Earth & Life (http://www.geofinds.com), (2006-11-20), 1(3): 1-14.

 

  
   

New insights into mantle processes during Miocene beneath Ethiopia: evidence from spinel lherzolite xenoliths in Simien shield volcano

 

Dereje Ayalew1, Nick Arndt2, Florence Bastien2

 

1. Department of Earth Sciences, Addis Ababa University, P.O.Box 729/1033,

Addis Ababa, Ethiopia (e-mail: ayalder@yahoo.com, dereayal@geol.aau.edu.et )

2. Laboratoire de Géodynamique des Chaînes Alpines, UMR 5025 CNRS, BP 53, 38041 Grenoble Cedex, France

 

Abstract  The ultramafic enclaves enclosed in Miocene alkali basalt (dated at 18.7 Ma) from the western flank of Simien shield volcano are spinel lherzolite. The spinel lherzolites have undergone an ancient phase of deformation followed by a younger phase of partial recrystallization, which requires high temperature. They were also subjected to a slight episode of partial melting. Thin section observations show reaction rims around minerals and olivine electron probe analyses recording Fo values down to 74.8 in olivine indicate that the studied material has been strongly modified by reaction with the host lavas. Calculations of equilibrium condition yield temperature and pressure estimates of 1009 °C and 14.0-15.4 kbar, respectively, typical values of the lithospheric mantle. The REE contents of clinopyroxene crystals are extremely low and look much more like a highly depleted mantle sample, which has suffered extensive melt extraction and are not or little enriched by a later metasomatic process.

Keywords: Simien shield volcano; spinel Lehezolite; lithospheric mantle; metasomatism; xenolith

  
 

1

  
     
 

TO  1，2，3，4，5，6，7，8，9，10，11，12，13，14       Contents of  v1n3