Permo-Triassic Clastic Rocks from the Ghomaride Complex and Federico Units (Rif Cordillera, N Morocco): An Example of Diagenetic-Metamorphic Transition

Abstract

A detailed characterization of the mineralogy corresponding to the low-grade diagenetic-metamorphic sequence of the clastic rocks from the Beni Mezala antiform has allowed the processes implied in their origin to be stablished, integrating them in the geodynamic evolution of the Rif-Betic mountain range during the Alpine orogeny. A progressive evolution towards chemical and textural equilibrium was observed. The illite “crystallinity” (CIS) ranges from diagenetic-lower anchizone in Ghomaride complex (CIS: 1.50–0.37 Δ°2θ) to epizone in the deepest Federico units (CIS: 0.29–0.21 Δ°2θ). The main phyllosilicates in the diagenetic samples are illite (2M1-1M polytypes) and kaolinite, with mixed-layer illite/smectite and chlorite, but the mixed layers disappear in the lower anchizone samples, which show sudoite and dickite. Pyrophyllite is also present in the upper anchizone (0.43–0.29 Δ°2θ) whereas the epizone samples show muscovite (2M1-3T polytypes), chlorite, paragonite, and intermediate micas. The chlorite geothermometers give a T range of 150 to 400 °C, and the b parameter of 2M1 micas (8.992–9.029 Å) indicates low to intermediate pressure regional metamorphism (1–3 kbar) although the wide range of phengitic substitution in micas and the 3T polytype suggest a wider pressure range in coherence with the clockwise PTt evolution for the Rif Cordillera during the Alpine orogeny.

The pressure–temperature–time–deformation history of the Beni Mzala unit (Upper Sebtides, Rif belt, Morocco): Refining the Alpine tectono‐metamorphic evolution of the Alboran Domain of the western Mediterranean

“…Regarding the metamorphic signature, the Alpujarride-Sebtide units record a low grade, subduction-type metamorphism on Permian-Triassic protoliths and a high-grade Barrovian-type metamorphism on pre-Alpine protoliths (Azañón & Crespo-Blanc, 2000;Azañón et al, 1998;Bouybaouene et al, 1995;El Maz & Guiraud, 2001;Goffé et al, 1996;Gueydan et al, 2015;Homonnay et al, 2018;Michard et al, 2006;Rodríguez-Ruiz et al, 2019;Ruiz Cruz et al, 2010;Vidal et al, 1999), respectively, with a marked downward increase in the palaeo-temperature gradients, commonly referred to the heat source provided by the intracrustal emplacement of the peridotite bodies (Negro et al, 2006).…”

“…The Sebtide Complex in the Rif belt crops out within four antiformal structures, Beni Mzala, Ceuta, Cabo Negro and Beni Bousera, from north to south (Kornprobst, 1974;Michard et al, 2006;Figure 2a). The Sebtides are divided into the Upper Sebtides (Federico units) and the Lower Sebtides (Filali units), which, traditionally described with subduction-and Barrovian-type metamorphic signature respectively (Bouybaouene et al, 1995;Bouybaouene et al, 1998;El Maz & Guiraud, 2001;Gueydan et al, 2015;Michard et al, 1997Michard et al, , 2006Negro et al, 2006;Rodríguez-Ruiz et al, 2019;Ruiz Cruz et al, 2010), form the envelope of the Beni Bousera units (migmatitic granulites and the Beni Bousera peridotites; Álvarez-Valero et al, 2014;Bouybaouene et al, 1998;Kornprobst, 1974;Melchiorre et al, 2017;Rossetti et al, 2020).…”

“…Their lithostratigraphic structure includes Upper Palaeozoic greywakes, Permo-Triassic reddish to greyish phyllites, Triassic quartzites and dolostones (Bouybaouene et al, 1995;Chalouan & Michard, 2004;Michard et al, 2006;Zaghloul, 1994). Each tectonic unit is characterized by its own thermo-baric evolution, typified by nearly isothermal or cooling exhumation path, with final equilibration under low greenschist facies conditions (M 2 retrogressive stage; Bouybaouene et al, 1995;Rodríguez-Ruiz et al, 2019;Ruiz Cruz et al, 2010;Vidal et al, 1999;Figure 2b). In TZ, the M 1 assemblage cookeite-pyrophyllite-phengite (low substituted) corresponds to LP-LT (0.3-0.4 GPa and 300°C) metamorphic conditions.…”

Low-Temperature Chlorite Geothermometry and Related Recent Analytical Advances: A Review

“…Therefore, and contrary to empirical equations, these two geothermometers predict that one chlorite composition corresponds to one temperature of formation, but that at any fixed temperature, a range of chlorite compositions exist and can be theoretically calculated, in agreement with observations of natural chlorites. These two geothermometers are easy to use, circumvent bulk rock composition effects, and perform well in diagenetic to low-grade metamorphic contexts (e.g., [55,[57][58][59][60][61][62]), particularly in the T range where Si-rich chlorites are observed. They were later derived in a graphical way by [23] to estimate T only from Si and R 2+ contents, making it as easy to use as empirical thermometers.…”

“…The effect of the nonconsideration of Fe 3+ has a subordinate role because temperature estimates are, in the first place, controlled by TK and DT substitutions [23,55], in particular at the Fe total content and XFe 3+ ranges usually observed for LT chlorites. Studies that compare T estimates from the thermometer of Bourdelle et al [55] with those of Inoue et al [28], Vidal et al [49,50], or Lanari et al [51], which require prior quantification of Fe 3+ content, conclude at the convergence of the two approaches [23,59,60,62,77], especially when T < 250 • C.…”