CHARACTERISATION OF THE LITHOSPHERE BENEATH THE NORTH-WESTERN ETHIOPIAN PLATEAU: SPINEL LHERZOLITES FROM INJIBARA (LAKE TANA PROVINCE)

Abstract

We have investigated xenoliths of spinel lherzolites in a Quaternary basanitic flow near Injibara (Tana Lake Province) to characterise the mantle beneath the north-western Ethiopian Plateau. Previous studies of mantle rocks from the Main Rift (e.g. Mega, South Ethiopia; Bedini et al., 1997), have indicated that peridotites suffered modal (apatite) or cryptic (enrichment/depletion in LILE) metasomatism. The metasomatic agents were identified as silicate melts migrating through the lithospheric mantle. At Injibara, lherzolites [Ol (46-69), Opx (19-31), Cpx (9-22), and Spl (2-7) in vol.%] show porphyroclastic to equigranular textures. These rocks suffered a modal metasomatic event, testified by growth of Ti-pargasite, and enrichment in major (Mg,Na,Ti) and trace elements (LREE,Rb,Ba,Th,U,Pb). Equilibration temperatures are 907-1015 °C (median values 960-970°C), in agreement with previous data obtained from Injibara mantle xenoliths (954-985°C, Conticelli et al., 1999). Fluid inclusions (<1-40 _m) occur only in the most preserved rocks, along intragranular trails within porphyroclasts, and as rare primary inclusions in Opx neoblasts. At room temperature, monophase (L) and biphase (L+V) inclusions are observed with rare birefringent solids, recognised as magnesite by Raman spectroscopy. Microthermometric data reveal the presence of CO2 that melts (TmCO2) instantaneously at @ -56.6°C. Most of inclusions homogenise to the liquid (ThLCO2) between –39.2 and 31°C, and only rare homogenise to the vapour (ThVCO2; 28.3 - 30.9°C). For a temperature of 970°C, a minimum pressure of 0.95 GPa is obtained by the highest density (1.11 g/cm3) fluid isochore. In conclusion, our data indicate that the mantle under Injibara (North-Western Ethiopian Plateau) underwent a different evolution with respect to the mantle beneath the Rift. In mantle rocks beneath Injibara, growth of amphibole and the LILE enrichment are strictly associated to the migration of a CO2-rich fluid, whereas in rift peridotites, apatite growth and variable LILE contents are related to interaction with silicate melts.