MESOZOIC BASALTIC MAGMATISM OF THE SIDI SAÏD MAACHOU BASIN (WESTERN MESETA, MOROCCO): PETROGRAPHY, GEOCHEMISTRY AND GEODYNAMIC IMPLICATIONS

Abdelkbir Hminna 1 , * Hafid Saber 2 and Abdelouahed Lagnaoui 2 . 1. Department of Geology, Sidi Mohamed Ben Abdellah University, Dhar El Mahraz, B.P. 1796, Fès-Atlas, 30003, Morocco. 2. Department of Geology, Chouaïb Doukkali University, Faculty of Sciences, B.P. 20, El Jadida, 24000, Morocco. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History

The late Triassic-early Jurassic volcanism of Sidi Saïd Maachou basin belongs to the costalMeseta and the Central Atlantic Magmatic Province (CAMP). The volcanic pile conformably overlies the red siltstones of Machraa Boujamaa Formation. This set includes a stack of several lava flows 40 to 80 m thick. The petrographic study shows that the textures vary from porphyritic to microlitic porphyritic. These igneous rocks have the geochemical characteristics of an intracontinental tholeiitic series of an anorogenic context announcing the Triassic distension. On the basis of their petrographic and geochemical characteristics, the lava flows of Sidi Saïd Maachou tholeiitic basalts approach those of the Triassic Basin of Berrechid-El Gara-Benslimane and the High Atlas. With these data, we attempt to place these volcanic rocks in global geodynamic context by making comparisons with other Upper Triassic-Early Jurassic volcanic rocks genetically related to the breakup of Pangea and the Central Atlantic Magmatic Province resulting from thinning of continental lithosphere.
This work is a contribution to the knowledge of CAMP magmatism of the Moroccan coastal Meseta, presenting new petrographic and geochemical data on the Sidi Saïd Maachou basin. The basal Chaabet Ben Kliba Formation consists of debris-flow conglomerates. They were deposited as alluvial fans, sandstones and siltstones according to Miall (1996), grading vertically into distal alluvial-fan to alluvial-plain and maybe floodplain sandstones and siltstones (Miall, 1996) and soil formation in overbank fines siltstones.
The Oued Oum Er Rbiaa Formation is represented by braided alluvial plain sandstones and mudstones (Sidi Mbarek Member 1), wet mudfla mudstones and siltstones (Sidi Mbarek Member 2), and shallow lacustrine laminated mudstones (playa lake system) (Machraa Abbass Member). The upper Machraa Boujamaa Formation starts with alluvial-fan conglomerates (Machraa Bou Aguida Member 1: M.B.A. 1, Fig. 1D) that grade into increasingly finegrained sandstone-siltstone cycles. Those cycles record the transition into alluvial and floodplain deposition (Machraa Bou Aguida Member 2) and gypsiferous mudstones which are typical playa sediments that are probably formed under semiarid to arid climatic conditions.  1C) and might continue to the North-Western corner, but it is covered by Mesozoic and Cenozoic deposits. Subsurface data are required to more precisely assess the dimension and tectonic structure of the Triassic strata in the Sidi Saïd Maachou area.

Field observations:-
In the Sidi Saïd Maachou area, a large number of profiles were realized through the well developed Triassic-Liasic basaltic pile from North to South along the Oum Er Rbiaa valley (Fig.1C). This makes it possible a good correlation and to specify the conditions for the emplacement of the product of this volcanic activity. However, we will limit the description to the most significant cuts. The stratigraphic data show a superposition of several flows lava intercalated between a lower red detrital strata with thin gypsiferous levels and higher red detrital layer alternating with limestone near the top (Fig. 2).
The base of the volcanic pile conformably overlies the red siltstones of Machraa Boujamaa formation. This set includes a stack of several lava flows that developed over a variable thickness from 40 to 80 m. In some places, we observe the intercalation of sedimentary levels among the lavas. The bottom of these flows is characterized by onion-skin weathering. Towards the middle of this volcanic pile begins a complex of both hard and altered lavas intersected by basaltic dykes (10 to  3). These dykes are observed almost throughout the volcanic pile. They are hard, compact and arranged laterally, vertically or sometimes also in sinuous form ( Fig. 3E and 3F) in relation to the volcanic lava flows. Silica veins are also present frequently. The assembly is surmounted by a succession of altered lavas flows, interrupted in its central part by a prismatic lava deposit of 2 m thick (Fig. 3C, D), which is disseminated into remarkable prisms or columns. The absence of pillow lavas and the presence of such prismatic flows are arguments in favor of sub-aerial conditions. Significant variations of thickness in volcanic profiles (30 to 80 m) can be explained either by differential subsidence of the ante-volcanic substratum during the effusion of the flows or by settling of these lavas on a substratum already structured either in graben or half-graben structures.

