3.a. Field observations

Metapelites from the CUU present a main NE-striking (N30 to N50) S2 foliation plane, gently to moderately dipping (20 to 50°) towards the NW (Fig. 2). This foliation is marked by the alternation of quartz–feldspars and garnet–biotite–sillimanite-rich levels as well as by the alternation of centimetric quartzo-feldspathic leucosomes and biotite–sillimanite mesosomes (Fig. 3a). This main D2 deformation episode, synchronous with partial melting of the metapelites, is also contemporaneous with the thrusting of the CUU over the MHLU (Homonnay et al. Reference Homonnay, Corsini, Lardeaux, Romagny, Münch, Bosch, Cenki-Tok and Ouazzani-Touhami2018).

Fig. 3.

Field observations. (a) Decametric sill parallel to the main S2 metapelite foliation plane. (b) Edge of the sill with magmatic fluidality and enclave alignments parallel to the main S2 foliation plane. (c) Contact between the micro-diorite sill and the metapelite. (d) Microgranular porphyric texture preserved in the core of the sill. (e) Magmatic mineral orientation (underlined by white dashed lines) within the micro-diorite sill.

In these metapelites we found several pluricentimetric to plurimetric magmatic sills parallel to the main S2 foliation plane (Fig. 3a,b,c). In these ‘micro-granitoid’ sills we recognized two types of enclaves: (1) enclaves of migmatitic metapelites (i.e. host rock, Fig. 4a) and (2) granular to microgranular magmatic enclaves (Fig. 4b). The cores of the magmatic sills are clearly not deformed and contain a well-preserved microgranular porphyric texture (Fig. 3d). On the other hand, the edges of the sills display preferred mineral orientation interpreted as a magmatic flow-fabric (Fig. 3b,c,e). This interpretation is supported by enclave alignments parallel to the main foliation of the surrounding rocks (Fig. 3b). These structural observations are typical for syn- to late-tectonic (i.e. D2 deformation event) magma emplacement. In such a case, the magmatic sills were emplaced in thermal equilibrium with the surrounding metapelites.

Fig. 4.

Enclaves observed within the micro-diorite sill. (a) Metapelite enclave. (b) Magmatic enclave with microgranular texture.

3.b. Petrographic observations

The magmatic sills present a microgranular porphyric texture, while magmatic fluidality is visible in thin-sections from the edges of the sills (Fig. 5a). The sills contain a primary magmatic assemblage essentially composed of feldspars, quartz, biotites and oxides (ilmenite and magnetite). Feldspars occur as subhedral to anhedral large-sized grains (1–5 mm) (Fig. 5b) and present a magmatic zonation. They are predominantly plagioclases (An₇–An₂₅), which exhibit polysynthetic twinning. K-feldspars (orthoclase, microcline) are observed in minor amounts and orthoclase shows Carlsbad twinning. In the groundmass, xenomorphic quartz grains show wavy extinctions. Biotite appears as small (< 1 mm) brown, strongly pleochroic grains in the groundmass or as minute inclusions in large plagioclase grains (Fig. 5b). In the largest biotite grains, needle-shaped rutile developed along cleavage planes. In addition, we frequently observed a ‘ghost’ mineral with a rectangular or rhomboidal shape (around 5 mm), which is totally replaced by an association of chlorite, muscovite and epidote (Fig. 5c). Although the nature of this mineral cannot be definitely determined, the secondary mineral association together with the mineral shapes strongly suggests a primary magmatic amphibole or a primary magmatic pyroxene. Indeed, in one thin-section we recognized one fragment of a rectangular-shaped pyroxene within the pseudomorphic aggregate. Magnetite occurs as more or less rhombohedral isotropic crystals in the groundmass, while ilmenite is found as elongated dark brown frequently altered grains. Accessory minerals are titanite, apatite, monazite and zircon.

Fig. 5.

