Tectonometamorphic evolution of an intracontinental orogeny inferred from P–T–t–d paths of the metapelites from the Rehamna massif (Morocco)

Abstract

New petrographic and microstructural observations, mineral equilibria modelling and U/Pb (monazite) geochronological studies were carried out to investigate the relationships between deformation and metamorphism across the Rehamna massif (Moroccan Variscan belt). In this area, typical Barrovian (muscovite to staurolite) zones developed in Cambrian to Carboniferous metasedimentary rocks that are distributed around a dome‐like structure. First assemblages are characterized by the presence of locally preserved andalusite, followed by prograde evolution culminating at 6 kbar and 620 °C in the structurally deepest staurolite zone rocks. This Barrovian sequence was subsequently uplifted to supracrustal levels, heterogeneously reworked at greenschist facies conditions, which was followed locally by static growth of andalusite, indicating heating to 2.5–4 kbar and 530–570 °C. The 206Pb/238U monazite age of 298.3 ± 4.1 Ma is interpreted as minimum age of peak metamorphic conditions, whereas the ages of 275.8 ± 1.7 Ma and 277.0 ± 1.1 Ma date decompression and heating at low pressure, in agreement with previous dating of Permian granitoids intruding the Rehamna massif. The prograde metamorphism occurred during thickening and associated horizontal flow in the deeper crust (S1 horizontal schistosity). The horizontally disposed metamorphic zones were subsequently uplifted by a regional scale antiform during ongoing N–S compression. The re‐heating of the massif follows late massive E–W shortening, refolding and retrograde shearing of all previous fabrics coevally with regionally important intrusions of Permian granitoids. We argue that metamorphic evolution of the Rehamna massif occurred several hundred kilometres from the convergent plate boundaries in the interior of continental Gondwanan plate. The tectonometamorphic history of the Rehamna massif is put into Palaeozoic plate tectonic perspective and Late Carboniferous reactivation of (Devonian)–Early Carboniferous basins formed during stretching of the north Gondwana margin and formation of the Palaeotethys Ocean. The inherited heat budget of these magma‐rich basins plays a role in the preferential location of this intracontinental orogen. It is shown that rapid transition from lithospheric stretching to compression is characterized by specific HT type of Barrovian metamorphism, which markedly differs from similar Barrovian sequences along Palaeozoic plate boundaries reported from Variscan Europe.

The Geological Evolution of the Variscan Jebilet Massif, Morocco, Inferred From New Structural and Geochronological Analyses

“…Michard et al () suggested that HT‐LP metamorphism accompanying this compressive event in the Rehamna and the Jebilet was controlled by an older thermal anomaly in relation with the Late Devonian‐Early Carboniferous intracontinental rifting stage. This hypothesis is also supported by Chopin et al () and Wernert et al () for the Rehamna massif. Our results have further confirmed this model for the Jebilet massif.…”

“…Maps (modified after Michard et al, ; Wernert et al, ) and three‐dimensional idealized model for the evolution of the Moroccan Meseta domain and the Jebilet Massif from the Late Devonian to the Early Permian (see text for discussion). The large gray arrows correspond to the crustal shortening direction.…”

“…The D 2 HT-LP metamorphism recorded in the Jebilet massif cannot be explained by crustal thickening. Moderate thickening associated to HT-LP metamorphism (Figure 11e) can be explained by an inherited HT thermal anomaly, developed during the Late Devonian-Early Carboniferous intracontinental rifting as proposed in the Rehamna and Jebilet massifs by Michard et al (2010), and for the Rehamna by Wernert et al (2016). In the case of the Jebilet massif, we therefore propose that the HT thermal anomaly related to the D 0 rifting episode was responsible for the bimodal and granodioritic magmatism around 340 Ma ( Figure 10) and induced a thermal softening of the crust.…”

“…Sedimentation started with the deposit of the Famennian (370 Ma) Rhira Formation and finished with the Late Visean-Namurian (325 Ma) Teksim Formation (Aarab & Beauchamp, 1987;Beauchamp, 1984; Figure 11. Maps (modified after Michard et al, 2010;Wernert et al, 2016) and three-dimensional idealized model for the evolution of the Moroccan Meseta domain and the Jebilet Massif from the Late Devonian to the Early Permian (see text for discussion). The large gray arrows correspond to the crustal shortening direction.…”

Paleozoic subduction of the southern Dunhuang Orogenic Belt, northwest China: metamorphism and geochronology of the Shuixiakou area

“…Regional metamorphism occurs commonly coupled with deformation, which results in several generations of folds and / or shearing at different stages of metamorphism (e.g. Li et al, 2005Li et al, , 2010Wernert et al, 2016). In the Shuixiakou area, three episodes of deformation (D1-D3) were distinguished in the metamorphic complex.…”