Petrogenesis of the main petrologic and chronologic volcanic phases in the Charyan Province, NW Libya

Abstract

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Gharyan Volcanic Province (GVP) is one of the four major Cenozoic volcanic provinces, together with Jabal as Sawda, Jabal Al Haruj, and Jabal Nuqay, in Libya. These four provinces define a NW to SE linear trend that shows a systematic decrease in age from ̃55 Ma in the GVP to the Holocene in the Jabal Nuquay. The oldest volcanics in GVP are the 55-50 Ma Old Lava Series (OLS) that ascended through a thin interval of the Mesozoic sedimentary rocks within the Wadi Ghan fault zone. The second eruptive cycle is represented by phonolitetrachyte intrusions (̃40 Ma) that have shapes of laccoliths and plugs. The Wadi Ghan fault zone appears to have had a conspicuous effect on their emplacement. The Late Volcanic Center (LVC) represents the youngest volcanic activity in the province (< 12 Ma). Lavas of the Young Lava Series (YLS) represent minor eruptions that have a distinctive appearance in the field and on Landsat images by occupying ancient wadies. Chemical characteristics of the basaltic rocks (OLS, YLS, basaltic sill and LVC) and the phonolites define two distinct groups that are appear to be genetically unrelated. The basaltic rocks show smooth REE patterns, with LREE/HREE fractionation increasing from YLS (LaN/YbN = 7 average), OLS (LaN/YbN = 14 average), LVC (LaN/YbN = 16 average), to the basaltic sill (LaN/YbN = 21 average). The chondrite-normalized HREE concentrations are fairly low (YbN = 13-22). Phonolites show very different REE patterns. The three analyzed samples have concave-upward REE patterns with low TbN/YbN ratios (0.6-0.8). The normalized REE concentrations are very high (LaN ̃1600, YbN ̃ 95) and the patterns have marked negative Eu anomalies (Eu/Eu*= 0.61). Several volcanic and sub-volcanic basaltic rocks incorporate ultramafic lherzolite xenoliths which are include four phases: olivine + orthopyroxene + clinopyroxene + spinel. These xenoliths represent a fertile lithospheric mantle with Cr/(Cr+Al) in spinel of ̃ 0.1. The origin of the GVP mafic rocks is consistent with