Le magmatisme terminal de la chaîne hercynienne : signification géodynamique d'une association magmatique identifiée dans le Carbonifère terminal du Massif Hercynien Central Marocain

Na parte oriental do Maciço Hercínico Central de Marrocos afloram alguns filões intercalados em formações do Carbónico superior do Sudeste da bacia de Azrou-Kénifra. Estes filões instalaram-se paralelamente à estratificação e foram deformados por dobramentos associados aos cavalgamentos que controlaram a sedimentação, o que demonstra o seu carácter sintectónico. A sua composição química permitiu distinguir dois tipos de sequências magmáticas sobrepostas nas séries de Tariwalt e Talgarat, respectivamente de idades V3b e Namuriano-V3c, provável. A primeira inclui rochas básicas de carácter transicional e afinidade alcalina, enquanto a segunda possui uma assinatura calco-alcalina. Estas sequências magmáticas apresentam estreitas semelhanças petrográficas e geoquímicas com as que ocorrem em ambientes de margem activa. As assinaturas geoquímicas e a evolução magmática destas rochas são compatíveis com os dados tectono-sedimentares que caracterizam a bacia de Azrou-Kénifra como uma bacia desenvolvida em regime compressivo, integrada num sistema de "avant pays" determinado por sequências de cavalgamentos propagando-se para NW

Preliminary petrological inferences on the high-grade metamorphic rocks exhumed by the South Rif Thrust, Prerif, Northern Morocco

The exhumation of deep crustal rocks along major shear zones is common, yet a highly debated subject, particularly when occurring during recent tectonic events. This is the case of the South Rif Thrust (SRT), a significant shear zone analogous to those described at the Betic Cordillera in Spain. The SRT separates two major geodynamic domains in Northern Morocco: a) the Prerif, to the North, mostly composed of Miocenic sedimentary units on top of a stratigraphic sequence continuously deposited since the Triassic; and b) the Western Meseta, to the South, mostly composed of Paleozoic metasedimentary units, correlated with the Iberian Variscan Belt [1]. Associated to the SRT, and exhumed by its activity, there is a dismembered and exotic high-grade metamorphic belt representative of the middle and lower crust. Also in this region, several thermo-mineral waters occur, whose deep circulation can be traced back to the SRT [2]. Detailed geological mapping, structural, stratigraphic and petrographic analyses on this dismembered and highly deformed exotic sequence reveal the presence of low- to high-grade metasediments (including migmatites and felsic granulites), but mostly high-grade metabasic and basic rocks, including amphibolites, mafic granulites and gabbros. Preliminary geothermobarometry in the mafic granulites provides an important characterization of the infra-crustal conditions of the pre-Alpine geodynamics and of the activity and exhumation along the SRT since the Miocene: a) the mafic granulites endured M1 metamorphic peak conditions of T = 1030 ºC at P = 8.5 kbar, which is consistent with typical conductive continental crust geothermal gradients (~30 ºC.km-1); b) M2 retrogression occurred by near isothermal decompression at T = 820 ºC and P = 3.5 kbar, implying an initial vertical uplift of >18 km of the granulite-facies rocks to very shallow levels; c) during this period, the geothermal gradient in the region surpassed 60 ºC.km-1; d) exhumation and retrogression continued by almost isobaric cooling at T < 750 ºC and P = 1.7 – 3.0 kbar with an M3 amphibolitization of the granulites after late water inflow. The overall metamorphic evolution of these deep crustal rocks is compatible with a clockwise P-T path, involving initial fast tectonic exhumation, followed by thermal readjustment to shallower levels. This is consistent with the currently observed geothermal gradients in the area (≤ 42 ºC.km-1) [2] which may still be a reflection of the events during the Miocene. These petrological constrains on the tectonic processes associated with the exhumation of this lower crust segment and the activity of the SRT during the closure of the Alboran Basin are key to understanding the circulation of deep hot waters, which are an important part of the economy of this region in Northern Morocco.publishe

Multi-stage metamorphism recorded in crustal xenoliths from Permian dykes of the region of Mrirt (Moroccan Central Massif)