Thin-section photomicrographs of the micro-diorite sills. (a) Magmatic mineral fluidality marked by white dashed lines (plane-polarized light). (b) Microgranular porphyric texture (plane-polarized light). (c) Typical ‘ghost’ mineral totally replaced by muscovite and chlorite ± quartz (cross-polarized). fsp – feldspar; bt – biotite; ox – oxide; qtz – quartz; ms – muscovite; chl – chlorite.

This primary magmatic mineral association of plagioclase + quartz + K-feldspar + biotite + Fe–Ti oxide + Fe–Mg-mineral (amphibole and/or pyroxene) indicates a moderately differentiated magmatic rock (micro-granodiorite or micro-diorite). The lack of primary magmatic muscovite or aluminium silicates together with the presence of amphibole or pyroxene, Fe–Ti oxides and titanite in the magmatic association is typical for calc-alkaline magmas, i.e. Hybrid Late Orogenic (H_LO) or Hybrid Continental Arc (H_CA) magmatic series in the mineralogical/petrogenetic classification of Barbarin (Reference Barbarin1990) and thus of mixed (crustal + mantle) origin.

Most magmatic minerals underwent secondary metamorphic transformations (Fig. 5c). Feldspars are partly or totally replaced by patches of albite and white mica. White micas also occur as radial aggregates along magmatic grain boundaries. Chlorite forms pseudomorphic patches within magmatic phases. In some samples, epidote developed at the expense of primary Fe–Mg magmatic minerals.

The magmatic enclaves display a magmatic mineral association of plagioclase, amphibole, pyroxene and minor quartz (Fig. 6). Plagioclase occurs as lath-shaped crystals that show polysynthetic twinning. Amphibole is anhedral, green coloured and pleochroic with, when observable, an extinction angle of about 20°. Millimetre-sized pyroxene grains occur as anhedral crystals and are largely replaced by secondary metamorphic minerals. Quartz grains, with irregular grain boundaries, are scattered in the matrix. Accessory magmatic minerals include magnetite and/or ilmenite, titanite and zircon. Secondary metamorphic minerals are well developed; plagioclases are frequently converted to fine-grained white mica, while amphibole and pyroxene are partly or completely pseudomorphosed by muscovite, chlorite and epidote.

Fig. 6.

Thin-section photomicrograph of a magmatic enclave (plane-polarized light). The micro-porphyric texture is underlined by crystals of amphibole (amph), pyroxene (px) and oxides (ox). The domains in white correspond to plagioclase and minor quartz.

The development of a secondary metamorphic mineral association (i.e. white mica + chlorite + quartz + albite ± epidote assemblage) in the magmatic sills and their enclaves attests to a severe metamorphic re-equilibration at the boundary between amphibolite- and greenschist-facies conditions (i.e. synchronous with the D3 tectonic event; see Homonnay et al. Reference Homonnay, Corsini, Lardeaux, Romagny, Münch, Bosch, Cenki-Tok and Ouazzani-Touhami2018 for discussion). Thus, both the magmatic sills and surrounding metapelites are affected by D3 metamorphism, and consequently, field observations together with petrographic analysis are compatible with syn- to late-D2 and pre-D3 magmatic sill emplacement.

3.c. Geochemical investigations

Because the magmatic sills are significantly affected by D3 metamorphism, in order to investigate the geochemical characteristics of these magmatic rocks we sampled the magmatic rocks only in the largest and best-preserved sill. In this sill, in view of outcrop quality, we found only two samples of micro-granitoids with well-preserved magmatic texture (SEB12-04 and CEU14-05) and one enclave (CEU17-23E), also very well preserved. These three samples were selected for major- and trace-element chemical analyses. Bulk rock compositions were obtained by inductively coupled plasma atomic emission spectrometry (ICP-AES) and inductively coupled plasma mass spectrometry (ICP-MS), for major and trace elements, respectively, on rock powders at the Geochemical and Petrographical Research Centre in Nancy (SARM laboratory, CNRS-CRPG; Table 1) following the procedure described in Carignan et al. (Reference Carignan, Hild, Mevelle, Morel and Yeghicheyan2001).

