A CONFINED OROGEN IN THE TIBESTI MASSIF, SOUTHERN LIBYA, DURING THE LATE NEOPROTEROZOIC

The Tibesti Massif of southern Libya has a very different Neoproterozoic history than those of Precambrian rocks to the east or the west. In both Egypt and Algeria, Neoproterozoic ages are mostly imposed on older rocks. In the Tibesti massif, however, pre-560-Ma continental crust is shown only by the presence of platform quartzites older than ~560-Ma magmatism. The presence of oceanic lithosphere in the Tibestis at 560 Ma is shown by two magmatic suites. The Ben Ghnema batholith is a continental-margin suite with an initial 87Sr/86Sr ratio of ~0.706. It apparently formed by westward subduction of oceanic lithosphere beneath a continental block now underlying the Murzuk basin. The Wadi Yebigue pluton is a post-orogenic granite with epsilon Nd from ñ0.5 to ñ2.5, and initial 87Sr/86Sr of 0.706. Seven of eight zircon fractions are close to concordant with an upper intercept at ~560 Ma. Lower Paleozoic platform sediments around the flanks of the Tibesti uplift show that Neoproterozoic activity ended within a few tens of million years after it began. We conclude that a Neoproterozoic ocean basin appeared only briefly in the Tibesti region and did not permanently displace continental rocks on either side. Thus it seems to be similar to the Neoproterozoic Aracuai orogen in Brazil, which has been defined as a "confined" orogen.