Elsevier

Geochemistry

Volume 81, Issue 1, April 2021, 125694
Geochemistry

Pyroxenite xenoliths and clinopyroxene megacrysts from the Cenozoic Jbel Saghro Volcanic Field (Anti-Atlas, Morocco): Petrography, mineral chemistry and equilibration conditions

https://doi.org/10.1016/j.chemer.2020.125694Get rights and content

Highlights

  • Compositional diversity of pyroxenite xenoliths and clinopyroxene megacrysts in Saghro nephelinites is shown.

  • Mg-number and Cr2O3 contents of clinopyroxene increase from kaersutite-bearing clinopyroxenites to olivine clinopyroxenites.

  • Chemical compositions of clinopyroxene are used to identify the parental melt of pyroxenite xenoliths and megacrysts.

  • Crystallization of pyroxenite xenoliths and clinopyroxene megacrysts at a wide range of pressure (0.3−0.8 GPa).

Abstract

A suite of mafic pyroxenite xenoliths and clinopyroxene megacrysts was brought to the surface by Cenozoic nephelinites of the Jbel Saghro Volcanic Field (Anti-Atlas, Morocco). The large population of samples was subdivided into five groups: (i) clinopyroxenites sensu stricto; (ii) olivine clinopyroxenites; (iii) mica-bearing clinopyroxenites; (iv) kaersutite-bearing clinopyroxenites; (v) clinopyroxene megacrysts. These xenoliths display a cumulate texture (adcumulate, heteradcumulate with poikilitic clinopyroxene including olivine). The clinopyroxenes have the composition of augite and show an appreciable variation of MgO (7.02–14.80 wt.%), TiO2 (0.58–5.76 wt.%) and Al2O3 (2.81–12.38 wt.%) contents in grains. The clinopyroxenes are characterized by convex upward chondrite-normalized REE patterns, they display very similar trace element compositions with low contents of incompatible elements such as Rb (0−0.9 ppm), Ba (0.1–8.3 ppm), Th (0.1−0.3 ppm), U (0.01−0.04 ppm) and Nb (1.3–3.2 ppm). REE contents of the calculated melts in equilibrium with the clinopyroxene megacrysts and clinopyroxene from pyroxenite xenoliths are similar to those of the nephelinites exposed in Jbel Saghro. Crystallization temperatures of pyroxenite xenoliths and clinopyroxene megacrysts range from 950 °C to 1150 °C. Clinopyroxene barometry yielded pressure of crystallization ranging from 0.4 to 0.8 GPa for pyroxenite xenoliths and 0.3 to 0.7 GPa for clinopyroxene megacrysts. This pressure range is in agreement with pyroxenite xenoliths and clinopyroxene megacrysts being crystallized from their parental melts at the lower and upper crust.

Introduction

Megacrysts of clinopyroxene, amphibole, and olivine commonly occur with pyroxenite xenoliths in alkali basalt, basanitic and nephelinitic lavas (Irving and Frey, 1984; Akinin et al., 2005; Shaw and Eyzaguirre, 2000). The megacrysts are considered to be either high-pressure phenocrysts crystallized from the host basalts (e.g. Binns et al., 1970; Irving and Frey, 1984; Schulze, 1987) or as disrupted fragments of pegmatitic veins and cumulates crystallised from mafic magmas (e.g. Schulze, 1987; Righter and Carmichael, 1993; Shaw and Eyzaguirre, 2000; Ashchepkov et al., 2011). Clinopyroxene megacrysts in alkaline volcanic rocks are an important source of information about the geochemistry of melts and the evolution of magmatic systems (e.g. Dobosi and Jenner, 1999).

Pyroxenites are a volumetrically small, but petrologically significant, mantle/crust rock type.

The pyroxenite xenoliths are interpreted as : (i) upper mantle or lower crustal cumulates (crystal segregation from melts) (e.g. Wilkinson, 1975; Wilshire and Shervais, 1975; Choi and Kim, 2012) or (ii) crystallised in magma chambers (e.g. Wilkinson and Stolz, 1997; Villaseca et al., 2019). The nature and origin of crustal pyroxenites is still largely debated (e.g. Downes, 1993), they constitute a valuable source of information about the nature and evolution of the upper mantle and lower crust (e.g. Downes, 1993; Orejana et al., 2006). Previous studies have reported pyroxenite xenoliths transported by alkaline melts in different regions from Morocco (Sirwa; Bondi et al., 2002, Middle-Atlas; Raffone et al., 2009). The only recent descriptions in the literature of pyroxenite xenoliths and clinopyroxene megacrysts exhumed by the volcanism in the Jbel Saghro are those of Ibhi. (2000).

The Saghro nephelinites host a large diversity of clinopyroxenite xenoliths and clinopyroxene megacrysts that could shed some light on melt transfer and crystallization process below the Saghro hyperalkaline volcanic system belonging to the so called CiMACI (Circum-Mediterranean Anorogenic Cenozoic Igneous province; Lustrino and Wilson, 2007). In this paper, we present the petrography, the mineral major and trace-element compositions for pyroxenite xenoliths and clinopyroxene megacrysts, as well as major elements for olivine megacrysts from Cenozoic Jbel Saghro Volcanic field. The main goals of this work are: (1) to establish equilibration conditions of the pyroxenite xenoliths and clinopyroxene megacrysts, (2) to estimate the parental melt of pyroxenite xenoliths and clinopyroxene megacrysts, and (3) to propose the origin and sequence of crystallization of the investigated samples at various depths.

