Abstract

The Sabaloka igneous complex of late Precambrian or Cambrian age consists of an elliptical ring dyke of porphyritic microgranite surrounding acid volcanic rocks in the southern half of the area and a basement composed mainly of biotite gneisses in the north. Sandstones of the Nubian formation locally overlie the igneous rocks and the basement. The complex is bisected and displaced by the Jebel Umm Marahik fault.

The igneous rocks have a density of 2.60 g cm−3 and the biotite gneisses 2.65 g cm−3. A gravity low of about 8 mgal in the southern half of the complex indicates a thickness of about 5.5 km of lighter rocks, only 2 km of which can be volcanic, the rest being inferred to be intrusive granitic rocks. A gravity low in the northern half of the ring is indicative of a subsurface granitic intrusion with vertical thickness about 3 km. The ring dyke is intruded into a complicated ring fracture zone, in part a graben up to 2 km wide in which the basement rocks have subsided to a depth of 3 km or more. The outer contact of the ring dyke is steep but has a variable direction of dip—inward, vertical or outward. Gravity evidence for the course and throw of the Jebel Umm Marahik fault is presented and another roughly diametrical fracture trending at 50º to the former is postulated.

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