Structures and stratigraphy in the central part of the Arabian Shield

A50 STRUCTURES AND STRATIGRAPHY ARABIAN SHIELD

IN THE CENTRAL

PART OF THE

P. ELSASS

B.R.G.M. P.O. Box 1492, Jeddah (Saudi Arabia)

Recent geological investigations in the Arabian shield, have included structural analysis for interpretation of relationships between various Proterozoie sequences and their attribution to accepted stratigraphic age groups, i.e. Hulayfah, Murdama, Shammar and Jibalah. This has revealed possible errors in earlier geological interpretation, particularly among the formations attributed to the Hulayfah group, which unconformably underlie Murdama group rocks in the central part of the shield. On the basis of deformation characteristics it appears that the Hulayfah group rocks represent two distinct lithostratigraphic sequences separated by a major unconformity. The younger sequence comprises subaerial volcanic rocks displaying a single strain-slip cleavage, and greenschist-grade metamorphism without slaty cleavage. The older sequence consists of volcanic to sedimentary formations metamorphosed in the greenschist facies with associated slaty cleavage, and also with a superimposed crenulation cleavage; the As Siham formation, which hosts the Sha'ib LaraTsah c o p p e r - ~ i n c prospect and comprises metabasalt, graphitic slate and metarhyolite, is an example. Another possible misinterpretation can be seen in the Murdama group sediments. In the Jabal Murdamah type section, sediments are characterized by two fold directions producing typical interference p a ~ e r n s and a major strain-slip cleavage. They can be shown to grade southward into the Arran and Juqjuq red sandstone and andesite continental formations attributed to the Hulayfah group. A third example is the fact that the red rhyolite underlying Jibalah group sediments, and previously attributed to the Shammar group, and the red rhyolite, breccia and conglomerate occupying the newly'discovered Bani Ghayy north-trending graben, do n o t display any cleavage. It is therefore inferred that they belong to a group younger in age than the Murdama to which the Shammar is supposedly equivalent. In view of the discrepancies revealed by this study, it is suggested that structural analysis should play a much greater role in interpreting the lithostratigraphy o f the Precambrian Shield.

SULPHIDES IN THE GABBRO DESERT, EGYPT

AKAREM

COMPLEX, SOUTH-EASTERN

A.M.A. H A F E Z and Z. ABDEL-KADER

Cairo University, Cairo (Egypt)

Cu--Ni--Co--Fe sulphide mineralization occurs in the Precambrian mafic-~ultramafic complex of Gabbro Akarem, considered to have been intruded along a major NE-trending fracture zone in the South-eastern Desert of Egypt. The mineralization is in the form of bands and disseminated blebs, concentrated mainly in olivine websterite and lherzolite