The Permian magmatic rocks from Morocco contain crustal xenoliths that sample the Variscan crust in a context of widespread magmatism. A series of such xenoliths was collected in Permian dykes of the Central Massif, in the region of Mrirt. The metapelitic xenoliths are silica poor to intermediate (44 < SiO2 <57 wt%) and alumina-rich (17 < Al2O3 < 34 wt%) and are notably enriched in some HFSE (Nb, Ta, Ti) and some transition elements (Cr, V, W, Ni). Their petrographic evolution depicts a multi-stage evolution from an early, subsolidus, metamorphic history related to regional metamorphism, of which biotite, garnet and sillimanite are the witnesses toward a late thermal evolution coeval with the entrapment in the magma, marked by pervasive partial melting and development of peritectic spinel and cordierite together with K-feldspar and ilmenite. The overall presence of corundum, which relates to the high Al2O3 content, accounts for an initial stage of partial melting and magma escape, prior to the entrapment in the magma. Textural and chemical observation suggests further xenolith digestion and melt flux from the xenoliths toward the magma during ascent. Thermodynamic modelling allowing the determination of the pressure-temperature history of each xenolith shows that the initial pressure varies from ca. 1.5 to ca. 6 kbar, which, considering lithostatic pressure, corresponds to sampling depths of ca. 5–25 km. The temperatures recorded by the parageneses coeval with partial melting in the presence of spinel are in the range 800–900 °C whatever the pressure, which accounts for rapid heating due to thermal equilibration with the magma. U–Th/Pb dating of monazite by EMPA gives a poorly resolved Permian age of 293 ± 25 Ma for metamorphism in the xenoliths, in accordance with the stratigraphic age of the host rock. The pressure conditions are similar to those recorded for regional metamorphism in the other Variscan outcrops in Morocco, namely the Jebilet, the Rehamna and the Aouli-Mibladen granitic complex, while the maximal temperatures are much higher. The xenoliths thus appear as typical of the Late Variscan geological evolution of the deep crust in the Moroccan Mesetas, where abundant magmatism was responsible for local crustal heating of a crust previously affected by regional metamorphism

Multi-stage metamorphism recorded in crustal xenoliths from Permian dykes of the region of Mrirt (Moroccan Central Massif)

The Permian magmatic rocks from Morocco contain crustal xenoliths that sample the Variscan crust in a context of widespread magmatism. A series of such xenoliths was collected in Permian dykes of the Central Massif, in the region of Mrirt. The metapelitic xenoliths are silica poor to intermediate (44 < SiO2 <57 wt%) and alumina-rich (17 < Al2O3 < 34 wt%) and are notably enriched in some HFSE (Nb, Ta, Ti) and some transition elements (Cr, V, W, Ni). Their petrographic evolution depicts a multi-stage evolution from an early, subsolidus, metamorphic history related to regional metamorphism, of which biotite, garnet and sillimanite are the witnesses toward a late thermal evolution coeval with the entrapment in the magma, marked by pervasive partial melting and development of peritectic spinel and cordierite together with K-feldspar and ilmenite. The overall presence of corundum, which relates to the high Al2O3 content, accounts for an initial stage of partial melting and magma escape, prior to the entrapment in the magma. Textural and chemical observation suggests further xenolith digestion and melt flux from the xenoliths toward the magma during ascent. Thermodynamic modelling allowing the determination of the pressure-temperature history of each xenolith shows that the initial pressure varies from ca. 1.5 to ca. 6 kbar, which, considering lithostatic pressure, corresponds to sampling depths of ca. 5–25 km. The temperatures recorded by the parageneses coeval with partial melting in the presence of spinel are in the range 800–900 °C whatever the pressure, which accounts for rapid heating due to thermal equilibration with the magma. U–Th/Pb dating of monazite by EMPA gives a poorly resolved Permian age of 293 ± 25 Ma for metamorphism in the xenoliths, in accordance with the stratigraphic age of the host rock. The pressure conditions are similar to those recorded for regional metamorphism in the other Variscan outcrops in Morocco, namely the Jebilet, the Rehamna and the Aouli-Mibladen granitic complex, while the maximal temperatures are much higher. The xenoliths thus appear as typical of the Late Variscan geological evolution of the deep crust in the Moroccan Mesetas, where abundant magmatism was responsible for local crustal heating of a crust previously affected by regional metamorphism