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>Magister Scientiae - MSc / The Sea Point contact, Cape Town, South Africa, exposes the contact between the Neoproterozoic Malmesbury Group metasedimentary rocks of the Pan-African Saldania belt and the intrusive S-type Peninsula Granite of the Neoproterozoic-Paleozoic Cape Granite Suite (CGS). The exposure outcrops over an area of approximately 170 m × 60 m with the northern end of the exposure being characterized by the country rock–microgranite intrusive contact. Heading further south, the outcrop transitions to the main contact zone, which is a predominantly gradational zone marked by sheets of compositionally variable granitic injections (collectively referred to as hybrid granite phases) concordant to the country rock structure, before reaching the main pluton area comprising the voluminous coarse-grained porphyritic granite. Using a combined study incorporating field, structural, geochemical, isotopic and U-Pb geochronological data, the intrusive contact is investigated to determine the construction history of the pluton and delineate possible emplacement mechanisms. The granitic phases, which vary from fine-grained leucocratic, medium-grained porphyritic to coarse-grained porphyritic, are peraluminous, magnesian to ferroan, and alkali-calcic. Based on the linear trends between the whole-rock major and trace element content of the granites vs. maficity (molar Fe + Mg), their initial Sr ratios and εNd(t) values, the granites of the study area are consistent with the currently proposed petrogenetic model for the CGS (e.g. Stevens et al., 2007; Villaros et al., 2009a; Harris & Vogeli, 2010); i.e., they are crustally derived and their chemical variability is controlled primarily by peritectic assemblage entrainment. The fractional crystallization of K-feldspar is identified as the primary mechanism for the local geochemical variability of the granites. The fractionation of K-feldspar as a mechanism of variability was evaluated using binary log-log diagrams of Ba, Sr and Eu and is interpreted to have taken place at levels close to the emplacement site after source entrainment processes. Although there is outcrop evidence, particularly in the main contact zone, to suggest that local assimilation and filter pressing took place, this was not reflected by the whole-rock and isotope geochemistry of the granites. This suggests that these processes are very localized and will need further rigorous testing to ascertain the extent to which they caused variability. Outcrop evidence for assimilation includes gradational country rock-granite contacts and the ductile behaviour of the country rock, whereas the occurrence of K-feldspar megacrysts embedded in the country rock at the main contact zone suggests melt accumulation and escape consistent with the filter pressing mechanism. In the case of the latter, the melt fraction of the granite was easily mobilized and driven out compared to the crystal fraction (K-feldspars) during the emplacement of the granites. Field relationships and the structural interpretation of the Malmesbury Group country rocks and the granites reveal that: (1) the various granites are late syn-tectonic and (2) were emplaced as incrementally assembled, repeated pulses of inclined granitic sheets more or less normal (i.e. at high angles) to the regional NE-SW shortening (D1) of the Malmesbury forearc during the Saldanian orogeny. Given the lack of a controlling shear zone in facilitating granite emplacement in the study area, the pre-existing planar anisotropies (bedding planes and foliations) in the country rock provided preferential pathways for magma emplacement and propagation during deformation. This implies that the tensile strength normal and parallel to the bedding and foliation anisotropy of the country rock was larger than the regional differential stress (σ1 – σ3, with σ1 ≥ σ2 ≥ σ3), allowing for magma emplacement relative to shortening. Sheet propagation is interpreted to have occurred through the balance of the following conditions: (1) density contrasts between host rocks and magmas, (2) the pressure differential along the subvertical fractures/sheets, and (3) the melt pressure equalling the lithostatic pressure to keep the magma pathways open and being sufficiently high such that it exceeds the sum of σ1 and the tensile strength of the rock parallel to σ1. The crystallization ages of the dated granite samples are identical within error and vary between 538.7 ± 3.6 Ma and 542.7 ± 2.9 Ma. They, therefore, cannot prove which granite phase intruded first and which one proceeded and so forth. Field relationships, however, suggests that the microgranites were first to intrude given their fine-grained nature and the localized chilled contacts they show with the country rock. The various coarser-grained and porphyritic phases were next to intrude, with their coarse grain-sizes and lack of chilled margins with the country rock suggesting that the time interval between their successive emplacements was not too long; this prevented the country rock from completely cooling down between each magma batch. Magma stoping and the ductile flow of the host material (owing to highly viscous magma flow) to accommodate granite emplacement are interpreted to be secondary emplacement processes.

