Carbonated alkali-silicate metasomatism in the North Africa lithosphere: evidence from Middle Atlas spinel-lherzolites, Morocco

Natali, C., Beccaluva, L., Bianchini, G., Ellam, R.M. , Siena, F. and Stuart, F.M. (2013) Carbonated alkali-silicate metasomatism in the North Africa lithosphere: evidence from Middle Atlas spinel-lherzolites, Morocco. Journal of South American Earth Sciences, 41, pp. 113-121. (doi: 10.1016/j.jsames.2012.06.014)

Abstract

<p>Mantle xenoliths from Pliocene-Quaternary volcanic diatremes in the Azrou area (Middle Atlas, Morocco) include protogranular to porphyroclastic spinel lherzolites with superimposed metasomatic textures involving pyroxenes and spinel as the main reacting phases.</p> <p>Thermobarometric estimates on these xenoliths show pressure (P)–temperature (T) equilibrium conditions in the range 1.1–1.4 GPa and 900–1090 °C.</p> <p>Bulk rocks have flat heavy rare earth element (HREE) patterns (0.6–2.2 times chondrite) and are variably enriched in light REE (LREE; LaN/YbN between 1.1 and 15.6).</p> <p>The constituent clinopyroxenes are characterized by flat HREE distributions (5.1–11.9 times chondrite) and variable LREE enrichment with LaN/YbN from 0.4 to 25, which generally conform to the bulk rock chemistry. Trace elements characteristics of the metasomatised clinopyroxenes suggest that the metasomatasing agents were highly alkaline carbonate-rich melts such as nephelinites/melilitites or, as extreme, silico-carbonatites.</p> <p>Sr–Nd isotopic composition analyses carried out on clinopyroxene separates yield 87Sr/86Sr = 0.70243–0.70335, 143Nd/144Nd = 0.51273–0.51325. The helium isotope composition of olivines ranges from 6.2 to 6.8 Ra. These values, coupled with the radiogenic lead isotopic composition available in the literature, suggest that the Middle-Atlas lithospheric mantle interacted with HIMU-like metasomatic components. These are in turn related to local mantle upwellings along pre-existing tectonic lineaments – located at the northern border of the West African Craton – that were reactivated as far-field foreland reaction of the Africa-Europe collisional system.</p>

• Item Type: Articles
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Ellam, Professor Rob and Stuart, Professor Fin
• Authors: Natali, C., Beccaluva, L., Bianchini, G., Ellam, R.M., Siena, F., and Stuart, F.M.
• College/School: College of Science and Engineering > Scottish Universities Environmental Research Centre
• Journal Name: Journal of South American Earth Sciences
• ISSN: 0895-9811