Subduction and Exhumation of the Lithosphere: The Contribution of Structural Geology, Petrology and Geochronology

A special issue of Geosciences (ISSN 2076-3263). This special issue belongs to the section "Structural Geology and Tectonics".

Deadline for manuscript submissions: closed (20 September 2020) | Viewed by 16182

Special Issue Editors

Dipartimento di Scienze della Terra, Università di Torino, Via Valperga Caluso 35, 10125 Torino, Italy
Interests: tectonics; structural geology; shear zone; fluids and deformation; shear zone and mineralizations; veins, petrochronology; Himalaya; Variscan belt; northern Apennines
Special Issues, Collections and Topics in MDPI journals
Dipartimento di Scienze della Terra, Università degli Studi di Torino, 10125 Turin, Italy
Interests: structural geology; tectonics; orogenic belts; regional geology; shear zones; petrochronology; metamorphic petrology; Himalaya; Southern European Variscan belt

Special Issue Information

Dear colleagues,

Deformed rocks, showing the overprinting of several tectonic and metamorphic episodes, commonly record different stages of the subduction and exhumation of the lithosphere. Such rocks are frequently exposed in crystalline basement complexes and carry fundamental insights on processes and their rates, acting during the evolution of the Lithosphere. Starting from the pioneering studies carried out on mountain belts (e.g., Alps, Scottish Highlands, and Himalaya), different approaches and techniques were developed to decipher and constrain this complex tectono-metamorphic history.

Presently, the achieved progress and knowledge stress the fundamental role played by a multidisciplinary approach for the study of basement rocks.

Detailed structural–geological field mapping, integrated with meso- and microstructural investigations, petrofabric analysis, petrochronology, and petrologic modeling are fundamental tools to infer helpful information on the pressure (P)–temperature (T)–deformation (D)–composition of the system (X)–time (t)–fluid activity (a) path of crystalline rocks. All this information is fundamental for the building up of tectonic models.

In this Special Volume of Geosciences, contributions dealing with structural–geological mapping, microstructural analyses, metamorphic petrology, geochronology, and thermochronology concerning basement rocks are welcome.

Prof. Dr. Chiara Montomoli
Dr. Salvatore Iaccarino
Dr. Antonio Langone
Guest Editors

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Keywords

  • tectonics
  • microstructures
  • metamorphic rocks
  • geochronology and thermochronology
  • P–T–D–X–t path

Published Papers (5 papers)

