
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/10281/404097
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