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

A striking feature of the recent Asela-Ziway magmatism (< 1.8 Ma) is the bi modality (basalts-pantellerites) of the oldest and youngest volcanic produc ts and the unimodal character (pantellerites) of the intermediate period pr oducts. The lavas of the Chilalo central composite volcano display basaltic to trachyandesitic compositions. Basalts, mugearites and are all enriched in LREE (7 < (La/Yb)(N) < 14) whereas a decreasing level of enrichment with increasing SiO2 is observed among the most differentiated samples (SiO2 > 68%). A strong negative Eu anomaly is also observed in felsic samples. The Asela-Ziway mafic samples and Chilalo rocks display smooth multi-element pa tterns with positive Ba and strong negative Cs anomalies. Negative Sr and T i anomalies appear in Chilalo intermediate lavas and become particularly im portant in the Asela-Ziway rhyolites, reflecting lower-pressure fractionati on 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 simila r range of Nd and Pb isotopic ratios: 0.51265 < Nd-143/Nd-144 < 0.51280, 18 .00 < Pb-206/Pb-204 < 18.59, 15.53 < Pb-207/Pb-204 < 15.58, 38.23 < Pb-208/ Pb-204 < 38.66. The Asela-Ziway mafic rocks and Chilalo samples display var iable but moderately radiogenic Sr-87/Sr-86 (0.70392-0.70510). Differentiat ion among these rocks is accounted for by fractionation of more primitive m elts plus variable degrees of contamination, at variable depths, by heterog eneous Pan-African crust. A two-stage model involving small degrees (simila r to 10%) of melting of the mafic lower crust (underplated magmas?) followe d by moderate degrees (similar to 40%) of low-pressure fractionation (mostl y Na-feldspar) is proposed to account for the genesis of the huge volumes o f pantelleritic ignimbrites. The high F content of apatite in Chilalo benmo reites as well as in waters coming from felsic and alkaline rocks in this s ector of the MER give evidence for anomalously high halogen concentration i n 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/an d scavenged from the surrounding crust. (C) 1999 Elsevier Science B.V. All rights reserved.