Abstract

Models for the late Proterozoic to early Palaeozoic evolution of the Arabian Shield require a knowledge of the fundamental basement in the region. In the south-central Arabian Shield there are several areas of gneissic granitoid rocks which could represent such pre-Pan African basement. Whole-rock Rb-Sr isochrons for 2 of these areas are presented. The Dahul gneisses comprise an extensive terrain of granitic gneisses, migrnatites, foliated and unfoliated granites, dykes, and pegmatites. 5 gneissic rocks from Lat. 22º11'N, Long. 44º03'E define an isochron indicating an age of 599±43 Ma (586 ±43 Ma) with an initial 87Sr/86Sr ratio of 0.7067 ± 20 and a MSWD of 0.13. Though the isotopic data could be compatible with certain models implying original ages of up to 1150 Ma, the field evidence and other relationships indicate that the age of c. 600 Ma was probably the age of intrusion. The Dahul gneisses therefore represent a major syntectonic batholith of Pan African age (s.s.) and of primitive derivation. The Juojuq gneisses are porphyroclastic rocks within the Najd fault zone. 6 gneisses from Lat. 21º03'N, Long. 43º 4 0 'E define an isochron with an age of 782 ± 26 Ma (766±26 Ma), an initial 87Sr/86Sr ratio of 0.70297 ± 12 and a MSWD of 1.99. The bulk compositions, the textures and the isotopic data indicate that the rocks were originally plagioclase-rich granitoid plutonic rocks of primitive derivation. The isotopic data are compatible with original emplacement between c. 780 and 870 Ma ago. This age range is slightly greater than the 760–775 Ma (744–759 Ma) determined for the nearby Halaban lavas. The gneisses might therefore represent pre-Halaban intrusives, emplaced into unidentified crust. Pre-Pan African basement has thus not yet been identified in the Arabian Shield.

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