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I N T R O D U C T I O N

A.M.S. AL-SHANTI

Dean, Institute of Applied Geology, Jeddah (Saudi Arabia)

The Arabian--Nubian Shield comprises the Precambrian basement terrain underlying large areas of the Middle East (mainly Saudi Arabia, Egypt, Sudan, Yemen, Somalia and Ethiopia). The recent intense interest and act ivi ty in searching for new mineral deposits in this region has led to a wealth of geological investigations by diverse individuals and groups f rom many parts of the world. In this context , exchange of current ideas is difficult. It is also very impor tan t because explora t ion for mineral resources depends so comple te ly on basic geological knowledge, and part icularly on tec tonic evolut ionary models. It is, there- fore, an appropr ia te t ime to bring together those scientists who are directly concerned with these problems in the Arabian--Nubian Shield. Such a meet ing has been arranged by the Inst i tute of Applied Geology, King Abdulaziz University.

The gross evolut ionary style of the African Shield, and especially that of the Pan-African belts, remains one o f the major unsolved geological questions. The Arabian--Nubian por t ion of the African Shield is, therefore, critical in view of its excel lent exposures and accessibility. There are current ly two main schools of thought on the origins of the complex terrains o f the Arabian- -Nubian Shield. One school considers the shield to have evolved as essentially intra-cratonic, ensialic belts with a basement o f older sialic material. This is the kind of model generally suggested for most of the African Shield. The o ther school holds that the Arabian-- Nubian Shield (if not a larger p ropor t ion of the African Shield) is not ensialic, but has evolved on oceanic crust f rom the welding together of a series of island arcs. F o r the reso- lu t ion of this fundamental problem, the critical factors appear to be : the nature of the ultra- basic belts in the shield (Are they remnants of oceanic crust?); the nature of the gneisses and granitic rocks (Do any of them represent original cont inenta l basement?) ; and the nature of the principal volcanic and sedimentary units of the shield (Were they deposited in oceanic or cont inental environments?) .

The fol lowing 60 abstracts ref lect these major questions. Amongs t them there are 5 related to ophiol i tes and possible oceanic crust, 2 related to gneisses, 10 concerning granites of various types, 14 related to gross stratigraphic sub-division of the shield, 13 on the types and distr ibut ion o f known mineral deposits, and 14 on s t ructure and tec tonic evolution.

TECTONIC E V O L U T I O N OF THE BASEMENT ROCKS IN THE S O U T H E R N AND C E N T R A L E A S T E R N D E S E R T OF EGYPT

M.L. A B D E L - K H A L E K

Institute of Applied Geology, Jeddah (Saudi Arabia)

The different classifications proposed by previous authors for the basement rocks of Egypt deal most ly with the distr ibution of l i thologic units, groups and format ions and their chronological sequence. Litt le a t ten t ion has been paid to structural t rends and events. In the present work, the tec tonic evolut ion o f parts of the central and southern Eastern Desert is examined, making use of the available satellite images and detailed field work in particular areas. Emphasis is given to the folding movements which affected the geosynclinal metamor- phites and are ref lected by major folds with their axes t rending NNW--SSE (Hafafi t and

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Beitan areas) and ENE--WSW (Barramiya area). Superposed structures are well demonstrated in several localities where the NNW--SSE trend is found to be the older. Thrust faulting may be associated with these structures.

The main masses of serpentinite are found in belts and sheet-like forms concordant with the geosynclinal metamorphites. Tracing their outcrops on a regional scale, it appears that some occur in arcs with their convex side towards the Red Sea. In the Barramiya area, the serpentinite belts represent parts of an arc and are highly compressed into an ENE-- WSW direction. These masses possibly represent Proterozoic subduction zones along the boundaries of crustal plates.

Major fractures cut through the basement rocks and extend for hundreds of kilometers. Their main trends are E--W and NW--SE and are probably of strike-slip type and of deep- seated nature. Block faulting is also conspicuous and the faults mostly run in a NNW--SSE direction parallel to the Red Sea fault system. A few gravity faults have ENE--WSW trends. Most of these structures are the result of rejuvenation of movement along older fractures.

THE RIEBECKITE GRANITES OF JABAL ABU KHARIF, A NEW GRANITIC PHASE IN THE BASEMENT COMPLEX OF EGYPT

M.A. A B D E L - M A K S O U D 1, A.H. S A B E T 2 a n d M.A. A B D E L - R A H M A N =

1 Cairo University, Cairo (Egypt) 2 Geological Survey o f Egypt, Cairo (Egypt)

The granitic rocks of the Abu Kharif area comprise three main types. Chronologically, they include the Late Proterozoic syntectonic grey granites, the late tectonic pink and red granites and a still younger phase of alkaline riebeckite-bearing granites. The first two types, as well as older rock units (including diorites and metavolcanics), are traversed by acidic, intermediate, and basic dykes -- the so-called post-granitic dykes. The riebeckite granites intrude th e dykes; consequently they represent a separate granitic phase emplaced after the dyke intrusion. This new observation indicates the inadequacy of the nomenclature "post-granitic" for the dykes that frequently traverse the late tectonic pink and red granites of the basement.

Based on field observations, the authors believe that this new granitic phase extends laterally in the Basement Complex where it is represented in part by syenites and nepheline syenites. The desilication of these granitic rocks was probably caused by the assimilation of calcareous materials deposited on the Basement before the Cretaceous.

The alkaline granitic rocks, in addition to the other basement rocks, were intensely faulted in two main stress periods. The older was applied in a NE direction and the younger in a NW direction. Both stress periods are younger than the emplacement of the alkaline granites and are believed to have formed during plate movements resulting in the formation of the Red Sea graben. Consequently this granitic phase was considered to be pre-rifting.

Three types of joints were observed: cross, sheet, and conjugate. Structural analysis reveals that the first two types are due to stresses resulting from thermal contraction of the granitic rocks, while conjugate joints were contemporaneously formed with the faulting.