Garnet mechanical behaviour is of great importance to understand the rheological evolution of rocks within the mantle and the lower crust. Well-constrained natural examples of plastically deformed garnets are scarce; consequently their identification and the physical parameters controlling their occurrence are still debated. In southern Madagascar, a granulitic metamorphic event has developed during a late Panafrican – Cambrian, east-west shortening (570 Ma). This has led to the development of vertical transpressive shear zones. Within these zones, we identified variations in garnet microstructure following the deformation sequence. In order to understand this evolution, we carried out a thorough microstructural description of samples using the following techniques: optical microscopy and SEM imaging, EBSD technique (localized lattice-preferred orientation), TEM for dislocation density, EMP for chemical analyses, as well as crystal size distribution, statistic grain boundary and shapes analyses (Lexa et al., 2005). The hand samples were quartzites or two-feldspars quartzofeldsphatic rocks bearing 10 modal percent of garnet. As strain increases, various garnet textures were observed: Type 1) millimetre-sized rounded garnets bearing two types of inclusions, i.e. elongated quartz ribbons and well oriented sillimanite parallel to the lineation; Type 2) elliptic very elongated and lobed garnets (1 to 8 aspect ratio); Type 3) smaller elongated pinch and swell garnets (1 to 3 aspect ratio); and finally Type 4) rounded small garnets (300 microns in diameter). Type 1 textures are due to multiple nucleation garnets and coalescence controlled by aluminous aggregates (biotite and sillimanite). As strain increases, these large skeleton garnets start to re-crystallise preferentially at the tip of lenticular quartz inclusions, giving Type 2 very elongated garnets with unique CPO. The latter then continues to re-crystallise by sub-grain rotation as underlined by the CPO in situ measurements of new re-orientated grains (Type 3). In the type 4, few large garnets remain and only smaller-sized rounded garnets are left. In these highly deformed rocks, fine sillimanite needles are locally preserved and tilted with respect to the main foliation. All garnets from Type 2 to 4 textures are chemically homogeneous. Data from garnets, quartz, and feldspars are compared for each microstructural type and progressive deformation. The observed microstructures are in accordance with garnet ductility coeval with the deformation of quartz K-feldspar and plagioclase and showing mixing of all phases (random distribution) as well as constant grain size (average diameter 200 microns). Our analyses show that under the high-temperature and dry conditions (850°C) all phases are mechanically active. This indicates convergence of strength minerals marked by contrasting (laboratory derived) rheologies.
The first attempt of pressure-temperature-deformation-time (P-T-d-t) path reconstruction for the Lower Units (Alpine Corsica, France) is presented in this work. The Lower Units represent, together with the Tenda Massif, fragments of the European continental margin involved into the east-dipping Alpine subduction. The new data of thermobarometry applied on metapelites and the new 40Ar/39Ar dating of syn-kinematic muscovite sampled into a metagranitoids allowed to define the P-T conditions and the age of the metamorphism of Venaco Unit, a Lower Unit located in the southernmost sector of the Alpine Corsica. The outcoming scenario indicates that the Venaco Unit reached the baric peak at ≈ 33 km of depth not before the Bartonian time. At 35.7 Ma (i.e., during the middle Priabonian), it was exhumed at shallower structural level (i.e., at ≈ 26 km of depth) mainly through the activation the top-to-the W shear zones. This retrograde path suggests that Venaco Unit experienced a fast exhumation, unlike the Tenda Massif which has already involved into subduction during the Ypresian and stationed at 25-30 km before its exhumation in the Priabonian.
Abstract We document the occurrence of micro-diorite magmatic sills, with magmatic enclaves, in the Ceuta Peninsula within metapelites from the Lower Sebtides units (Internal Rif). All magmatic rocks show a primary magmatic mineralogy and geochemical signature diagnostic for high-K calc-alkaline to shoshonitic island arc magmatism. Moreover, these rocks are significantly affected by secondary metamorphic transformations under greenschist- to amphibolite-facies conditions, regionally dated at c . 21 Ma. Geometric relationships between the sills and the main regional foliation, developed under intermediate-pressure granulite-facies conditions at c . 28 Ma, demonstrate that the sills emplaced during the late stage of this main tectonic event. New U–Pb in situ analyses of monazite performed on the micro-diorite sills provide an age of 20.64 ± 0.19 Ma, coherent with this chronological framework and interpreted as the age of greenschist-facies re-equilibration. The discovery of pre-Miocene high-K calc-alkaline to shoshonitic arc-related magmatism is clearly consistent with the subduction context proposed for the Alboran Basin evolution, according to geophysical investigations. In this framework, the Lower Sebtides units could be considered as part of the upper plate of the subduction system, while the Upper Sebtides must be regarded as the lower subducted plate.
Petrological study on andesitic schists sampled in the Acceglio zone, allows, for the first time, the Alpine P–T path of this metamorphosed continental unit to be quantified. Three successive metamorphic stages are distinguished, the first one under eclogite facies conditions, with a pressure of 13.5 ± 1.5 kbar and a temperature of 450 ± 25 °C. A second stage, characterized by a moderate temperature increase at the beginning of the decompression, corresponds to a reequilibration near the boundary between blueschist and eclogite facies conditions. The third stage, characterized by the costability of lawsonite and albite, documents a reequilibration at the boundary between blueschist and greenschist facies conditions. These new data confirm the heterogeneity of the metamorphic conditions observed in the Piemont zone of the western Alps. They are in agreement with structural data and allow us to interpret the Acceglio domain as a tectonic extrusion of eclogitized continental crust within the less metamorphic calcschists of the Piemont zone. L'étude pétrologique des schistes andésitiques permiens de la terminaison nord de la zone d'Acceglio permettent, pour la première fois, de contraindre le trajet pression–température de cette unité continentale alpine. Trois stades métamorphiques sont enregistrés successivement, avec (1) des conditions éclogitiques, avec une pression de 13,5 ± 1,5 kbar et une température de 450 ± 25 °C, (2) une augmentation modérée de la température en début de décompression, avec rééquilibrage dans des conditions proches de la transition entre faciès éclogites et schistes bleus, (3) la costabilité de la lawsonite et de l'albite, indiquant un rééquilibrage à la limite des conditions entre les faciès schistes bleus–schistes verts. Ces données sont une nouvelle illustration de l'hétérogénéïté des conditions métamorphiques au sein de l'arc interne des Alpes occidentales et confortent les données structurales qui conduisent à interpréter la zone d'Acceglio comme une écaille tectonique de croûte continentale éclogitisée extrudée, au sein des Schistes lustrés piémontais, moins métamorphiques.