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Research

21 pages, 25438 KiB  
Article
Permian Hydrothermal Alteration Preserved in Polymetamorphic Basement and Constraints for Ore-genesis (Alpi Apuane, Italy)
Geosciences 2020, 10(10), 399; https://doi.org/10.3390/geosciences10100399 - 05 Oct 2020
Cited by 5 | Viewed by 3136
Abstract
The reconstruction of the polymetamorphic history of basement rocks in orogens is crucial for deciphering past geodynamic evolution. However, the current petrographic features are usually interpreted as the results of the metamorphic recrystallization of primary sedimentary and/or magmatic features. In contrast, metamorphic rocks [...] Read more.
The reconstruction of the polymetamorphic history of basement rocks in orogens is crucial for deciphering past geodynamic evolution. However, the current petrographic features are usually interpreted as the results of the metamorphic recrystallization of primary sedimentary and/or magmatic features. In contrast, metamorphic rocks derived by protoliths affected by pre-metamorphic hydrothermal alterations are rarely recognized. This work reports textural, mineralogical and geochemical data of metasedimentary and metaigneous rocks from the Paleozoic succession of the Sant’Anna tectonic window (Alpi Apuane, Tuscany, Italy). These rocks were recrystallized and reworked during the Alpine tectono-metamorphic event, but the bulk composition and some refractory minerals (e.g., tourmaline) are largely preserved. Our data show that the Paleozoic rocks from the Alpi Apuane were locally altered by hydrothermal fluids prior to Alpine metamorphism, and that the Permian magmatic cycle was likely responsible for this hydrothermal alteration. Finally, the Ishikawa Alteration Index, initially developed for magmatic rocks, was applied to metasedimentary rocks, providing a useful geochemical tool for unravelling the hydrothermal history of Paleozoic rocks, as well as a potential guide to the localization of hidden ore deposits in metamorphic terranes. Full article
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20 pages, 8703 KiB  
Article
Garnet-Rich Veins in an Ultrabasic Amphibolite from NE Sardinia, Italy: An Example of Vein Mineralogical Re-Equilibration during the Exhumation of a Granulite Terrane
Geosciences 2020, 10(9), 344; https://doi.org/10.3390/geosciences10090344 - 31 Aug 2020
Cited by 4 | Viewed by 2953
Abstract
A complex system of mono- and polymineralic centimeter-thick veins occurs within the ultrabasic amphibolites of Montigiu Nieddu hill in northeastern Sardinia, and they are filled with garnet, amphibole, chlorite, and epidote. Some garnet-rich veins are margined by an amphibole layer at the interface [...] Read more.
A complex system of mono- and polymineralic centimeter-thick veins occurs within the ultrabasic amphibolites of Montigiu Nieddu hill in northeastern Sardinia, and they are filled with garnet, amphibole, chlorite, and epidote. Some garnet-rich veins are margined by an amphibole layer at the interface with the host rock and/or show replacement of epidote concentrated in the vein core. Together with homogeneous matrix garnet (Grt1), millimetric, euhedral, and strongly zoned garnet porphyroblasts occur within these veins. The estimated pressure–temperature conditions (P = 1.0–1.7 GPa, T = 650–750 °C) for the formation of Grt1 match the metamorphic peak and early exhumation derived previously for the host rocks and confirm that the garnet veins also formed under high-pressure (HP) conditions. The igneous protolith of the host rocks experienced HP metamorphism in a subduction zone and underwent exhumation in an exhumation channel. The vein system in the ultrabasic amphibolites formed by cyclic hydrofracturing as rapid and transient events such as crack-seal veining. The growth of multiple vein-filling mineral assemblages indicates the formation of separate vein-producing cycles. Full article
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29 pages, 10442 KiB  
Article
Submagmatic to Solid-State Deformation Microstructures Recorded in Cooling Granitoids during Exhumation of Late-Variscan Crust in North-Eastern Sicily
Geosciences 2020, 10(8), 311; https://doi.org/10.3390/geosciences10080311 - 13 Aug 2020
Cited by 12 | Viewed by 2797
Abstract
Late-Variscan granitoid rocks of trondhjemitic and granitic composition, intruded in migmatitic paragneisses in the north-eastern Peloritani Mountains (southern Italy) at ~310 Ma and ~300 Ma, respectively, exhibit a range of deformation microstructures developed under a shear regime at decreasing temperatures. Non-coaxial deformation is [...] Read more.
