Crustal control in the genesis of Plio-Quaternary bimodal magmatism of the Main Ethiopian Rift (MER): geochemical and isotopic (Sr, Nd, Pb) evidence

Abstract

A striking feature of the recent Asela–Ziway magmatism (<1.8 Ma) is the bimodality (basalts–pantellerites) of the oldest and youngest volcanic products and the unimodal character (pantellerites) of the intermediate period products. The lavas of the Chilalo central composite volcano display basaltic to trachyandesitic compositions. Basalts, mugearites and benmoreites are all enriched in LREE (7<(La/Yb) N<14) whereas a decreasing level of enrichment with increasing SiO 2 is observed among the most differentiated samples (SiO 2>68%). A strong negative Eu anomaly is also observed in felsic samples. The Asela–Ziway mafic samples and Chilalo rocks display smooth multi-element patterns with positive Ba and strong negative Cs anomalies. Negative Sr and Ti anomalies appear in Chilalo intermediate lavas and become particularly important in the Asela–Ziway rhyolites, reflecting lower-pressure fractionation of plagioclase, alkali feldspar and titanomagnetite. Strong negative Ba and P anomalies, reflecting alkali feldspar and apatite fractionation, are also observed in these rhyolites. Mafic and felsic samples display a similar range of Nd and Pb isotopic ratios: 0.51265< 143 Nd / 144 Nd <0.51280, 18.00< 206 Pb / 204 Pb <18.59, 15.53< 207 Pb / 204 Pb <15.58, 38.23< 208 Pb / 204 Pb <38.66. The Asela–Ziway mafic rocks and Chilalo samples display variable but moderately radiogenic 87 Sr / 86 Sr (0.70392–0.70510). Differentiation among these rocks is accounted for by fractionation of more primitive melts plus variable degrees of contamination, at variable depths, by heterogeneous Pan-African crust. A two-stage model involving small degrees (∼10%) of melting of the mafic lower crust (underplated magmas?) followed by moderate degrees (∼40%) of low-pressure fractionation (mostly Na-feldspar) is proposed to account for the genesis of the huge volumes of pantelleritic ignimbrites. The high F content of apatite in Chilalo benmoreites as well as in waters coming from felsic and alkaline rocks in this sector of the MER give evidence for anomalously high halogen concentration in these rocks. In this area, melting of the lower crust was induced by the thermal anomaly created by the Afar mantle plume and may have been enhanced by the presence of halogens, either originating from the deep mantle or/and scavenged from the surrounding crust.