Petrographic study:-
The basalts of Sidi Saïd Maachou are largely altered. Indeed, only a few samples were collected in relatively fresh or slightly altered levels. These basaltic rocks are characterized by three main textures: porphyritic, porphyritic intersertal to dolerite, and a slightly microlitic porphyry probably vacuolar microlitic texture. Three volcanic facies are defined according to the textures of the rocks :

Facies 1:-
The rocks show a porphyritic microlitic texture, composed mainly of plagioclase, clinopyroxene, olivine and opaque minerals (Fig. 4A, B). The groundmass phases are represented by glass containing fine crystal of clear feldspars, and opaque minerals.
1514 Olivine accounts for about 8% of the total volume of the rock. Its size varies from 0.2 to 1 mm. It appears as euhedral to subhedral crystals, sometimes isolated but more often grouped in clusters. During hydrothermal alteration, it is completely transformed in either chlorite or iddingsite. However, the preservation of their shape is generally easy to recognize.

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Pyroxene represents approximately 25% of the total volume of rock. The crystals are subhedral with sizes from 0.1 to 0.7 mm. They appear intact and show augite optical characteristics. It exhibits a change in color, may be due to a chemical zoning.
Plagioclase is the most dominant phase (40%). It comes in laths ~0.1 to 0.7 mm long, either as phenocrysts with polysynthetic twins or as microlithes. Some of them are included in the pyroxene. Sometimes it has a symplectic structure, which suggests that these rocks may be the result of a fractional crystallization process.
Opaque minerals are very abundant (approximately 20%) and are present either in small crystals, square to subrounded, or sometimes in sharp crystals and skeletal appearance indicating fast cooling.
The mesostasis corresponds to a cryptocrystalline or microcrystalline material. It generally consists of a multitude of microlites and ferromagnesian plagioclase.

Facies 2
This type of basalts presents a porphyry dolerite texture to intersertal whose crystal size reaches a few hundred microns (Fig. 4C, D). The mineral paragenesis is composed of plagioclase, pyroxene and opaque minerals.
Plagioclase is the predominant mineral in these rocks (50%). They appear as phenocrysts (40%) and as subhedral microlites (less than 5%) which sometimes they overlap phenocrysts of pyroxene.
Occasionally plagioclase is partially transformed into sericite. Pyroxene is less abundant than plagioclase and appears as anhedral to subhedral phenocrysts (10%) and also as microlites (8%). Some are twinned and chloritized the center and the rim. Some are transformed completely into chlorite.
Opaques are abundant and are represented by ilmenite and/or magnetite. They form, along small crystals of plagioclase and pyroxene, the rest of the groundmass. The matrix is also characterized by the presence of reddish glass, filling the interstices between the crystalline minerals.

Facies 3:-
The basalts of this facies are characterized by show microlitic vacuolar texture. The lava flow of particular composition shows generally the vacuoles clogged with opaque and plagioclase (Fig. 4E, F). The whole rock is formed by plagioclase microlites embedded in a fine-grained groundmass made up of plagioclase, pyroxene (present also as phenocrysts: 5% ; and microlites: 50%) and opaque minerals.
Vacuoles occur in large subhedral cavities formed by expanding gas. They are clogged with phenocrysts of plagioclase and pyroxene.