Table 1.

Chemical analyses: major elements (wt %), minor and trace elements (ppm)

In the total alkalis versus silica classification (i.e. TAS diagram after Le Maitre et al. Reference Le Maitre, Bateman, Dudek, Keller, Lameyre, Le Bas, Sabine, Schmid, Sorensen, Streickeisen, Woolley, Zanettin and Le Maitre1989), the ‘micro-granitoid’ samples are micro-diorites, while the microgranular enclave is a less differentiated micro-gabbrodiorite (Fig. 7a). Moreover, with respect to SiO₂ and K₂O (i.e. in the classification of Peccerillo & Taylor, Reference Peccerillo and Taylor1976), all these magmatic rocks have a geochemical signature typical for high-K calc-alkaline magmas (Fig. 7b).

Fig. 7.

Rock classification diagrams using major elements. (a) TAS diagram for igneous rocks after Cox et al. (Reference Cox, Bell and Pankhurst1979). (b) Rock series discriminant diagram from Peccerillo & Taylor (Reference Peccerillo and Taylor1976).

However, the identification of the geochemical characteristics of magmatic rocks re-equilibrated during regional metamorphism is still delicate because of the mobility of the chemical elements during fluid-assisted metamorphism (Bridgwater, Reference Bridgwater1989; Ferry, Reference Ferry1994). Numerous studies have already shown that major elements as well as some trace elements, mainly large ion lithophile elements (LILE) (e.g. Rb, Cs, Sr, Ba) and light rare earth elements (LREE) (e.g. La, Ce, Nd), are considered to be the most mobile elements under greenschist- and amphibolite-facies conditions, whatever the considered lithology (Bridgwater, Reference Bridgwater1989; Potdevin et al. Reference Potdevin, Cotonian and Bertrand1989; Lawrie, Reference Lawrie1992; Ague, Reference Ague1994, Reference Ague2003; Roser & Nathan, Reference Roser and Nathan1997; Brewer & Menuge, Reference Brewer and Menuge1998; Masters & Ague, Reference Masters and Ague2005; Zheng et al. Reference Zheng, Chen and Zhao2009). Therefore, in order to improve the characterization of the geochemical signature of the studied samples, we selected diagrams suitable for magmatic rocks, using only minor and trace elements considered immobile during metamorphic processes.

First, in the Winchester & Floyd (Reference Winchester and Floyd1977) magmatic rock classification including its modification by Pearce (Reference Pearce and Wyman1996), based on immobile element ratios such as Zr/Ti versus Nb/Y, the studied samples range in the field of gabbroic and dioritic rock types (Fig. 8a). Second, we used the accurate Th–Co diagram proposed by Hastie et al. (Reference Hastie, Kerr, Pearce and Mitchell2007) for magmatic rocks transformed under greenschist-facies conditions (Fig. 8b) and the Th/Yb versus Ta/Yb diagram (Fig. 8c) from Pearce (Reference Pearce, Hawkesworth and Norry1983). In these diagrams, the three samples are micro-gabbrodiorites and diorites, whereas their compositions still have a geochemical signature typical for the high-K to shoshonitic series.

Fig. 8.

(a) Zr/Ti versus Nb/Y diagram from Pearce (Reference Pearce and Wyman1996) modified after Winchester & Floyd (Reference Winchester and Floyd1977). (b) Th–Co discriminant diagram for altered volcanic island arc rocks, using immobile trace elements, from Hastie et al. (Reference Hastie, Kerr, Pearce and Mitchell2007). (c) Th/Yb versus Ta/Yb diagram after Pearce (Reference Pearce, Hawkesworth and Norry1983).

