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and transformation of hematite to magnetite and thence to specular hematite; and (V) weak alterat |on of the hanging-wall rocks.

PETROGRAPHY AND SEQUENCE OF GRANITES IN THE JABAL SAYID COMPLEX, SAUDI ARABIA

A.R.A. TURKISTANY

Institute of Applied Geology, Jeddah (Saudi Arabia)

The Jabal Sayid granite complex in the western part of the Arabian Shield has been mapped at a scale of 1:15,000 and studied petrographically. These studies lead to the recognition of the following five types of granite, arranged from oldest to youngest.

(1) Red biotite granite is the oldest type in the complex as indicated by field relation- ships. It is rich in quartz (40%) and microcline (30%), with abundant biotite but no other mafic minerals. The red colour is characteristic.

(2) The porphyritic biotite granite consists mainly of subhedral microcline phenocrysts (40%) and quartz (25%). Mafic minerals are biotite, actinolite and chlorite. In places it forms porphyry dykes cutting the red biotite granite.

(3) Riebeckite granite is found as large rafts in the younger hornblende granite at several localities. The rock is grey and sometimes porphyritic. It is rich in quartz (44%), microcline (28%) and blue amphibole (probably riebeckite), which is the only mafic mineral.

(4) Hornblende granite is the main component of the Jabal Sayid complex, and cuts the older granites. It is light grey and very massive. It contains abundant euhedral glassy quartz phenocrysts (43%), microcline perthite (30%) and needles of dark-green hornblende. Some rocks have minor aegirine. In several places the rock has been intensely oxidized and the amphibole needles have been pseudomorphed by Fe oxides.

(5) Aegerine granite is the youngest component, forming dykes cutting the hornblende granite. It is greenish and sometimes porphyritic. It is rich in quartz (47%), and microcline (25%). Aegirine is the main mafic mineral together with some hornblende.

OUTLINE OF THE GEOLOGY AND MINERALIZATION OF THE NUBIAN SHIELD EAST OF THE NILE VALLEY, SUDAN

J.R. VAIL

Portsmouth Polytechnic, Portsmouth (England)

The Red Sea Hills physiographic province and the interior plains to the Nile in the west are made up of Basement Complex rocks in which four major stratigraphic--metamorphic units have been recognized. The oldest, western unit is a quartzo-feldspathic grey gneiss group, overlain by amphibolite-grade metasedimentary quartzofeldspathic psammites, limestones and pelites with amphibolite sheets. In Sudan, these rocks are called the Kashebib Group, in Egypt Meatiq gneisses, and in Saudi Arabia they probably correspond to the Hall Group. The older rocks are overlain and interfolded with a predominantly andesitic volcanic group referred to as the Greenschist Assemblage, in which serpentinites occur, especially in the Eastern Desert. The rocks correspond to the Nafirdeib Group sequences in Sudan, the Shadli and other volcanics in Egypt, and probably the Bahah,