Abstract

The Precambrian Tibesti massif of southern Libya and northern Chad is separated from other massifs to the east and west by relatively deep basins of Paleozoic and younger sedimentary rocks. It is also bordered on the north by a northward-thickening wedge of Phanerozoic sedimentary rocks and connects southward, under shallow sedimentary cover, with the older Precambrian terranes of central Africa.

The northwestern part of the Tibesti massif contains the Ben Ghnema batholith, a complex assemblage of plutons intruded along a north-south axis between metamorphic wall rocks. To the west, the wall rocks are felsic, platformal, and metasedimentary; to the east is a more intensely folded, mafic basinal sequence of metasedimentary and metavolcanic rocks. The northern part of the batholith, about 40 by 120 km, contains a typical calc-alkalic plutonic assemblage. AFM and other variation diagrams for the Ben Ghnema batholith are virtually identical to those for the Sierra Nevada and Southern California batholiths. Furthermore, the Ben Ghnema batholith exhibits compositional variations from tonalite and granodiorite in the east to adamellite and granite in the west, identical to the west-to-east variation in the Sierra Nevada. The age of the Ben Ghnema batholith is 550 m.y. (Pan-African) by K-Ar and Rb-Sr methods.

By analogy with the Sierra Nevada batholith, the Ben Ghnema batholith may have formed above a subduction zone carrying oceanic crust westward under a continental margin located in the northwestern Tibesti massif. This conclusion probably implies eastward extension of the West African craton across areas now occupied by the Ahoggar massif and the Murzuk basin. Events of Pan-African time in these areas were probably related to intracratonic reactivation. It is also possible that an oceanic basin extended eastward from the Tibesti massif to the western margin of the Arabian shield in Saudi Arabia, where Greenwood and others (1976) have postulated northeastward subduction of oceanic crust before and during Pan-African time.

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