Petrology, mineralogy and isotopes in olivine mela-nephelinites, basanites and carbonatites from Uwaynat region, south east Libya: inferences on their genesis

In the Uwaynat region, there are hundreds of volcanic plugs, dykes and lava flows, ranging in composition from carbonatites to olivine mela-nephelinites, alkali-basalts and minor phonolites and trachytes. Olivine mela-nephelinites, basanites and alkali basalts are the most abundant rock types. These rocks are from ring complexes and mafic plugs located between the Uwaynat Inlier and the Al Kufrah basin, in the Libyan desert. These volcanic rocks range from 28.2 to 26.7 Ma in age. Olivine mela-nephelinites and basanites are characterised by extremely high titania (up to 6 wt.%) and incompatible trace elements contents. Minor ferro-carbonatites are also present and they are also characterised by high content of titania and related high field strength elements and higher content in incompatible trace and rare earth elements than olivine mela-nephelinites and basanites. No large variations in the initial Sr and Nd isotope compositions are observed. This may suggest none or very little contribution of crustal rocks-magma interaction. A similar isotopic signature for the source of olivine mela-nephelinites and ferro-carbonatites is inferred. Olivine mela-nephelinites and basanites are not genetically related to each other by fractionation but they represent primitive magmas generated by different degrees of partial melting of a metasomatised upper mantle source. Incompatible trace elements in olivine mela-nephelinites may be modelled with small degrees of partial melting of such a source. Close relationships are also observed with the slightly older volcanic rocks from Gilf Gebir (SW Egypt), suggesting that Uwaynat magmatism is possibly related to a northeastward migration of the African plate above an intracontinental hot spot.