Intervention of carbonate components in petrogenesis of the pyroxene nephelinites from the Jbel Saghro (Anti-Atlas, Morocco) Available to Purchase

Introduction. – The Jbel Saghro alkaline complex was emplaced close to the eastern edge of the Moroccan Anti-Atlas. Within the northern part, two types of nephelinite were recognized [Ibhi and Nachit, 1999 and Ibhi, 2000]. The first type (olivine-rich nephelinite) constitutes the main volcanic mass south of the Bou Gafer granit (fig. 1), where the volcanism had been active at least during 2 Ma, between 9.6 and 7.5 ± 0.1 Ma [Berrahma et al., 1993]. The second group outcrops in the north (Foum El Kouss). It consists of pyroxene nephelinites which are younger (2.9 ± 0.1 Ma) [Berrahma et al., 1993], and bears carbonatitic xenoliths, melteigitic pyroxenites and metasomatised peridotite xenoliths. Geochemically, the pyroxene nephelinite is highly enriched in LILE compared with the first one. The mineralogical and geochemical characteristics may be explained by the incorporation of carbonatitic and melteigitic pyroxenite segregates of carbonatitic affinity.

Petrology and mineralogy. – Nephelinites. – The chemical analyses of minerals were done using the microprobe SX 50 of the micro-analysis laboratory (University of Nancy I) and of CAMPARIS (Paris VI). Chemical compositions of minerals are presented in table I.

The petrographical and mineralogical studies show that these nephelinites could be subdivided into two groups :

• – olivine nephelinites (according to the terminology of Le Bas [1987]) are more or less rich in coloured minerals : olivine (Fo_80–85), Ti-rich augite (3.8 to 4.5 wt. % TiO₂) with relatively low Na₂O (0.5 to 0.9 wt. %) and oxide (Ti-magnetite). Olivine phenocrysts are always present while augite exists only in the form of micro-phenocrysts. The groundmass is made up of augite, nepheline and Fe-Ti oxide micro-crystals;

• – clinopyroxene-rich nephelinites with strongly zoned phenocrysts;the green core of phenocrysts is Fe-rich diopside (11.4 to 13.4 wt % FeO and high Na₂O up to 2,2 % wt. %). The rim is Ti-rich augite, similar to the augite micro-phenocrysts from olivine nephelinites. The olivines (Fo_78–82) are present in the form of sub-automorph crystals of a composition less magnesian than that of the lower flow. The groundmass is formed by nepheline, plagioclase, sanidine and Ti-magnetite micro-crystals.

Nature of enclaves

Carbonatites. – Pyroxene nephelinite are characterized by the presence of calcite carbonatitic xenoliths. Their size is variable (a few millimeters to a few centimeters) and their texture is generally granular to micro-granular.

Carbonate (table II) is a low-Mg (less than 0.4 wt. % MgO) calcite with high SrO (up to 3.4 wt. %) and relatively high BaO (1.2 wt. %). Rare Ba-Ti biotite, containing up to 21.5 wt. % BaO and 13.8 wt. % TiO₂, occurs in the groundmass of most samples, along with SrO-rich (1.8 wt. %) fluorapatite (4 wt. % F). The pyrochlore is a niobozirconolite of a structural formula CaZr(Ti, Fe, Nb)₂O₇, generally associated to the magnetite and the apatite [Williams, 1996]. The mean for Nb₂O₅ of 4 analyses is 20.1 wt. % (range 17.5 to 20.9 wt. %), and for Zr O₂ the mean is 23.2 wt. % (range 21.7 to 25.5 wt.). The clinopyroxene is diopside with Na₂O up to 0.7 wt. % and Al₂O₃ up to 1.5 wt. % (table II).

The presence of Sr-rich calcite and pyrochlore establish the carbonatitic nature of the xenolith [Ngwenya and Bailley, 1990].

According to the geothermometers of Stormer and Carmichael [1971], revised by Andersen and Austrheim [1991], the temperatures calculated for the exchange reaction F / OH between biotite and apatite, are situated between 650 and 665°C.

Mineralogical studies do not allow the pressure of inhaduction for carbonatites, however the absence of dolomite suggests that their crystallization took place at pressures lower than to 3 kbar, according to the remarks by Le Bas [1987].

Peridotites. – The peridotite xenoliths contained in the nephelinites of Jbel Saghro are all, according to Hart’s [1977] nomenclature, of a porphyroclastic texture with a granuloblastic tendency.

