Petrographic and structural study of the Tadaout-Tizi n’Rsas dyke swarms.
Magnetic fabrics indicate the tectonic events that affected the Tadaout-Tizi n’Rsas dykes.
Characterization of the variscan deformation and kinematic faults of Tadaout-Tizi n’Rsas anticline.
rock Magnetism, coupled with complementary methods, has long been applied to understand the modes of mineralization emplacements and the evolution of some ore deposits (Essalhi, 2009). Moreover, rock magnetism allows to evaluate the direction of fluid flow (Sizaret
The magmatic system of TTR anticline is located in the NW part of the rural commune of Taouz, about 40 km to the west of the Moroccan-Algerian international frontier. The magmatic system of this study is a part of the Tafilalet region ones. This magmatic system is injected in the Paleozoic formations folded by Variscan orogeny that shows polyphase tectonics with reactivation of paleofaults and creation of new major folds (Baidder
Tafilalet is a Moroccan region with a big mining vocation; since antiquity, it has been the site of a big lead-zinc production. Nowadays, the exploitation concerns mainly the barite ore bodies. The main vein fields of Tafilalet are M’Fis, Shayb Arras, Njakh, Bouizrane, Bou Mayz, Tijekht and TTR. Geographically, the majority of these vein fields are concentrated in areas with abundant magmatic activity (Pouclet
Several studies have been carried in Tafilalet region. The oldest ones are the studies undertaken in the framework of the geological mapping of this area, corresponding to 1/200 000 map sheet of Tafilalet-Taouz (Destombes and Hollard, 1986) recently revised by Destombes, (2006). Numerous works have been consecrated to the lithostratigraphic, pleontologic and sedimentoligical studies (Destombes
In this paper, AMS study was coupled with structural field works to define the deformation having affected the dykes of TTR and to relate the deformation with the Variscan tectonic evolution of the Eastern Anti-Atlas.
Tafilalet region is situated at the easternmost border of the Anti-Atlas. Generally, the Moroccan Anti-Atlas consists of Precambrian basement covered by Paleozoic sedimentary sequence. Precambrian basement is exposed in several inliers oriented ENE-WSW (Choubert, 1943, 1947; Thomas
Paleozoic series of the eastern Anti-Atlas are affected by the Variscan orogeny. This later appear in the development of numerous folds and the reactivation of paleofaults (Soualhine
According to Destombes and Hollard (1986) (geological map of Tafilalet-Taouz, scale 1 :200 000e), the Tafilalet province shows a large distribution of magmatic rocks located at the Ordovician, Silurian, Devonian and Carboniferous formations, but they don’t present any indication of ages. However, recent publications about dykes and sills which crosse the folded paleozoic series of the Anti-Atlas yielded an age of 200-195 Ma. The majority of these magmatic rocks follow the NE-SW direction faults, and are attributed to the Central Atlantic Magmatic Province (CAMP) events (Hailwood and Mitchell, 1971; Hollard, 1973; Sebai
The Cambrian series of Tafilalet show a Cambrian magmatic activity, limited in the Jbel Tazoult n’Ouzina massif (Pouclet
According to several authors (Makkoudi, 1995; Álvaro
At the north part of Tafilalet basin, the Widane Chebbi (C in Fig. 1b), Erg Chebbi (B in Fig. 1b) and Marzouga (G in Fig. 1b) sites show the presence of a laccolith system and sills. All of them took place in the Devonian and the Lower Carboniferous formations (Benharref
The methodology followed in this work consists of mapping, sampling and measuring the fracture system that affects the magmatic bodies of TTR. Thirty-six samples were sampled in two dykes (principal dyke and the dyke crossing the Ordovician formations) to prepare the thin sections for microscopic observation. The dyke crossing the Devonian formations has not been studied because of its advanced alteration.
Also, measurements of AMS were performed. The AMS technique consists of measuring the magnetic susceptibility (k) of a rock sample in different directions and defining the intensity and the direction of the three principal axes k1 ≥ k2 ≥ k3 of the anisotropy of the magnetic susceptibility ellipsoid. The parameters of Jelinek, (1981) were used to characterize the AMS ellipsoid for each station (Table 1).
Sampling method consists of collecting oriented samples from the roof, the center and the walls of the two chosen dykes. Two techniques of sampling were adopted; the first one consists to extract cylindrical cores using a portable drilling machine (a gasoline-powered rock drill), and the second one consists of extracting oriented samples (handblocks) that were core drilled in the laboratory by a settled apparatus. Twenty-three samples coming from six sites were collected in two dykes (principal dyke and crossing Ordovician formations dykes) using the second method. Each drill core has been cut perpendicularly to the length into 2 or 3 specimens. AMS measurements were conducted on fifty-two standard size specimens with 25 mm-diameter and 22 mm high. The AMS measurements were carried out in low applied field of 300A/m (≈ 0.38 nT) using the Susceptometer Kappabridge KLY- 3S (AGICO) working in a weak alternating field in the magnetic laboratory of the Geology Department of the Faculty of Sciences of Chouaïb Doukkali University (El Jadida, Morocco).