Classification of rocks:-
On the basis of the TAS diagram (Fig. 5A), analyzed samples are classified as sub-alkaline rocks (Irvine and 1517 Baragar, 1971;Miyashiro, 1978) and are mainly represented by basalts to basaltic andesites (Le Maitre et al., 1989).
In the diagram of Winchester and Floyd (1976), which involves elemens considered as immobile during alteration and metamorphism (Ti, Zr, Nb and Y), the rocks plot in the field of subalkaline basalts (Fig. 5B).
These rocks are characterized by a low content of potassium (K 2 O: 0.48-0.99 wt%), which situate the majority of these rocks in the field moderately to weakly potassic "Low-K" defined by Peccerillo and Taylor (1976) and Gill (1981 ; Fig. 5C).  Peccerillo and Taylor (1976) showing the weak to moderately potassic character of Triassic volcanic rocks. D-AFM diagram (Kuno, 1968) showing the tholeiitic character of Triassic magmatic rocks.
In order to determine the geochemical affinity of basalts in the study area, a number of discriminating triangular diagrams were used. This is the AFM diagram (A = Na 2 O + K 2 O; F = FeOt ; M = MgO) of Nockolds and Allen (1956), Al 2 O 3 -FeO t -MgO diagram of Besson and Fonteilles (1974).
The AFM diagram (Fig. 5D) separates the tholeiitic from calcalkaline evolutionary trends, mostly based on the different iron enrichment in the residual melts (Kuno, 1968). The studied rocks clearly plot in the tholeiitic field, being characterized by iron enrichment (Fig. 5D).

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We used the diagram of Miyashiro (1974) of TiO 2 vs. FeOt/MgO (Fig. 6A). The FeOt/MgO ratio in the residual melt increases with the fractional crystallization. The chemical data projection of these rocks on the diagram of TiO 2 according to FeOt/MgO (Fig. 6A) shows a slight enrichment in the titanium during the differentiation of the characteristic of tholeiitic series. This diagram also separates the isotitanium field and the anisotitanium field characteristic of rocks of non-orogenic zones.
Geotectonic Site:-Placed in the geotectonic discriminant diagrams (Fig. 6B, C and D), the Triassic lavas shows the characters of basalts with Anorogenic to sub-alkaline affinity. In the diagrams of Ti O 2 -MnO-P 2 O 5 (Mullen, 1983) (Fig. 6B), Nb-Zr-Y (Meschede, 1986) (Fig. 6D), the representative points of the samples studied are mostly situate in the MORB area and/or intra-continental tholeiites.
The projection of these rocks on the Zr/Y vs. Zr diagram, Pearce (1980), shows that the basalts of Sidi Saïd Maachou occupy largely the field of intraplate basalts (Fig. 6C).

Conclusion:-
Petrographic and geochemical study of basaltic complex of Sidi Saïd Maachou area, consisting of stack doleritic and basaltic flows, clarifies the geochemical significance, magmatic affinity and geodynamic context of establishment of these basalts.
Petrographic study shows a porphyry to microlitic structure and texture.
The geochemical study (major and trace elements) allowed us to make the following conclusions: -The igneous rocks have the characteristics of an intra-continental tholeiitic series.
-The magmatism is part of an anorogenic context announcing the Triassic distension. This volcanism had close links with the opening of the central Atlantic, compared to diabase of the Eastern coast of North America (Bertrand and Westphal, 1977). For all their petrographic and geochemical characteristics, the lava flows of Sidi Saïd Maachou tholeiitic basalts approach those of the Triassic Basin of Berrechid-El Gara-Benslimane and the High Atlas.
With these data, we attempt to place these volcanic rocks in global geodynamic context by making comparisons with other Upper Triassic-Early Jurassic volcanic rocks genetically related to the breakup of Pangea and the Central Atlantic Magmatic Province resulting from thinning of continental lithosphere (CAMP, e.g., Marzoli et al.,1999Marzoli et al., , 2004Callegaro et al., 2014).

Acknowledgements:-
The authors thank reviewers for constructive remarks and the journal's editor for helpful comments that improved this work. Also, many thanks to Faculty of Sciences of Chouaïb Doukkali University El Jadida for logistical support during field trips.