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

Abstract

Al Haruj basalts represent the largest volcanic province in Libya (∼16,000 km 2). Field, petrographic and mineral compositional studies of the NW corner of this province have been undertaken in order to better understand their petrogenetic and geotectonic evolution. These basalts have been distinguished into two phases of different field and mineralogical characteristics. Clinopyroxene compositions classified the earlier phase (AHB-I) as subalkaline, tholeiitic basalts and the later, more voluminous, phase (AHB-II) as alkaline ones. Clinopyroxene and chromian spinel compositions suggest anorogenic within plate affinity for basalts of the two phases. Chromian spinel of the Al Haruj basalts shows compositional similarities to those of the ocean island basalts (OIB). 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.