Metamorfismo granulítico de alta-presión en la Unidad de Filali (Macizo de Beni Bousera, Marruecos): implicaciones para una evolución metamórfica Varisca

Los paragneises migmatíticos basales de la unidad de Filali (Rif Interno, Marruecos) contienen inclusiones de granulitas básicas de alta-presión. Tres episodios metamórficos que documentan una descompresión isotérmica han sido distinguídos en estas litologías: (1) un episodio granulítico de alta-presión (810-890°C, 13.3-15.7 kbar); (2) un episodio transicional entre las facies de las granulitas y anfibolitas (712-832°C, 10.3-13 kbar); y (3) un estadio anfibolítico (690-770°C, 7.7-9 kbar). Esta descompresión isotérmica está probablemente asociada a un colapso extensional tardi-Varisc

2.46 Ga kalsilite and nepheline syenites from the Awsard pluton, Reguibat Rise of the West African Craton, Morocco. Generation of extremely K-rich magmas at the Archean–Proterozoic transition

The Awsard pluton in the Moroccan part of the Reguibat Rise, West African Craton, contains the oldest kalsilite-bearing rocks discovered to date, with a SHRIMP zircon U/Pb age of 2.46 ± 0.01 Ga. The pluton is composed of nepheline syenites, kalsilite syenites and minor silica-saturated syenites, all with the same primitive Sr and Nd initial isotope compositions and Nd model age that cluster around 87Sr/86Sr2.46Ga ≈ 0.7029, ɛNd2.46Ga ≈ −1.4, and TDM ≈ 2.75 Ga. Silica-saturated syenites are in fact contact fenites that grade into nepheline syenites, but the two feldspathoidal syenites are true magmatic rocks that crystallized from two coeval highly fractionated K-rich magmas with sharply different chemical compositions. Chemical, isotopic and textural evidence suggests that the two magmas originated by liquid immiscibility within an already fractionated alkaline potassic magma of asthenospheric origin that split in two phases, a nephelinitic melt rich in HFSE + REE, and a kalsilitic melt poor in HFSE + REE. The Awsard pluton, spatially associated to carbonatites and other alkaline rocks, does not mark a Late Archean fossil subduction zone but represents the first manifestation of a Paleoproterozoic alkaline province located in the Reguibat Rise, the full extent and importance of which is yet to be determined

Kalsilite-bearing plutonic rocks: The deep-seated Archean Awsard massif of the Reguibat Rise, South Morocco, West African Craton

Kalsilite-bearing igneous rocks are extremely rare,most known examples are volcanic. The few previously recorded kalsilite-bearing plutonic rocks are all Phanerozoic and mostly limited to the small ultrapotassic massifs of the Baikal–Stanovoi Rift in the Siberian Craton, the Greenland Batbjerg Massif, and the Kola Peninsula Khibiny Complex. We have found that the Archean to Proterozoic transition in thewestern Reguibat Rise of theWest African Craton is marked by several small massifs predominantly composed of kalsilite syenites, i.e., synnyrites. The largest massif is Awsard, a deep-seated intrusive body mainly composed of 2.46 Ga synnyrites and K-rich nepheline syenites with mantle-like Sr and Nd (whole-rock) and O (zircon) isotope composition. Apart from some superficial resemblance to the Baikal–Stanovoi synnyritiferous complexes, Awsard has no known equivalent in the geological record. It is the oldest, the deepest and the largest known occurrence of synnyrites. Awsard comprises solely felsic syenites with εNd(t) notably more primitive than their Siberian counterparts. The synnyrites contain kalsilite and rare nepheline as primary phases with no leucite or leucite pseudomorphs. Kalsilite and nepheline form large discrete grains that, in places, are accompanied by spectacular Ks–Or or Ne–Or symplectites. The symplectites are magmatic, generated by simultaneous crystallization of the two phases and the imbalance between the growth rate of the feldspar and the diffusivity of silica and alkalis in the melt. To explain why Awsard lacks mafic rocks and associated carbonatites, typical of other synnyrite massifs, we propose that ascending water-poor (H2O b 0.65 wt.%) mafic ultrapotassic magmas solidified at a pressure of 10–16 kbar underneath the already stabilized Archean crust of the region. In these conditions leucite began to crystallize when the temperature dropped to around 1100 °C. As a result of their low density, leucite crystals floated and formed a cap at the top of the intrusion. Then, the chamber was replenished with awater-rich andmore sodic ultrapotassicmagma that originated in the samemetasomatized mantle-source region. After prolonged fractional crystallization this second magma released an aqueous vapor phase that migrated upwards and melted the leucite cap thus producing a low-density hydrous magma of leucite-like, synnyritic, composition. This leucite-like magma, and the late residual melts from the second pulse that replenished the chamber, ascended and intruded the already cratonized lower crust of the western Reguibat Rise. There, the magmas crystallized outside the leucite stability field to produce the synnyrites and the nepheline syenites, respectively. There is no evidence that the metasomatic refertilization of the mantle required to produce the initial ultrapotassic mafic magmas was related to subduction fluids. On the contrary, it seems to have been caused by incompatible-element enriched hydrous fluids released from delaminated lower crustal fragments

Validating structural styles in the flysch basin northern rif (Morocco) by means of thermal modeling

Vitrinite reflectance and a micro-Raman spectroscopy parameters data set have been acquired on dispersed organic matter of the Maghrebian flysch basin and the Tangiers unit across a NE-SW section in the north-western Rif belt (North Morocco). Thermal maturity shows increasing values from the hinterland to the external unit (from NE to SW). Paleo-thermal indicators show that the internal flysch basin (i.e., the Mauretanian unit) is less mature than the external one, (i.e., the Massylian unit), with Ro % and Ro eq. Raman values ranging from 0.64% to 1.02% (from early mature to late mature stages of hydrocarbon generation). 1D thermal modeling estimates the overburden now totally eroded ranging from 3.1 km to 6.0 km, and has been used as constraint to reconstruct the complete thrust wedge geometry in Miocene times. The reconstructed geometry accounts for high shortening (about 63%) due to the development of an antiformal stack in the frontal part of the wedge made up by the flysch succession. This stacking is interpreted as a consequence of the western translation of the Alboran Domain in the core of the Betic-Rif orogenic system