Abstract We developed a 2D numerical model that simulates the evolution of a subduction in order to analyze which different metamorphic facies and deformation fabrics can be recorded within a subduction complex and whether the subduction velocity can influence their distribution. Regions with pressure and temperature (P‐T) conditions characteristic of different metamorphic facies have been explored in the different domains of the subduction system to verify the existence of associations that may result as diagnostic of specific areas of the subduction system at different times. Distribution patterns of strain rates and relative deformation fabrics have also been investigated to verify whether they can be recorded under different metamorphic facies conditions. Our results show that three domains characterized by contrasting metamorphic conditions can be simultaneously observed in different regions of the subduction complex, in contrast with the widespread idea that different metamorphic series are representative of peculiar geodynamic scenarios, but in agreement with the commonly observed coexistence of contrasted P‐T conditions in orogenic belts. In addition, we verified that mylonitic fabrics can develop more frequently in metamorphic facies characterized by high Pressure/Temperature (P/T) ratios, while coronitic fabric can be better preserved in metamorphic facies with low‐to‐medium P/T ratios.
<p>The Variscan belt is the result of the Pangea accretion, a prominent feature of the European continental lithosphere (von Raumer et al., 2003) . The debate on the number of oceans and the geodynamic evolution of the Variscan belt is still open (Faure et al., 2009; Franke et al., 2017). Two scenarios have been proposed:</p><ol><li> <p>Monocyclic scenario: assumes a single long-lasting south-dipping subduction of a large oceanic domain. Armorica remained more or less closed to Gondwana during its northward drift, in agreement with lack of biostratigraphic and paleomagnetic data that suggests a narrow oceanic domain (lesser than 1000 km; Matte, 2001; Lardeaux, 2014);</p> </li> <li> <p>Polycyclic scenario: this geodynamic reconstruction envisages two main oceanic basins opened by the successive northward drifting of two Armorican microcontinent and closed by two opposite subductions (Lardeaux, 2014; Franke et al., 2017). The northern oceanic basin is identified as the Saxothuringian ocean, while the southern basin is identified as the Medio-European ocean (Lardeaux, 2014).</p> </li> </ol><p><span>Models of single and double subduction have been developed to verify which scenario better fits with Variscan P-T evolutions from the Alps and the French Central Massif (FCM). From the comparison between model predictions</span> <span>and natural Variscan P-T-t estimates results that data from the Alps with high P/T ratios better fit with the double subduction model, supporting that a polycyclic scenario is more suitable for the Variscan </span><span>belt evolution</span><span>. Differently, data from the FCM with high P/T ratios that fit with both models have</span> <span>poorly constrained</span> <span>geological ages and, therefore, are not suitable to actually discriminate between mono- and polycyclic scenarios (Regorda et al., 2020). Moreover, the predictions of the models open to the possibility that rocks of the Upper Gneiss Unit of the FCM could derive from tectonic erosion of the upper plate and not only from the ocean-continent transition of the lower plate.</span></p><p><strong><span>References</span></strong></p><p><span>Faure M., Lardeaux J.-M. and Ledru P.; 2009: </span><em><span>A review of the pre-Permian geology of the Variscan French Massif Central</span></em><span>. Comptes Rendus Geoscience, </span><strong><span>341</span></strong><span>, 202-213.</span></p><p><span>Franke W., Cocks L.R.M. and Torsvik T.H.; 2017: </span><em><span>The Palaeozoic Variscan oceans revisited</span></em><span>. Gondwana Research, </span><strong><span>48</span></strong><span>, 257-284.</span></p><p><span>Lardeaux J.-M.; 2014: </span><em><span>Deciphering orogeny: a metamorphic perspective. Examples from European Alpine and Variscan belts. Part II: Variscan metamorphism in the French Massif Central &#8211; A review</span></em><span>. Bull. Soc. g&#233;ol. France, </span><strong><span>185(5)</span></strong><span>, 281-310.</span></p><p><span>Matte P.; 2001: </span><em><span>The Variscan collage and orogeny (480-290 Ma) and the tectonic definition of theArmorica microplate: A review</span></em><span>. Terra Nova, </span><strong><span>13(2)</span></strong><span>, 122-128.</span></p><p><span>R</span>egorda A., Lardeaux J-.M., Roda M., Marotta A.M. and Spalla M.I.; 2020: <em>How many subductions in the Variscan orogeny? Insights from numerical models</em>. Geoscience Frontiers, 10.1016/j.gsf.2019.10.005.</p><p><span>von Raumer J. F., Stampfli G.M. and Bussy, F.; 2003: </span><em><span>Gondwana-derived microcontinents &#8211; the constituents of the Variscan and Alpine collisional orogens</span></em><span>. Tectnophysics, </span><strong><span>365</span></strong><span>, 7-22.</span></p>