Two types can be mineralogically distinguished (table III) :

1. – the first one does not contain any trace of destabilisation. It is characterized by an assemblage of minerals in equilibrium and with composition typical of mantle lherzolites : olivine (Fo_90–91), orthopyroxene (En_90–92), diopside (Ca_46–59 Fe_05–07 Mg_43–47) and spinelle (mg* = 82 and 100 × Cr / (Cr+Al) = 10), which can be considered as primary ;

2. – the second type, which occurs only in pyroxene nephelinites, is characterized by the presence of millimetric and pale-green reactional aggregates which are scattered throughout the sample and filled by a microgranular mineral assemblage. These aggregates are interconnected by a microveinlet network. The microgranular mineral assemblage consists of green diopside (containing up to 0.67 wt. % Al₂O₃ and 2,2 % wt. Cr₂O₃) rich in fluid inclusions of CO₂, olivine (Fo_{90 – 91}), chromite (100 × Cr / (Cr+Al) = 72 to 79) and interstitial anorthoclase (Ab_52–56 , Or_41–45 , An_01–02). The scanning electronic microscope equally shows the presence of very small apatite crystals in these aggregates.

Melteigitic pyroxenite cumulates.– A melteigitic pyroxenite inclusion has been found in a pyroxene nephelinite. Major phases are Na, Fe rich (4 wt. % Na₂O and 20 wt. % FeO) diopside, nepheline (Ne₆₉ – Ks₂₇ – Qz₀₄) and SrO rich (1.5 wt. %) fluorapatite (3.5 wt. % F). Carbonate globules are common in these xenoliths. The carbonate is SrO rich (2.3 to 5.0 wt. %), FeO, MgO and LREE barely detectable with the electron microprobe.

Geochemistry. – Major and trace element analyses for Jbel Saghro nephelinite and carbonatite xenoliths are presented in table IV. Major elements were analyzed by ICP and trace elements by ICP-MS with LabRobStation system (rocks and minerals analysis service, Nancy). The nephelinites are strongly SiO₂ undersaturated (< 43 wt. %) and they contain 15 to 25 % of normative nepheline.

Globally, the two types of nephelinites show similar trends, which suggests a possible common source. According to this hypothesis, the LILE increase observed in the pyroxene nephelinites can be explained by a decrease of partial melting rate, which would be in agreement with its higher under-saturation in SiO₂. However, in comparison with the olivine nephelinites, the pyroxene nephelinites are clearly enriched in HREE (fig. 2) and in the less incompatible elements (fig. 3) while the Th, U, Rb, and K concentrations are similar. This observation argues against the previous hypothesis and suggests a more complicated petrogenetic process for the pyroxene nephelinite.

Discussions and conclusions. – The petrological study of peridotite xenoliths from the pyroxene nephelinite shows that the lithospheric mantle of this region was metasomatised. Metasomatism is represented by extensive petrological and mineralogical changes [Ibhi et al., 1999c]. The reactions produce aggregates, which are predominantly composed of high-Cr diopside, alkali feldspar, chromite and apatite. The paragenesis described in these samples and the experimental data on the peridotite-carbonate systems [Brey et al., 1983] suggest that the reacting fluid was carbonate rich. The abundance of CO₂ inclusions observed in these peridotites also favours this interpretation.

It remains to be seen whether a carbonatitic origin is possible for these pyroxenite cumulates. Their mineralogy (diopside + apatite + Ti-magnetite + nepheline + calcite) is well known in the pyroxenites of carbonatitic complexes [Le Bas, 1977; Bouabdli, 1994], they can be considered as melteigitic. The petrogenetic relationship between carbonatites and pyroxene nephelinites has been previously emphasised by Le Bas [1987].

Globally, the pyroxene nephelinites are caracterized by: (i) the presence of phenocrysts of highly reverse zoned clinopyroxene : green core of Na, Fe rich diopside partially resorbed and pink rim of augite (table I), this one is rich fluid CO₂ inclusions; (ii) the presence of small carbonatite xenoliths, (iii) a considerable enrichment in HREE and in the less incompatible elements while the Th, U, Rb, and K concentrations are similar. This shows that there is an intervention of carbonatite segregates in the petrogenesis of these pyroxene nephelinite.

The intervention of a carbonatitic component during the petrogenesis of the Jbel Saghro pyroxene nephelinite can be geochemically evidenced by the variations of ratios implying trace elements fractionated by carbonates [Hamilton et al., 1989; Brenan and Watson, 1991]. Thus, the decrease of Hf / Sm and the increase of Ba / Th and Sr / Th between olivine nephelinite and pyroxene nephelinite are in good agreement with this carbonatitic influence.