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.
Highlights
The establishment of metamorphic paths in very low- and low-grade metamorphic stratigraphic sequences of clastic rocks is not an easy task as textural relations among minerals are not obvious; intergrowths of phases, a general lack of equilibrium, and changes in the mineral paragenesis are characteristic below the biotite isograd [1,2,3,4,5]
The X-Ray Diffraction (XRD) analysis and petrographical and electron microscopy studies showed that quartz, feldspars and K-micas are the principal phases in all samples (Tables 1 and 2)
We suggest that the Federico units can be considered intermediate units, and in the case that they were considered the cover of Sebtide complex, it is evident that Tizgarine unit shows more affinity for the ghomaride units than for the deepest Federico ones, as evidenced by the lithological characteristics, the progressive trends of phyllosilicates assemblages, and chemical compositions of the K-white micas
Summary
The establishment of metamorphic paths in very low- and low-grade metamorphic stratigraphic sequences of clastic rocks is not an easy task as textural relations among minerals are not obvious; intergrowths of phases, a general lack of equilibrium, and changes in the mineral paragenesis are characteristic below the biotite isograd [1,2,3,4,5] For these reasons, researchers have relied on criteria, such as polytypism, crystallite size values, or chemical characterization of the metastable sheet silicates, that are only qualitative indicators of the stages the phyllosilicates have reached through a series of metastable mineral reactions during low-temperature metamorphism [6]. The data derived from the application of different geothermometric methods, under these metamorphic conditions, may contribute to the knowledge of the pressure (P)–temperature (T) paths and their tectonic significance [11,12]
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