Ever since the first plate-tectonic model for the Appalachians was proposed, the Laurentian margin has been interpreted as having experienced a collision-related dynamo-thermal event during the Middle Ordovician Taconic orogeny. In the western Newfoundland Appalachians, evidence for this collision is well-preserved in the Dashwoods subzone. Nevertheless, rocks of the neighbouring Corner Brook Lake block (CBLB), which is located in the heart of the Laurentian realm, did not show evidence for such an event. Instead, it was affected by Early Silurian Salinic deformation and associated peak metamorphism. Even though this difference in Early Palaeozoic tectonic history between the Dashwoods and the CBLB is widely known, it has not been satisfactorily explained. To better understand the Early Palaeozoic history of the region, in particular to test and better explain the lack of a Taconic dynamo-thermal event in the CBLB, field mapping, microscopic work, and U-Pb and 40Ar/39Ar geochronological studies were undertaken in the western and northern part of the Dashwoods subzone, and in the southern part of the CBLB. In addition, the kinematic history of the Baie Verte-Brompton Line - Cabot Fault Zone (BCZ), the tectonic zone that separates the two unique tectonic fragments, was studied. The western and northern parts of the Dashwoods subzone contain variably foliated igneous units of Middle Ordovician age (ca. 460 Ma) that are associated with the regionally voluminous Notre Dame continental arc. A ca. 455 Ma conjugate set of late syn-tectonic pegmatite dykes in the BCZ demonstrates a dextral sense of shear along the BCZ (DBCZ-1) during the Late Ordovician to earliest Silurian, and constrains the minimum age of the main phase of ductile deformation in the Dashwoods subzone. The fault-bounded CBLB has been affected by a single west-vergent deformational event, constrained between ca. 434 and ca. 427 Ma. More importantly, no evidence – neither petrographic nor geochronological – is present that would indicate that the CBLB was affected by a significant Taconic dynamo-thermal event. Hence, the CBLB and Dashwoods could not have been juxtaposed until after the late Early Silurian. Furthermore, the basement to the CBLB is devoid of any Grenville (sensu lato; ca. 1.0-1.3 Ga) U-Pb ages, which is in sharp contrast with crystalline basement elsewhere in the region, such as the Long Range Inlier. Therefore, it is highly unlikely that the CBLB represents the para-autochthonous leading edge of the Laurentian craton in the Newfoundland Appalachians, as commonly accepted. The CBLB is interpreted as a suspect terrane that has moved over 500 km parallel to the strike of the orogen. Docking to the external Humber Zone is likely to have occurred during the Early Silurian. Final juxtaposition with the Dashwoods took place after the late Early Silurian (post-Salinic) as a result of protracted dextral movement along the BCZ (DBCZ-2 and DBCZ-5). Current tectonic models for the Newfoundland Appalachians mainly focus on well-documented Early Palaeozoic orthogonal convergence of various terranes with the Laurentian margin, but large-scale orogen-parallel movements have rarely been considered. The possibility of large-scale strike-slip tectonics documented here, in addition to the convergent motions, may have significant implications for the tectonic interpretation of the Early Palaeozoic evolution of the Newfoundland Appalachians.

Ever since the first plate-tectonic model for the Appalachians was proposed, the Laurentian margin has been interpreted as having experienced a collision-related dynamo-thermal event during the Middle Ordovician Taconic orogeny. In the western Newfoundland Appalachians, evidence for this collision is well-preserved in the Dashwoods subzone. Nevertheless, rocks of the neighbouring Corner Brook Lake block (CBLB), which is located in the heart of the Laurentian realm, did not show evidence for such an event. Instead, it was affected by Early Silurian Salinic deformation and associated peak metamorphism. Even though this difference in Early Palaeozoic tectonic history between the Dashwoods and the CBLB is widely known, it has not been satisfactorily explained. To better understand the Early Palaeozoic history of the region, in particular to test and better explain the lack of a Taconic dynamo-thermal event in the CBLB, field mapping, microscopic work, and U-Pb and 40Ar/39Ar geochronological studies were undertaken in the western and northern part of the Dashwoods subzone, and in the southern part of the CBLB. In addition, the kinematic history of the Baie Verte-Brompton Line - Cabot Fault Zone (BCZ), the tectonic zone that separates the two unique tectonic fragments, was studied. The western and northern parts of the Dashwoods subzone contain variably foliated igneous units of Middle Ordovician age (ca. 460 Ma) that are associated with the regionally voluminous Notre Dame continental arc. A ca. 455 Ma conjugate set of late syn-tectonic pegmatite dykes in the BCZ demonstrates a dextral sense of shear along the BCZ (DBCZ-1) during the Late Ordovician to earliest Silurian, and constrains the minimum age of the main phase of ductile deformation in the Dashwoods subzone. The fault-bounded CBLB has been affected by a single west-vergent deformational event, constrained between ca. 434 and ca. 427 Ma. More importantly, no evidence – neither petrographic nor geochronological – is present that would indicate that the CBLB was affected by a significant Taconic dynamo-thermal event. Hence, the CBLB and Dashwoods could not have been juxtaposed until after the late Early Silurian. Furthermore, the basement to the CBLB is devoid of any Grenville (sensu lato; ca. 1.0-1.3 Ga) U-Pb ages, which is in sharp contrast with crystalline basement elsewhere in the region, such as the Long Range Inlier. Therefore, it is highly unlikely that the CBLB represents the para-autochthonous leading edge of the Laurentian craton in the Newfoundland Appalachians, as commonly accepted. The CBLB is interpreted as a suspect terrane that has moved over 500 km parallel to the strike of the orogen. Docking to the external Humber Zone is likely to have occurred during the Early Silurian. Final juxtaposition with the Dashwoods took place after the late Early Silurian (post-Salinic) as a result of protracted dextral movement along the BCZ (DBCZ-2 and DBCZ-5). Current tectonic models for the Newfoundland Appalachians mainly focus on well-documented Early Palaeozoic orthogonal convergence of various terranes with the Laurentian margin, but large-scale orogen-parallel movements have rarely been considered. The possibility of large-scale strike-slip tectonics documented here, in addition to the convergent motions, may have significant implications for the tectonic interpretation of the Early Palaeozoic evolution of the Newfoundland Appalachians.