Section snippets

Geological background

The Jbel Saghro Volcanic field (JSVF) is located in eastern part of Moroccan Anti-Atlas representing the northern boundary of the West African Craton (Fig. 1). The Anti-Atlas contains two volcanic edifices: the Sirwa stratovolcano in central Anti-Atlas mainly composed by alkaline to hyperalkaline rocks (basalts, phonolites, and trachytes) and the Jbel Saghro volcanic Field (JSVF) with alkaline nephelinitic, tephritic and phonolitic lavas and domes in the eastern part of Anti-Atlas (Berrahma and

Sample descriptions

The samples studied in this work were collected from pyroclastic deposits and lava flows from four areas: Foum el kouss (clinopyroxenites ss, mica-bearing clinopyroxenites, olivine clinopyroxenites, clinopyroxene and olivine megacrysts), Assaka (kaersutite-bearing clinopyroxenites), Tlassem (clinopyroxenites ss and clinopyroxene megacrysts), and Tazlaft Tamzant (clinopyroxene megacrysts). Fifteen pyroxenite xenoliths and five megacrysts were selected from a collection of 40 xenoliths and 20

Analytical methods

Major element compositions of mineral were determined with a CAMECA SX Five FE electron microprobe at the Castaing centre (Université Paul Sabatier, Toulouse, France). Analyses have been performed using 15 kV accelerating voltage, 10–20 nA beam current, and 10 s/background and peak counting times. The concentrations trace-elements in clinopyroxenes were analysed in situ on 120 μm thick polished sections by LA-ICP-MS at the Laboratory Géosciences environnement Toulouse (GET), Observatoire Midi

Host rocks

Pyroxenite xenoliths and megacrysts are found in the nephelinite and phonotephrite lava flows from the JSVF. Microscopic observations reveal that the nephelinites (olivine nephelinites and pyroxene nephelinites) have microlitic textures with olivine and clinopyroxene phenocrysts (from 200 to 700 μm in size) within a microcrystalline matrix composed of clinopyroxene, nepheline, Ti-magnetite with minor amounts of apatite, biotite, perovskite, and melilite. The phonotephrite has more or less a

Clinopyroxene

Major element composition data for the clinopyroxenes in the nephelinites, pyroxenite xenoliths, and megacrysts are presented in Table 2. For samples F310, TLS1, and F423, the clinopyroxenes from the host volcanic rocks were analyzed.

Trace elements in clinopyroxene

REE and trace element analyses of clinopyroxenes were performed (listed in Table 6) on one biotite-bearing clinopyroxenite (sample F34), two clinopyroxenites ss (samples F440 and TLS8), and one Cpx megacryst (TLS1).

REE patterns of clinopyroxenes from the studied samples are practically identical (Fig. 8a). They are, characterized by enrichment in MREE and depletion in HREE (with (La/Yb)N and Ce/Yb)N ratios varying between 4.36–8.14 and 5.98–10.29), similar to those of clinopyroxenes from

Discussion

The upper mantle beneath the Jbel Saghro Volcanic Field (JSVF) consists of peridotites (mainly spinel-bearing lherzolites (Ibhi and Nachit, 1999a; Ibhi, 2000) while this study shows that pyroxenites occur in the crust. Moreover, the pyroxenites and Cpx-megacrysts in alkaline volcanic rocks provide important information on the evolution of melts and magmatic systems (e.g. Dobosi and Jenner, 1999; Berly et al., 2006; Dantas et al., 2009).

The petrography and mineral chemistry data presented in the

Conclusion

Cenozoic nephelinites and tephrites of the Jbel Saghro volcanic field carry a suite of pyroxenite xenoliths and clinopyroxene megacrysts which can be divided into five groups according to their petrological and mineralogical characteristics: (i) clinopyroxenites ss; (ii) olivine clinopyroxenites; (iii) mica-bearing clinopyroxenites; (iv) kaersutite-bearing clinopyroxenites; and (v) clinopyroxene megacrysts. The estimated pressures, ranging from 0.1 to 0.8 GPa, suggest that the pyroxenite

CRediT authorship contribution statement

Abdelghani Soukrati: Investigation, Formal analysis, Writing - original draft, Writing - review & editing, Visualization. Nasrrddine Youbi: Supervision, Conceptualization, Writing - original draft. Michel Grégoire: Writing - review & editing, Validation. Julien Berger: Validation. Moulay Ahmed Boumehdi: Project administration. Abderrahmane Ibhi: Resources. Khalid Rkha Chaham: Writing - original draft.

Declaration of Competing Interest

The authors declare that they have no known competing financialinterestsor personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by the French Centre National of Scientific Research (CNRS) and the Moroccan National Centre of Scientific and Technical Research (CNRST) to MG and NY. IRSES MEDYNA programme is thanked for the two months scholarship awarded to the first author for his scientific stay at the Geosciences Environment Toulouse laboratory. We are particularly indebted to Philippe de Parseval and Sophie Gouy for their help during the electron microprobe analyses. We thank the

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