Mineral chemistry of Al Haruj low-volcanicity rift basalts, Libya: Implications for petrogenetic and geotectonic evolution

Field and mineralogical features of the Al Haruj and Libyan volcanics in general are typical of the low-volcanicity rifts. The Libyan continental rift zone is probably related to the larger Afro-Arabian rift system. The low Mg# of clinopyroxene phenocrysts (59-81 and 50-73 for AHB-I and AHB-II, respectively) and olivine groundmass crystals (60-71 and 53-66 for AHB-I and AHB-II, respectively), implying the evolved nature of these rocks with the AHB-II are relatively more fractionated. Clinopyroxene, olivine and chromian spinel compositions suggest that the AHB-II were crystallized at higher temperature and pressure, probably at greater depth, compared to the AHB-I. It is proposed that the tholeiitic basalts (AHB-I) represent pre-rift stage magma, probably generated by higher degree of partial melting of asthenospheric mantle source, at shallow depth, whereas the dominant alkaline basalts (AHB-II) represent the rift stage magma formed by low degree of partial melting of the same source, at greater depth.