The MS data and petrographic study has been coupled in this work to explain the obtained MS value. However, the AMS data and structural field works studies are associated in order to analyze the relationships between these magmatic bodies and the different deformation phases (at a larger regional scale with respect to previous works).
The TTR dykes intersect the Paleozoic sedimentary formations ranging from the Ordovician to Devonian. These magmatic bodies are located in the eastern part of the WNW-ESE oriented anticline of TTR, outlying to the OJTF (Fig. 2). The dykes are located in three zones; (i) the principal dyke trending NE to ENE and dipping 70° to the southeast that took place on the right edge of the Ziz valley. This dyke follows the direction on OJTF branch that put the Ordovician formations into contact with the Middle-Upper Devonian ones (Figs. 3a-b). (ii) the dyke crossing the Ordovician formations located approximately 900 m westward of the principal dyke, it is trending N15° to N20° and dipping 50° to the southeast (Fig. 3b), and (iii) in the Jbel Aoufilale, at the northern limit of the study area, outcrops a dyke crossing the Upper Devonian formations with a NE-SW direction and dipping 70° to the north-west (Figs. 2, 3c, 4a-4c). Generally, the orientations of the studied dykes span from N15° to N70°, with a dominance of the NE-SW direction, with thicknesses ranging from 1 m to 2 m (Table 2).
The TTR doleritic dykes are macroscopically similar; they all show a greenish color with oxidation marks and feldspar crystals. They also show centimetric phenocryst of orthoclase, biotite, pelitic fragments of the host, and rounded amygdales calcite (Figs. 4e-f).
The microscopic observation shows that these dykes have a microlite to microlite porphyry texture with rarely microgranular texture. They are composed of plagioclase feldspar, alkali feldspar, amphibole, pyroxene, sphene (titanite), apatite and biotite in the primary paragenesis, and mesotype (natrolite), actinolite, hematite and calcite in the secondary paragenesis (Fig. 5). The plagioclase feldspar is the most abundant mineral in this facies. It appears in the form of elongated millimetric phenocrysts (c.5mm), more or less automorphic and it sometimes shows a fan form microlites. The plagioclase feldspar is occasionally altered and shows an elongated form (Figs. 5a, 5c-d, 5g-h, 5j-l). Towards the centre of the dyke, alkaline feldspar is increasingly abundant. It often appears as an elongated phenocrysts form (Figs. 5d, 5i). With two planes of cleavage, the amphibole (hornblende) is present in the form of small lozenges. In general, the amphiboles are altered into chlorite or transformed to iron oxides that follow the cleavage planes (Figs. 5b-5c, 5e). The pyroxene is not abundant; it appears automorphic, biotitized and chloritized (Figs. 5c- 5d, 5f). The mesotype (natrolite) is a secondary mineral in this facies. It has a globular form and rarely has an elongated form. This mineral is colorless in the transmitted light with parallel nicols (PN) and with a white to gray color in the transmitted light with crossed nicols (CN). It shows a low relief and low refringence. The mesotype often shows small inclusions which seem to show a cleavage plane (Figs. 5g-i). The other secondary mineral is actinolite which is present in the elongated form, often on feldspars and shows a greenish-blue color in CN and light green in PN, which reflects the effect of the alteration into the chlorite (Figs. 5a-b, 5d, 5g, 5k-l). Accessory minerals include sphene (titanite) with a high relief lozenge form and very apparent irregular cracking. It shows a yellowishpink color in CN and uncolored in PN, and it is often altered into the chlorite (Fig. 5j). They also include greyblue apatite which has an elongated form. It’s present as inclusions in feldspars (Figs. 6a, 6d, 6g). The automorphic crystals of the biotite shows a reaction aureole with the alkaline feldspars. This means that the biotite is anterior to the alkaline feldspars (Fig. 5l). The ferromagnetic minerals (hematite and other iron oxides not identified in transmitted light) often have a reddish-brown color. The hematite appears as veins or disseminated or as amygdales of calcite (Figs. 5b, 5d, 5h, 5k). Iron oxides have a nodule structure reflecting the transformation products of biotite and amphiboles.
