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Genèse d'un pluton composite tardi-hercynien. Le Massif du Tichka, Haut Atlas occidental (Maroc)

Abstract : The Hercynian Tichka plutonic complex in Morocco has been studied, for the following reasons, in order to define and detail the mechanisms by which it was derived: it offers a vertical section of about 2500m with an exceptional quality of the outcrop allowing continuous observation and sampling; it exhibits a large compositional range from gabbros to leucogranites with' spectacular relationships between coeval diorites and granitoids. Therefore, field work, petrographical, mineralogical, geochemical and isotopic (Rb-Sr and Sm-Nd) studies have been used to constrain the genesis of the Tichka complex. Furthermore, the study of magmatic, structures and rheological properties of the magmas permits a better understanding of the mechanisms of ascent and emplacement of this pluton. Our genetic model appears to have a more general application to the genesis of similar plutonic associations of various ages and from various tectonic settings. The Tichka complex was emplaced 291 ± 5Ma ago (Rb/Sr isochron) in lower Cambrian sedimentary rocks adjacent to the South Atlasic Fault, locally known as the Tizi n'Test Fault. The depth of emplacement of the pluton has been estimated, with geobarometers applied on amphiboles from plutonites, to about 7 km in agreement with (1) the estimated stratigraphic thickness of the volcano-sedimentary pile of country rocks, (2) the P-T conditions of crystallization of minerals in hornfels at the immediate vicinity of the pluton. Five major rock-types, with a heterogeneous distribution through the whole complex, have been identified based on structural, petrographical and geochemical criteria: gabbros, diorites (subdivided into mafic, common and heterogeneous diorites), granodiorites-tonalites, monzogranites and leucogranites (subdivided into biotite ± amphibole leucogranites, mainly to the S, and two mica leucogranites, to the NE). Except for the younger two-mica leucogranites, all rocks are coeval. Study of magmatic flow-structures indicate that : (1) the complex is made of three well-defined. and sub-contemporaneous composite intrusions bounded to the south by meta-volcanosedimentary septa and one subsequent intrusion with a poorly-defined geometry, made of two-mica leucogranites and located in the NE ; (2) the magmatic source moved about from South-West to North-East of the complex relative to the country-rocks; (3) the role played by the thrusting component along the Tizi n'Test fault during the emplacement of the four intrusions is important; (4) the inner part of the complex was rising faster than the outer parts. The last intrusion, made of two-mica leucogranites, consists of small diapiric bodies centred around the kilometre-scale diapir of the Tasghimout mountain and was emplaced into entirely crystallized, but not entirely cooled, granitoids. Study of viscosities and densities of magmas suggests that (1) the intrusions could have risen as diapirs and (2) an inversion of the viscosities of the dioritic and granodioritic magmas could have occurred at about 850°C. The spectacular striped structure of the southern part of the complex had been acquired in a magmatic chamber at a depth of more than 15 km. As a whole, the Tichka complex shows characteristics of a subalkalic metaluminous transitional association with a sodic signature acquired in a post-collision uplift environment. The two mica leucogranites have alumino-potassic characteristics. Major and trace element data as well as Rb-Sr and Sm-Nd isotopic results rule out any genetic process involving a unique homogeneous source. The Sm-Nd isotopic heterogeneity, even at the scale of one unit, implies the existence of contrasted source rocks as well as the combination of different mechanisms. The gabbro and diorite magmas are assumed to be derived from the melting, at different degrees, of a depleted upper mantle source in a within-plate geotectonic environment, whereas combined wall rock assimilation - fractional crystallisation model is proposed for the evolution of the diorite suite. In contrast, the granitoids are mainly the result of the anatexis of a dominant basic to intermediate igneous heterogeneous continental crust. The two mica leucogranites are S-type granites. The combination of geochemical and Sr, Nd isotopic data has provided strong constraints on the petrogenetic model proposed for the genesis and evolution of the Tichka plutonic series. A two step model involving different processes appears to be suitable with all available data: (1) about 290 Ma ago the relaxation of the regional strain, in relation with the tectonic dextral shear activity of the Tizi n'Test fault, may have induced the progressive partial melting of the upper mantle producing successively gabbroic and dioritic magmas. During its ascent within the continental crust (or perhaps in a deep-seated crustal magma chamber) the dioritic magma was underwent to a combined wall rock assimilation and fractional crystallization (AFC) process whereas the preceding gabbroic magma, due to its limited volume, has retained its primary mantle signature; (2) the ascent of these hot magmas in a compositionally zoned continental crust would have induced partial melting to different degrees and at different levels, generating various independent granitoid magmas. Each major plutonic unit was emplaced without a long residence time in a large magma chamber where total chemical and isotopic homogenization would otherwise have occurred. The magmatic history of the Tichka massif ends with the late generation and emplacement of two-mica leucogranites derived from the melting of upper continental crustal rocks. Such an intricate history involving nested intrusions in relation with major intra-lithospheric fauIts, multiple magmatic sources, various mechanism of genesis as fractional crystallization, contamination, hybridization, is similar to those of other Variscan plutons in Morocco as well in the West-European Belt .
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Dominique Gasquet. Genèse d'un pluton composite tardi-hercynien. Le Massif du Tichka, Haut Atlas occidental (Maroc). Sciences de l'environnement. Université de Nancy 1, 1991. Français. ⟨tel-00690900⟩

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