Petrogenesis of the Main Petrologic and Chronologic Volcanic Phases in the Gharyan Province, NW Libya.

Abstract:

Cenozoic Libyan volcanic fields are manifestations of tremendous episodic outpourings of basaltic lavas within the East Saharan Craton. The volcanic fields are confined to a NW-SE trend (N140°E) that stretches from the Mediterranean coast in the north to Tibesti (Libya-Chad border) in the south. The four major volcanic fields (Gharyan, As-Sawda, Al-Haruj, and Nuquay) show a systematic decrease in age starting from ~55 Ma in Gharyan (NW) to the Holocene in Nuquay (SE). This apparent trend in ages along with characteristics resembling oceanic island basalts (OIB’s) prompted several authors to attribute their origin to the African plate moving over a hot spot (e.g. Conticelli et al. 1995; Woller and Fediuk 1980; Hegazy 1999).

 In the Gharyan province (GVP), the igneous activity was indeed episodic and lasted for at least 50 Ma. The large span of ages of these volcanic rocks within the same volcanic field makes the hot spot model at least equivocal. Whole rock analyses for the major petrologic and chronologic units suggest that the basaltic and phonolitic suites within the GVP had different primary sources. The basaltic rocks show smooth REE patterns. LREE/HREE fractionations of the eruptive pulses are inconsistent with their ages, suggesting that they represent different melt fractions generated from the same mantle source. Phonolites show very different REE patterns. The patterns are concave-upward with low Tb^N/Yb^N ratios (0.6-0.8). The origin of the GVP basaltic rocks is consistent with melts generated from metasomatized lithospheric mantle across the garnet-spinel transition zone. The most primitive (>7 wt % MgO) basalts were used to model mantle melting processes and indicate 3-10% melting of an amphibole-bearing, spinel/garnet mantle source. Rather than being related to a hot spot, the genesis of the Libyan lavas appears to have been caused by reactivation of lithospheric megastructures with asthenospheric upwelling, in relation to the Africa-Europe convergence.