Trace-element discriminant diagrams are often considered an important way to determine the tectonic environment of magma formation (Pearce, Reference Pearce, Hawkesworth and Norry1983; Pearce et al. Reference Pearce, Nigel, Harris and Tindle1984). However, trace-element compositions are also a function of source components and the crystallization history of the melts (Rollinson, Reference Rollinson1993; Frost et al. Reference Frost, Barnes, Collins, Arculus, Ellis and Frost2001). Since trace-element diagrams can discriminate various tectonic settings, the composition of the studied samples was plotted on some of these diagrams (Fig. 9). In the discriminant diagrams proposed by Pearce & Cann (Reference Pearce and Cann1973) and Pearce (Reference Pearce and Thorpe1982) (Ti/Zr/Y in Fig. 9a and Cr versus Y in Fig. 9b, respectively), the three samples plot in the calc-alkaline and volcanic arc fields. In the Th/Hf versus Ta/Hf diagram (Schandl & Gorton, Reference Schandl and Gorton2002), samples plot in the active continental margin field (Fig. 9c). Moreover, the two micro-diorites typically have an intermediate rock composition. Consequently, we felt it was also interesting to confront their chemical signatures with discriminant diagrams established for granitoids. In the Nb versus Y diagram from (Pearce et al. Reference Pearce, Nigel, Harris and Tindle1984), the two samples plot in the volcanic arc and syn-collisional granitoid field (Fig. 9d), while using the more accurate Ta versus Yb diagram proposed by Pearce et al. (Reference Pearce, Nigel, Harris and Tindle1984) to distinguish between volcanic arc and syn-collisional granitoids, our samples plot in the volcanic arc granitoid field (Fig. 9e).

Fig. 9.

(a) Ti/Zr/Y discriminant diagram from Pearce & Cann (Reference Pearce and Cann1973). A – field of island arc tholeiites; B – field of MORB, arc tholeiites and calc-alkaline magmas; C – field of calc-alkaline magmas; D – field of within-plate magmas. (b) Cr versus Y discriminant diagram between MORB, volcanic arc basalts (VAB) and within-plate basalts (WPB) from Pearce (Reference Pearce and Thorpe1982). (c) Th/Hf versus Ta/Hf diagram from Schandl & Gorton (Reference Schandl and Gorton2002). (d) Nb versus Y tectonic discriminant diagram for granitoid rocks (Pearce et al. Reference Pearce, Nigel, Harris and Tindle1984). (e) Ta versus Yb tectonic discriminant diagram for granitoid rocks (Pearce et al. Reference Pearce, Nigel, Harris and Tindle1984).

Altogether, the geochemical characters of the studied sills are consistent with a high-K calc-alkaline magma formed within a continental magmatic arc.

4.a. Analytical method

U–Th–Pb dating was performed on monazites and one xenotime from the micro-diorite SEB12-04. In situ analyses were achieved on thin-sections, in order to keep the microstructural context of the analysed minerals. Prior to LA-ICP-MS in situ U–Th–Pb analyses, minerals were examined with backscattered electrons (BSE) (Fig. 10), using a scanning electron microscope (SEM) at the CRHEA in Sophia Antipolis (Valbonne, France), to reveal the internal structure of the minerals. The operating conditions were 15 kV accelerating voltage and a 17 nA beam current. In situ U–Th–Pb analyses were performed at Geosciences Montpellier (University of Montpellier, France) by laser ablation (LA)-ICP-MS using a Lambda Physik CompEx 102 Excimer laser coupled to an Element XR sector field ICP-MS at the AETE-ISO regional facility (OSU OREME). Laser spot sizes were 15 μm. Before analysis, the laser was first fired to clean the sample surface (ten pulses). The targeted mineral was subsequently ablated under ultrapure helium in a 15 cm³ circular-shaped ablation cell with an energy density of 12 J/cm² at a frequency of 2 Hz during 45 s. A gas blank was measured during 15 s before each analysis. Analyses of unknown monazite grains were bracketed by analyses of the Manangotry monazite standard (Poitrasson et al. Reference Poitrasson, Chenery and Shepherd2000). Repeated analyses of this reference mineral were used to correct for inter-element fractionation and for mass bias factor corrections (see details in Bosch et al. Reference Bosch, Garrido, Bruguier, Dhuime, Bodinier, Padròn-Navarta and Galland2011). Quoted ratios (see Table 2) correspond to measured ratios corrected for background and mass discrimination (+ elemental fractionation for the ²⁰⁶Pb/²³⁸U ratios). All ages have been calculated using the U and Th decay constants recommended by Steiger & Jäger (Reference Steiger and Jäger1977). Analytical data were plotted and ages were calculated using the IsoplotEx program (Ludwig, Reference Ludwig2003). Individual analyses in Table 2 and in Concordia plots (Fig. 11) are ± 1σ errors, and uncertainties in ages are quoted in the text at the 2σ level.