The TTR dykes contain, in variable quantities, a number of pegmatitic enclaves and rock fragments embedded in groundmass. These enclaves contain orthoclase feldspar, bitotite, calcite and pelite coming from the hosted formations (Fig. 6). The orthoclase feldspar phenocrysts have centimetric size and a pink color. They often include biotite and mesotype (natrolite). The orthoclase feldspar crystals are altered, and shows fractures filled with iron oxides (Figs. 6a, 6d, 6e). In addition, the biotite phenocrysts have centimetric size and they are easily recognized. They appear in the form of ranges with a brown color, very paleochroics and sometimes chloritized inside the orthoclase feldspar minerals, microcline and forming amygdales of micropegmatite with mesotype grains (Figs. 6a, 6e). Pelitics xenoliths are chloritized, oxidized and often show primary rock inclusions (Fig. 6b). In addition, the calcite xenoliths are essentially expressed as large automorphic to subautomorphic crystals including grained of opaque minerals (Fig. 6f).
The observation of the MS values allows an evaluation of the magnetic mineral content of the dykes. The histogram in (Fig. 7) shows the MS intervals for the two dykes. All the studied samples have a high and positive magnetic susceptibility more than 10-3 (SI), indicating a dominance of mineral with high susceptibility and therefore ferromagnetic minerals in both dykes.
The overall magnetic behavior of TTR dykes is controlled by: (i) the type and the quantity of contained magnetic minerals, and (ii) the effects of alteration of primary minerals to iron oxides.
A number of parameters can be calculated to characterize the AMS ellipsoid, but the most widely used are (Tarling and Hrouda, 1993): the mean magnetic susceptibility (Km), the anisotropy degree (P′), and the shape parameter (T). The volumetric MS averages and its associated parameters, as well as their graphical projection was done using Anisoft 5.1 AGICO program (Chadima and Jelinek, 2009) (Table 1).
The parameter P’= Exp (2[(lnk1/km)2 + (lnk2/km)2 + (lnk3/km)2])1/2 defined by Jelinek (1981) was calculated for every sampled site of the studied dykes. P’ describes the anisotropy degree of the magnetic fabric. It varies from P’=1 for an isotropic (i.e. spherical) AMS ellipsoid to infinity. The shape of the AMS ellipsoid is defined by the parameter T = [ln (k2/k3) – ln (k1/k2)] / [ln (k2 /k3) + ln (k1/k2)] (Jelinek, 1981). When –1≤ T <0 the AMS ellipsoid is defined as prolate (or linear) and as oblate (or planar) when 0 < T ≤ +1 (Jelinek, 1981).
The diagrams in Fig. 8 represent the degree of anisotropy (P') as a function of the shape parameter (T). In the principal dyke, there is a dominance of the "oblate" form of the AMS ellipsoid. This form indicates a dominance of the magnetic foliation and therefore planar structures compared to linear structures. However, the samples with negative T values cannot be overlooked, as they represent more than a third of the studied samples. The degree of anisotropy (P') is low since it is still less than 1.2 where the anisotropy can be linked to an intense tectonic episode. We can then affirm that the deformation is weakly expressed in this dyke (Fig. 8a).
For the dyke that outcrops in the Ordovician formations, the P'-T diagram reveals that the majority of studied samples have a negative value of shape parameter (Fig. 8b). The shape of the AMS ellipsoid is globally "prolate". A magnetic lineation is thus well expressed. The degree of anisotropy P' is generally low, indicating low anisotropy (the magnetic fabric is close to the sphere). This low value of P' indicates that the deformation is weak in this dyke.
As mentioned above, the magnetic fabric of the main dyke is dominated by magnetic foliation. This can be seen in the projection of the AMS ellipsoid axes. Also, we notice the dominance of a vertical magnetic foliation NNW-SSE trending at the site 2, in contrast to site 3 where the foliation is rather horizontal as well as oriented NNW-SSE (Fig. 9, sites 2 and 3). This foliation would reflect the preferential orientation of the magnetic grains, inherited from the late phase of the Variscan deformation. For the case of the dyke outcropping in the Ordovician formations, the magnetic lineation is better expressed (sites 4 and 6) than the foliation (site 5), and this may be related to the observed shearing and very extensive alteration in this locality (Fig. 9).
The WNW-ESE trending TTR anticline; has an Ordovician core. It is characterized by a long north limb and a short southern one (asymmetric fold). This anticline is affected by some kilometric faults and by disharmonic folds. The eastern part of this anticline is limited by the ENE-trending fault (the OJTF), with the same orientation as the main dyke of the study area. Structural analysis of different faults along the TTR anticline shows three families of faults: the NE-SW family which is dominant and the ENEWSW and NNE-SSW families which are less importance (Ait Daoud
The reactivation of the ENE-WSW OJTF during the deformation is marked in the principal dyke that follows it. However, the OJTF reactivation is marked by a N60, 90° cleavage in the principal dyke located between the Ordovician and the Devonian formations (Fig. 10a). This post-setting up deformation is not only limited to the cleavage. It is also marked by the presence of strike-slip faults (left lateral and right-lateral fault) crossing the magmatic bodies of the study area (Figs. 10b-e, 11). All of these faults are oriented NE-SW or ENE-WSW to E-W (Fig. 11). From a microscopic point of view, the deformation is recorded by the pegmatitic enclaves of the biotite (Fig. 10f).
