The granitic dome of Jabal Uwaynat on the Howar-Uweinat-Uplift

Norbert Brügge, Germany
Dipl.-Geol.
 
At Howar-Uweinat Uplift, four episodes of deformation and related metamorphism and anatexis occurred until the final cratonisation during the Late Pan-African event was completed. From that time on, probably six independent magmatic episodes can be recognized up to the Quaternary. This anorogenic type of magmatism is mainly related to a fracture system which originated in the Late Precambrian as a result of intraplate block faulting. Periodical reactivation of these older fracture zones throughout the Phanerozoic gave way to the different types of plutonic and volcanic rock assemblages.

Granitic ring complexes

The oldest intrusive rocks in the Western Desert are among the most prominent features of the Jabal Arkenu and Jabal Uwaynat region in Libya. By their general geologic setting, macroscopic appearance and bulk composition, at least four principal suites can be recognized (RICHTER 1986):

  • Grey-green, calc-alkaline granitoides
  • Red, alkaline granites
  • Porphyritic, calc-alkaline granitoides
  • Alkaline syenite to alkali-granitic ring complexes

Although radiometric age data of the intrusive rocks are scare or not available, textural and cross-cutting relations indicate a chronostratigraphic succession as given above. While the first two rock units are believed to be of Precambrian age, a Paleozoic an d Cenocoic age of the latter two has been proved by KLERKX & RUNDLE (1976), and SCHANDELMEIER & DARBYSHIRE (1984).

Settings of red Rapakivi granites and grey-green granitoids of Precambrian age can be observed in the Djebel Kamil complex  as batholiths at the Egytian-Sudanese borderline.
The Paleozoic porphyritic granitoids are ongly present in the Djebel Kamil complex where they form medium to small-sized, irregularly shaped intrusions. The age of these granites was determined with 430 - 480 Ma (Ordovician age). They have intruded migmatites and orthogneiss.

The young syenite-alkali granite ring complexes of Tertiary age are only touched by Egyptian territory. The most prominent one is Jabal Uwaynat, the intrusive part of the massive, however, is almost completely situated in Libya. Another ring complex is Jabal Babein to the north of Jabal Uwaynat, 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 Djebel Kissu is rising as a prominent landmark above the gravel plains. Less considered is a further complex in Egypt, southern of the Gilf Kebir, the "unnamed plateau".
The dessected oval structure of the Jabal Babein, with its longer axis trending north-northwest, is emplaced in Precambrian gneiss and red granite. It is partly obscured by sand of the Arkenu dune. 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.



Kissu granite


Djebel 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.


The six ring complexes

In the Jabal Uwaynat (Libya) we can see the unusual circular structure. These structure are the surface expression of a granitic intrusion. This Granite intruded into the 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 Uwaynat 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.



Jabal Uwaynat: Left the Libyen part with the granitic intrusions



Area of the observations


We have now seen contact - metamorphosis and destructions of the deposits in the uplifted Basement and sandstones of the Paleozoic at the edges of the structure and inside. All following photos prove, that the granites intruded in the time after deposit of Paleozoic sandstone layers.


Typical erected sandstones nearby the granitic dome


1.) Weathering granite

2.) Well-banded cherts in rocks of the Proterozoic Basement

3.) Crushed and heated rocks of the Proterozoic Basement

4.) Erected and splitted Paleozoic sandstone

5.) By heat hardened Paleozoic sandstones