Trace Elements in Continental-Margin Magmatism: Part II. Trace Elements in Ben Ghnema Batholith and Nature of the Precambrian Crust in Central North Africa

The Ben Ghnema bacholith is in the northwestern part of the Tibesti massif of southern Libya. It was formed as a series of separate plutons about 550 m.y. ago (Pan-African time). The batholith is lithologically and compositionally similar to the Sierra Nevada batholith, thus suggesting that the Ben Ghnema batholith was also formed by subduction of oceanic crust under a cratonic margin. Areal variation from tonalite and granodiorite (on the east side) to granite (on the west) indicates that ocean basin occurred to the east of the batholith and craton to the west. Several compositional properties indicate that the plutons of the Ben Ghnema batholith represent a series of partial melts instead of a sequence formed by fractional crystallization. This conclusion is supported by the pronounced increase of K ₂ O with SiO ₂ , the lack of variation in Zr/Y ratios, and the lack of fractionation of heavy rare-earth elements (REE) in the more silicic rocks. The dominant rock type of the Ben Ghnema batholith is an adamellite (65% to 70% SiO ₂ ). The average composition of the adamellite is 68.1% SiO ₂ ; 0.49% TiO ₂ ; 15.7% Al ₂ O ₃ ; 3.8% total iron as Fe ₂ O ₃ ; 1.4% MgO; 3.0% CaO; 3.3% Na ₂ O; 4.2% K ₂ O; 156 ppm Rb; 220 ppm Sr; 25 ppm Y; 169 ppm Zr; 11 ppm Nb; 564 ppm Ba; 25 ppm V; 87 ppm Cr; and 6 ppm Ni. This rock contains slightly more K ₂ O and Rb and less Sr than typical calc-alkalic subduction-zone suites. This composition is consistent with intrusion of the batholith into the margin of a thick, well-formed, possibly Archean craton (the East Saharan craton of Bertrand and Caby).