Thursday, March 19, 2015

Ring Intrusion, Jebel Uwaynat, Sudan/Egypt/Libya

Circular structures are always eye-catching from space since they are such unusual features of the terrestrial landscape. In this respect, the Earth is quite different from all the other terrestrial planets and satellites, on which circular impact structures are by far the most common (and sometimes the only) surface features. This elliptical feature forms the Jebel Uwaynat on the triple point of Sudan, Egypt, and Libya (the topographic feature was used to define the geographic point). The structure is the surface expression of an ancient granite intrusion intruded into the Precambrian metamorphic basement along elliptical ring fractures. The granite is clearly more resistant to erosion than the basement it intrudes, which is mantled by the sand sea of the Sahara. Structures of this sort are common in Sudan and Egypt; they were formed 600 — 700 million years ago during the so-called pan-African event, which affected much of North Africa. Little is known in detail about the Uwaynat structure, which has rarely been visited by geologists. (Uwaynat is spelled various ways, including Aouenat.)

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.

Africa Geoscience Review


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