In the area southwest of Hagar al Garda, and up to 50 km distance, there are a number of upstanding plugs. One such is 1.2 km in diameter and consists of trachyte but has a lenticular body of syenite within it which contains radiating aegirine needles.
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The two most useful general sources of information on the igneous rocks of Libya are the geochemistry section of the third volume of “The geology of Libya” (Salem and Busrewil, 1980) and Goudarzi’s (1970) account of the geology and mineral resources, which includes a 1:2,000,000 geological map (Conant and Goudarzi, 1970) that is particularly useful for demonstrating the distribution and broad extent of the volcanic rocks. A number of pertinent papers are in the seventh volume of “The geology of Libya” (Salem et al., 1991)
This is a poorly exposed granitic intrusion which may be alkaline but no details have been traced.
This also is a poorly exposed granitic intrusion which may be alkaline but of which no details have been traced.
The Archenu complex intrudes Precambrian metamorphic basement rocks and overlying Cambro-Ordovician sediments.
This is by far the largest intrusive complex in Libya covering 430 km2. The occurrence extends eastwards into Egypt and Sudan, but the greater part is located within Libya.
A 4 km diameter ring complex occupies part of Chazzi Hill and extends north of the hill as a negative topographic feature. The outermost ring is a coarse nepheline-biotite syenite inside which is a mesocratic nepheline syenite.
Sandara Hill is a volcanic plug standing some 200 m above the desert floor. The Nubian Sandstone at its foot dips steeply outwards for 50 m from the contact before becoming horizontal again. The principal rock type is a nosean-bearing phonolite.
80 km west of Archenu (No. 097-00-012) two circular intrusions of carbonatitic rocks dome gently dipping Cretaceous sandstones and siltstones. The two intrusions are some 5 km apart and both are about 3 km in diameter but dome the associated sediments over 9-10 km.
An extensive volcanic field east of Gharyan, sometimes referred to as the Jebel Nefusa volcano (e.g. Bausch, 1978), extends over 3000 km2 and consists of plateau lavas, which are rarely more than a few 10s of metres in thickness, and valley fillings.
Several dykes and sills of phonolite cut granites and metamorphic rocks in this area. Two main textural varieties are present both of which contain alkali feldspar, nepheline, aegirine and zeolite with eudialyte present in one variety. Two chemical analyses are available.