Fig. 10.

Backscattered electron (BSE) and microscopic images of monazite grains in the micro-diorite sill. Locations of U–Th–Pb analyses are circled in BSE images. (a) Homogeneous monazite grain. (b) Monazite grain exhibiting light grey core and dark grey rim. (c) Monazite grain with light grey patches and inclusion. (d) Microscopic photo of a monazite (mnz3) surrounded by chlorite (chl) and muscovite (ms).

Table 2.

Results of the U–Th–Pb LA-ICP-MS analysis of monazite and xenotime from the micro-diorite sill SEB12-04

Pb* – radiogenic lead.

Fig. 11.

(a) Concordia diagram for U–Pb analyses on monazites and xenotime. (b) Tera-Wasserburg diagram close-up of young U–Pb analyses.

4.c. Age interpretations

Most of the dated minerals yield an age of c. 21 Ma. In the Ceuta Peninsula, this age is now well documented and is associated with the D3 tectonic event, developed at the boundary between amphibolite- and greenschist-facies conditions, and which corresponds to the development of ductile extension (Homonnay et al. Reference Homonnay, Corsini, Lardeaux, Romagny, Münch, Bosch, Cenki-Tok and Ouazzani-Touhami2018). Interestingly, in the investigated micro-diorite, the dated monazites and xenotime are systematically in textural equilibrium with the secondary metamorphic phases in the groundmass as well as in inclusions within magmatic minerals, the latter being totally replaced by muscovite and chlorite (Fig. 10d). Consequently, monazite ages at c. 21 Ma cannot be interpreted as the ages of magma crystallization but, on the contrary, are related to the late-D3 stage of metamorphism characterized by metamorphic re-equilibration, associated with fluid circulation, during early Miocene time. Resetting of monazite ages during greenschist-facies metamorphism and related fluid activity is clearly demonstrated (e.g. Williams et al. Reference Williams, Jercinovic, Harlov, Budzyń and Hetherington2011; Seydoux-Guillaume et al. Reference Seydoux-Guillaume, Montel, Bingen, Bosse, de Parseval, Paquette, Janots and Wirth2012) and was already outlined on metamorphic rocks from the Internal Rif (Montel et al. Reference Montel, Kornprobst and Vielzeuf2000; Janots et al. Reference Janots, Negro, Brunet, Goffé, Engi and Bouybaouène2006; Michard et al. Reference Michard, Negro, Saddiqi, Bouybaouene, Chalouan, Montigny and Goffé2006; Negro et al. Reference Negro, Beyssac, Goffe, Saddiqi and Bouybaouene2006; Chalouan et al. Reference Chalouan, Michard, Kadiri, Negro, Frizon de Lamotte, Soto, Saddiqi and Michard2008; Rossetti et al. Reference Rossetti, Theye, Lucci, Bouybaouene, Dini, Gerdes, Phillips and Cozzupoli2010; Gueydan et al. Reference Gueydan, Pitra, Afiri, Poujol, Essaifi and Paquette2015; Homonnay et al. Reference Homonnay, Corsini, Lardeaux, Romagny, Münch, Bosch, Cenki-Tok and Ouazzani-Touhami2018) and also in the Internal Betic (Acosta-Vigil et al. Reference Acosta-Vigil, Rubatto, Bartoli, Cesare, Meli, Pedrera, Azor and Tajčmanová2014).