The Jbel Saghro alkaline complex was emplaced close to the eastern edge of the Moroccan Anti-Atlas. Two types of nephelinite were recognized in the northern part [1]: a) olivine-rich nephelinite (between 9.6 and 7.5 ± 0.1 Ma), b) pyroxene nephelinites (2.9 ± 0.1 Ma) bearing metasomatised peridotite xenoliths [2]. In the nephelinites of Jbel Saghro there are mantle spinel lherzolites and spinel harzburgites enriched in LREE relative to HREE and have U-shaped REE patterns, with strongly depleted middle REE. Since [3], U-shaped REE patterns are generally attributed to chromatographic effects resulting from interstitial percolation of small melt fractions through LREEdepleted peridotites. However, the Foum El Kouss (North of the South-atlasic-fault) xenoliths display more complex distributions of trace elements, are enriched in LILE (Rb, Ba, Th, LREE and Sr) and shows negative anomalies of HFSE (Nb, Ta, Zr and Hf). Such trace-element distribution is generally attributed to carbonate melt metasomatism [4]. In pyroxene nephelinite there are megacrysts of green diopside rich in fluid inclusions of CO2, olivine, chromite (Cr/(Cr+Al) = 0.72 to 0.79), interstitial anorthoclase (Ab52 - Ab56, Or41 - Or45, An01 - An02) and apatite phases. The Mössbauer spectroscopy gives more precise evaluation of Fe3+ in spinels [5] and the oxygen fugacity in peridotitic xenoliths which were higher (log (fO2) – log (fO2) FMQ averaging 0.18 to 0.25). [1] Ibhi, Nachit, Abia & Hernandez (2002) Bull. Soc. Géol. France 173, 37–43. [2] Ibhi & Nachit (1999) Géol. Méditerranéenne XXXVI, 1/2, 19–28. [3] Navon & Stopler (1987) J. Geol. 95, 285–307. [4] Rudnick, McDonough & Chappel (1993) Earth Planet. Sci. Letters 114, 463–475. [5] Ibhi, Khatib & Hernandez (1999) Ann. Chim. Sci. Mat. 24, 487–492.
Upper mantle and metasomatic processes on Eastern Moroccan Anti-Atlas
AIT TOUCHNT, Ahmed;VACCARO, Carmela
2010
Abstract
The Jbel Saghro alkaline complex was emplaced close to the eastern edge of the Moroccan Anti-Atlas. Two types of nephelinite were recognized in the northern part [1]: a) olivine-rich nephelinite (between 9.6 and 7.5 ± 0.1 Ma), b) pyroxene nephelinites (2.9 ± 0.1 Ma) bearing metasomatised peridotite xenoliths [2]. In the nephelinites of Jbel Saghro there are mantle spinel lherzolites and spinel harzburgites enriched in LREE relative to HREE and have U-shaped REE patterns, with strongly depleted middle REE. Since [3], U-shaped REE patterns are generally attributed to chromatographic effects resulting from interstitial percolation of small melt fractions through LREEdepleted peridotites. However, the Foum El Kouss (North of the South-atlasic-fault) xenoliths display more complex distributions of trace elements, are enriched in LILE (Rb, Ba, Th, LREE and Sr) and shows negative anomalies of HFSE (Nb, Ta, Zr and Hf). Such trace-element distribution is generally attributed to carbonate melt metasomatism [4]. In pyroxene nephelinite there are megacrysts of green diopside rich in fluid inclusions of CO2, olivine, chromite (Cr/(Cr+Al) = 0.72 to 0.79), interstitial anorthoclase (Ab52 - Ab56, Or41 - Or45, An01 - An02) and apatite phases. The Mössbauer spectroscopy gives more precise evaluation of Fe3+ in spinels [5] and the oxygen fugacity in peridotitic xenoliths which were higher (log (fO2) – log (fO2) FMQ averaging 0.18 to 0.25). [1] Ibhi, Nachit, Abia & Hernandez (2002) Bull. Soc. Géol. France 173, 37–43. [2] Ibhi & Nachit (1999) Géol. Méditerranéenne XXXVI, 1/2, 19–28. [3] Navon & Stopler (1987) J. Geol. 95, 285–307. [4] Rudnick, McDonough & Chappel (1993) Earth Planet. Sci. Letters 114, 463–475. [5] Ibhi, Khatib & Hernandez (1999) Ann. Chim. Sci. Mat. 24, 487–492.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
