Abstract :
Al-Haruj Tertiary basalts cover an area of about 50,000 km2 in central Libya. This occurrence is the largest among the other Libyan volcanic rocks. These volcanics are previously differentiated into six phases separated by a long hiatus. The studied phase is also distinguished into two units based on its weathering features, morphology and primary igneous structures. In petrographic bases, these basalt exhibit intergranular, inters ertal, and porphyritic textures which might suggest slight slow cooling history of their magmas. Geochemically, both units are alkaline to subalkaline, and representing two distinguished clustering groups, without real gap between them indicating of two consequence magmatic process, and suggesting an increasing degrees of partial melting of mantle source. They pertain to the within-plate basalts. The linear variation relation of the studied basalts is considered a direct evidence of the importance of low-pressure fractional crystallization in their evolution.The chemical data are normalized to the primitive mantle where the observed positive Nb anomaly reflects the existence of a residual Nb-bearing phase during partial melting. Moreover, the abundance of Ba, Rb and Sr are controlled by source composition, residual mineralogy, degree of partial melting and extent of fractional crystallization. The presence of subalkaline basalts in studied rocks suggests that a deep mantle upwelling (plume or hot spot) is the responsible for the initiation of rifting and the magma may has been generated from an OIB mantle source within the plume that may contain little contribution from the as theno spheric MORB-source components
