The Ghirian tertiary volcanic province of Tripolitania, Libya

Abstract

The Ghirian volcanic province comprises three distinct units. An early (50-55 my.) phase of extensive plateau lava (basaltic hawaiite) eruption was followed by the emplacement of small phonolite domes ( N 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 lherzolite 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.

Miocene to recent alkaline volcanism between Al Haruj and Waw an Namous (southern Libya)

“…Woller and Fediuk (1980) analysed a lherzolite xenolith found in a basalt intrusion from Jabal as Sawda. It contains olivine (Fo 90), orthopyroxene, clinopyroxene and spinel and is likely to be of the same type as those described by Almond et al (1974) in Gharyan lavas and analysed by Busrewil and Wadsworth (1980b). More recently, Beccaluva et al (2008) analysed in detail these lherzolite mantle xenoliths from Gharyan that contain up to 35 vol.% orthopyroxene and up to 20 vol.% clinopyroxene.…”

“…WAN samples display the lowest SiO 2 (Fig. 3) and Al 2 O 3 but highest normative Ne contents and Fe 2 O 3 and among the highest Na 2 O, MgO and P 2 O 5 compositions among all analysed Libyan and Tibestian volcanic rocks (Libya, Almond et al 1974;Busrewil and Wadsworth 1980a, b, 1982-1983Woller and Fediuk 1980;Asran and Aboazom 2004;Farahat et al 2006;Beccaluva et al 2008;Cvetković et al 2010;Tibesti, Chad, Gourgaud and Vincent 2004). Note that the less differentiated Gharyan samples (Almond et al 1974) are similar to the foidite analysed here, however (Fig.…”

“…Dashed outline: Jabal as Sawda (Woller and Fediuk 1980). Dashed-dotted outline: Gharyan (Almond et al 1974;Busrewil and Wadsworth 1980b). Dotted outline: Emi Koussi, Tibesti, Chad (Gourgaud and Vincent 2004).…”

“…• Gharyan (or Ghirian) (about 3,000 km 2 ), South of Tripoli, with thin basaltic lava flows (55-50 Ma), phonolitic domes (40 Ma) and young volcanic centres (12-1 Ma) consisting of basanites, mugearites and hawaiites (trachybasalts) (Almond et al 1974;Busrewil and Wadsworth 1980b). • Jabal As Sawda (or Soda), 840 m high, 6,000 km 2 (but probably 10,000 km 2 before erosion) and 1,200 km 3 , consisting of basaltic lavas dated between 16 and 8 Ma (Ade-Hall et al 1974;Woller and Fediuk 1980;Busrewil and Esson 1991).…”

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

“…Geochemical studies indicate that these rocks have a clear anorogenic within plate affinity (e.g. Almond et al, 1974;Busrewil and Wadsworth, 1980;Woller and Fediuk, 1980;Hegazy, 1999).…”