Late-Variscan granitoid rocks of trondhjemitic and granitic composition, intruded in migmatitic paragneisses in the north-eastern Peloritani Mountains (southern Italy) at ~310 Ma and ~300 Ma, respectively, exhibit a range of deformation microstructures developed under a shear regime at decreasing temperatures. Non-coaxial deformation is documented by sigmoidal feldspar porphyroclasts, mica fish, and asymmetric boudins affecting tiny andalusite crystals. Late-Variscan shearing during granitoid cooling is constrained by largely represented chessboard patterns in quartz and, especially, submagmatic fractures in plagioclase, indicating deformation at high-temperature conditions (T > 650 °C), in the presence of melt. Submagmatic deformation was extensively superseded by deformation at lower temperatures. Examples of solid state-high temperature deformation-related microstructures (T > 450 °C) include feldspar bulging, quartz grain boundary migration, and subgrain rotation recrystallization. Finally, low temperature subsolidus microstructures (T < 450 °C) consist of quartz bulging, mica kinks, and feldspar twinning and bending. A complete sequence of deformation, operating from submagmatic to low-temperature subsolidus conditions is recorded in both the older and younger granitoids, suggesting a duration of ~20 Ma for shear zone activity during post-collisional exhumation of the Variscan middle crust in southernmost Italy. Full article
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29 pages, 10058 KiB  
Article
Kinematics and Timing Constraints in a Transpressive Tectonic Regime: The Example of the Posada-Asinara Shear Zone (NE Sardinia, Italy)
Geosciences 2020, 10(8), 288; https://doi.org/10.3390/geosciences10080288 - 29 Jul 2020
Cited by 25 | Viewed by 3548
Abstract
Detailed geological field mapping, integrated with meso- and microstructural investigations, kinematic of the flow and finite strain analyses, combined with geochronology, are fundamental tools to obtain information on the temperature–deformation–timing path of crystalline rocks and shear zone. The Posada-Asinara shear zone (PASZ) in [...] Read more.
Detailed geological field mapping, integrated with meso- and microstructural investigations, kinematic of the flow and finite strain analyses, combined with geochronology, are fundamental tools to obtain information on the temperature–deformation–timing path of crystalline rocks and shear zone. The Posada-Asinara shear zone (PASZ) in northern Sardinia (Italy) is a steeply dipping km-thick transpressive shear zone. In the study area, located in the Baronie region (NE Sardinia), the presence of mylonites within the PASZ, affecting high- and medium-grade metamorphic rocks, provides an opportunity to quantify finite strain and kinematic vorticity. The main structures of the study area are controlled by a D2 deformation phase, linked to the PASZ activity, in which the strain is partitioned into folds and shear zone domains. Applying two independent vorticity methods, we detected an important variation in the percentage of pure shear and simple shear along the deformation gradient, that increases from south to north. We constrained, for the first time in this sector, the timing of the transpressive deformation by U–(Th)–Pb analysis on monazite. Results indicate that the shear zone has been active at ~325–300 Ma in a transpressive setting, in agreement with the ages of the other dextral transpressive shear zones in the southern Variscan belt. Full article
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32 pages, 7131 KiB  
Article
Porphyroclasts: Source and Sink of Major and Trace Elements During Deformation-Induced Metasomatism (Finero, Ivrea-Verbano Zone, Italy)
Geosciences 2020, 10(5), 196; https://doi.org/10.3390/geosciences10050196 - 21 May 2020
Cited by 10 | Viewed by 2974
Abstract
Petrographic and geochemical data for mylonites from a metric-scale shear zone in mantle peridotites from the Finero massif (Southern Alps) record large mineralogical and geochemical modifications compared to surrounding coarse-grained ultramafic rocks, which were pervasively deformed in presence of hydrous melts. The mylonites [...] Read more.
Petrographic and geochemical data for mylonites from a metric-scale shear zone in mantle peridotites from the Finero massif (Southern Alps) record large mineralogical and geochemical modifications compared to surrounding coarse-grained ultramafic rocks, which were pervasively deformed in presence of hydrous melts. The mylonites are composed by olivine and orthopyroxene and, less frequently, clinopyroxene, phlogopite, and pargasite porphyroclasts enclosed in a fine-grained matrix of phlogopite and olivine, with subordinate amounts of orthopyroxene, clinopyroxene, pargasite, and chromite. P-T estimates indicate that deformation occurred under granulite- to upper-amphibolite facies conditions. Field relationships and U-Pb dating indicate that the shear zone was active during Lower Jurassic and/or later, in an extensional setting at the western margin of the Adria plate, which led to the opening of the Alpine Tethys. The major and trace element composition of the porphyroclasts in the mylonites significantly differ from those in the hosting coarse-grained ultramafics. Porphyroclasts were chemically active during deformation acting as source (diffusion-out) or sink (diffusion-in) for some trace elements. The chemical modifications were promoted by the interaction with aqueous fluids and the composition varied from mantle- (enriched in Ni, Co, Li, Na, REE, Y, and Sr) to crustal-derived (enriched in Zn, K, Al, Ti, and Fe). Full article
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