Cette thèse présente la reconstruction de l’histoire tectono-thermique du massif varisque des Jebilet (Maroc) à fort potentiel minier, depuis son évolution pré-orogénique au Dévonien supérieur-Carbonifère inférieur jusqu’à sa structuration pendant l’orogénèse varisque-alléghanienne au Carbonifère supérieur-Permien inférieur. Pour répondre à cette problématique, ce travail s’organise autour de deux approches : (1) l’une métrologique appliquée à la géothermométrie Raman sur la matière carbonée (RSCM) et (2) l’autre intégrant une étude structurale, géochronologique et une analyse de la thermicité.L’approche métrologique a permis de valider l’applicabilité du géothermomètre RSCM (1) dans un contexte de métamorphisme polyphasé, (2) pour des roches carbonatées et des skarns des Jebilet et (3) de proposer un nouveau paramètre Raman RSA permettant de mieux préciser les températures supérieures à 500°C et d’étendre l’applicabilité de la méthode jusqu’à des températures maximales qui atteignent les 700°C.A partir de l’approche intégrée, trois épisodes tectono-thermiques ont été mis en évidence. Le premier épisode D₀ correspond à une tectonique extensive permettant l’ouverture du bassin des Jebilet au Dévonien supérieur-Carbonifère inférieur. Cette tectonique extensive est accompagnée par une anomalie thermique supérieure à 500°C déduites des mesures de géothermométrie RSCM (TRSCM) et par une importante activité magmatique bimodale et granodioritique datée dans ce travail entre 358 ± 7 et 336 ± 4 Ma. Au Carbonifère supérieur débute la phase compressive structurant le massif des Jebilet avec la mise en place de nappes superficielles au Namuro-Westphalien (D₁), suivie par la phase varisque majeure (D₂). L’analyse structurale a permis de montrer une évolution progressive du régime de déformation de D2 depuis une compression coaxiale à une transpression dextre compatible avec un raccourcissement horizontal WNW-ESE à NW-SE. D₂₁ est associée à deux événements thermiques, le premier syn-tectonique de moyenne température (300°<TRSCM<400°C), et le second syn- à post-tectonique de haute température (600°<TRSCM<660°C).Ce contexte tectono-thermique serait l’expression de processus géodynamiques impliquant à partir du Dévonien supérieur une délamination de la lithosphère rhéïque par « slab break-off » ou « slab roll-back » qui entrainerait (1) la remontée de courant asthénosphérique chaud, et (2) la rotation horaire de Gondwana et son amalgamation progressive avec Laurussia structurant au Carbonifère supérieur-Permien inférieur la chaine varisque-alléghanienne. / This thesis presents the reconstruction of the tectono-thermal history of the Paleozoic Jebilet massif (Morocco), from its pre-orogenic evolution at the Upper Devonian-Lower Carboniferous to its structuration during the variscan-alleghanian orogeny at the Upper Carboniferous-Lower Permian. To address this issue, this work is organized around two approaches: (1) one metrological applied to the Raman Spectroscopy of Carbonaceous Matterial (RSCM) and (2) the other integrating a structural and geochronological study and a thermicity analysis.The metrological approach allowed to validate the applicability of the RSCM geothermometer (1) in a context of polyphase metamorphism, (2) for carbonate rocks and skarns of Jebilet and (3) to propose a new parameter Raman RSA allowing to better specify temperatures above 500°C and extend the applicability of the method to maximum temperatures of up to 700°C.From the integrated approach, three tectono-thermal episodes were highlighted. The first episode D₀, corresponds to an extensive tectonic allowing the opening of the Jebilet basin at the Upper Devonian-Lower Carboniferous. This opening is accompanied by a HT thermal anomaly as shown by the important bimodal and granodioritic magmatic activity dated between 358 ± 7 Ma and 336 ± 4 Ma and the TRSCM higher than 500°C recorded by the rocks. During Upper Carboniferous, the compressive phase structuring the Jebilet massif begins with the emplacement of superficial nappes (D₁), followed by the variscan major phase (D2). Structural analysis showed a gradual evolution of D₂ deformation regime from coaxial compression to dextral transpression consistent with WNW-ESE to NW-SE horizontal shortening. D₂ is associated with two thermal events, the first is syn-tectonic with TRSCM between 300 and 400°C, and the second is syn- to post-tectonic with TRSCM between 600 and 660°C.This tectono-thermal context would be the expression of geodynamic processes involving from the Upper Devonian a delamination of the Rheic lithosphere by "slab break-off" or "slab roll-back" which would induce (1) the rise of hot asthenospheric current, and (2) the clockwise rotation of Gondwana and its gradual amalgamation with Laurussia structuring the variscan-alleghanian belt during the Upper Carboniferous-Lower Permian.