In the study area, the movements observed in the ENEWSW to E-W-trending faults are generally the strike-slip ones (right and left lateral), that are confirmed by the presence of horizontal striaes in the mirror surface, and also the net displacement of the dykes (Figs. 10b-e, 11). These movements are probably expressed by the reactivation of these faults during the NW-SE and NE-SW main shortening stage of the Variscan orogeny.
The eastern part of the TTR anticline exhibits a magmatic system hosted within the Paleozoic formations ranging from the Ordovician to Devonian. It is clearly obvious that these magmatic bodies are essentially distributed in the neighborhood of the major fault that delimited the TTR anticline to the east. This fault, called the OJTF, corresponds to the eastern branch of the AAMF. It should also be noted that this fault is followed by the magma flow that gave rise to the dykes in the study area.
From a petrographic point of view, the magmatic bodies of TTR are of three types: (i) doleritic basalts, (ii) lamprophyric dolerites, and (iii) camptonites (Benharref
The magmatic age of different outcrops of magmatic bodies in Tafilalet region is very difficult to resolve, because the field works combined with ASM data for the dykes of the TTR site (in this paper) are disaccords with the geochronological study in Jbel Dboa site; located at 14 km NE of the study area (Najih
Tectonic analysis of the collected data allowed us to distinguish the main deformation phases that the TTR anticline underwent (Fig. 12): (i) Eovariscan phase responsible for extensive tectonics confirmed by the presence of synsedimentary faults and slumps in the Devonian formations of the TTR. In addition we noted the presence of limestone strata with tamnopores along the OJTF. This deformation phase is responsible for tectonic tilting blocks in the whole Anti-Atlas (Baidder, 2007; Baidder
In our study area, the reactivation of the OJTF and other major faults during the Variscan events has a high importance in the deformation of these magmatic bodies. Moreover, the deformation recorded by these magmatic intrusions is related to the strike-slip movement of these faults during the Variscan shortening, and cannot be related to any other posterior deformation. The ENE to E-W directional left-lateral faults crossing the TTR are related to the late-Variscan shortening (Michard
Based on the work of Pouclet
Geochemically, the TTR site, especially the sills in the eastern part and the dyke on the right edge of the Ziz valley (principal dyke), show a higher melting degree (Pouclet
In the Eastern Anti-Atlas, the Maider basin shows similar magmatic activities with numerous dykes, sills and laccoliths intruded in the Upper Devonian, Tournaisian and Lower Visean strata (Clariound, 1944). An example is the Mecissi volcanic system, which corresponds to a sodic alkaline gabbro rich in kaersutite and Ti-biotite, with interstitial analcite (Salmon
The magmatic bodies of TTR are part of a magmatic complex in Tafilalet region. The discovery of a new dyke in the study area enriches the petrographic and structural studies of these bodies. Petrographically, we note the presence of pegmatitic enclaves with bitotite, calcite, orthoclase feldspar and pelite fragments in the TTR dykes, as well as the appearance of new minerals discovered for the first time in Tafilalet area such as sphene and mesotype which reinforces the idea of magmatism with a sodic alkaline affinity.
The reactivation of the OJTF at the time of Variscan shortening generates a deformation posterior to the set-up of the TTR magmatic dykes. Firstly, these deformations are in the form of cleavage and strike-slip faults affecting the different magmatic bodies of the TTR site. Secondly, the AMS results confirmed the deformation by the dominance of magnetic foliation NNW-SSE trending that reflect the preferential orientation of the magnetic grains, inherited from the late phase of the Variscan deformation. We assume that these deformations cannot be, in any way, explained by late Permian, Triassic or Alpine tectonics, in particular the strike-slip movement marked along the NESW and ENE-WSW to E-W faults. The TTR magmatism has probably a pre-Variscan magmatic event.
We would like to thank Mr. Hicham SI MHAMDI (FST Errachidia) and Mr Abdelaaziz TAKI (Technical high school Errachidia) for their English text amelioration. Our thanks also invited to Mr. Ismail ASSOUSSI and Mr. Abdeslam BOUTAIB for the thin sections’ preparation. The authors also wish to thank the two reviewers for their constructive feedback, and insightful comments and recommendations.