Classification of granitic intrusions on the Howar-Uweinat-Uplift

Norbert Brügge, Germany

The oldest intrusive rocks on the Uweinat-Howar Uplift are of Precambrian age. By their general geologic setting, macroscopic appearance and bulk composition, at least two principal suites can be recognized (RICHTER 1986):

  • Grey-green, calc-alkaline granitoides

  • Red, alkaline granites

Settings of red alkaline granites and grey-green granitoids of Precambrian age can be observed as batholiths in the Jebel Kamil and Jebel Nazar  in Egypt.

  • Porphyritic, calc-alkaline granitoides

are of  Paleozoic age and are only present in the Jebel Kamil and Jebel Nazar complexes where they form medium to small-sized, irregularly shaped intrusions. The age of these granites was determined with 430 - 480 Ma (Ordovician). They have intruded migmatites and orthogneiss.

  • Syenite-alkali granite ring complexes

are of Tertiary age. The most prominent one is Jabal Uwainat, the intrusive part of the massive is almost completely situated in Libya. Another ring complex is Jabal Babein to the north of Jabal Uweinat, nearby the border between Libya and Egypt. Ring complexes lying completely inside Libya are Jabal Arkenu and Jabal Bahari. In the southeast, in Sudan, the ring complex of Jebel Kissu is rising as a prominent landmark above the gravel plains.
The dessected oval structure of the Jabal Babein, with its longer axis trending north-northwest, is emplaced in Precambrian gneiss and red granite. The highly complex intrusion is essentially composed of outer quartz syenite ring-dyke and a central white, medium to fine-grained, granite. The syenite shows little variation, and is a pink, coarse-grained, equigranular rock containing major perthitic alk-feldspar, subordinate plagioklase with minor biotite and quartz. Basic to acidic cone sheets are intercalated and, in many places, variably composed dykes have penetrated the complex.
Jabal Bahari appears to consist of a single ring dyke of medium to coarse-grained pink granite, while Jabal Arkenu is mainly made up of medium to coarse-grained nepheline syenite beside subordinate syenite with trachyte and pyroclastic phonolithe in the centre. Jebel Kissu is built up by a steep outer ring of coarse-grained syenite, deeply weathered to large boulders, and a centre of sheeted, fine-grained quartz syenite. The complex is cut by frequent dykes of intermediate to basic composition and variable thickness.

In the Jabal Uweinat (Libya) we can see the unusual circular structure. These structure are the surface expression of a granitic intrusion. This granite intruded into the Archaen/Proterozoic metamorphic Basement as well as Paleozoic sandstone strata along a ring fracture. Three igneous activities has occured probably in the Early Oligocene age (42-46 Ma).
The composite intrusion of the Jabal Uweinat is roughly circular with a diameter of about 23 km, and is flanked on the northern side by three smaller overlapping intrusive rings which are aligned along a north-northeast-trending axis. The main complex is composed of deeply weathered alkaline rocks, forming very large well-rounded blocks. The outer rim is formed by coarse-grained quartz syenite which steeply slopes toward the surrounding basement. The rock is assembled of quartz up to 12% and major euhedral perthitic alk-feldspar beside minor aegirine. Toward the centre, in the south and west, the outer intrusive ring is followed by a highly complex zone of coarse-grained quartz syenite and coarse to fine-grained alkali granite. This zone forms the outer rim in the north. According to KLERKX & RANDLE (1976), similar to the syenite the alkali granite is essentially composed of subhedral alk-feldspar but with an increased amount of interstitial quartz and aegirine. The central depression of the ring complex is occupied by a reddish, fine-grained quartz syenite with trachytic matrix that encloses rare phenocrysts of K-feldspar. The rock is locally cut by trachytic dykes and sills. A steep inner slope around the central quartz syenite depression is build up by alternating cone-sheets of syenite, consisting largely of perthite with no quartz.