The Ghirian tertiary volcanic province of Tripolitania, Libya

The Ghirian volcanic province comprises three distinct units. An early (50–55 m.y.) phase of extensive plateau lava (basaltic hawaiite) eruption was followed by the emplacement of small phonolite domes (∼ 40 m.y.). After a long period of inactivity, a rejuvenation phase (< 12 m.y.ago) led to the formation of scattered minor volcanic centres, generally of basanitic composition, and carrying Iherzolite fragments. 39 new analyses illustrate the distinctive chemical characteristics of each episode. Differences in volume and composition between early and late basic volcanicity can be related to regional episodes of strong mantle convection in the early Tertiary and weak convection in the late Tertiary. The phonolites may be fractionates of an intervening injection of basic magma trapped at moderate depth. Continuing structural control was exerted on a regional scale by the zone of uplift and fracturing connecting the Ghirian area with Tibesti.