PHANEROZOIC TECTONO-THERMAL HISTORY OF THE NUBIAN MASSIF, EASTERN DESERT, EGYPT, AND ITS RELATIONSHIP TO THE OPENING OF THE RED SEA AS REVEALED BY FISSION-TRACK STUDIES

Seventy nine fission-track ages are reported on apatite, sphene, and zircon recovered from Eo-Cambrian to Cretaceous ring complexes and Precambrian granites, Eastern Desert, Egypt. With respect to the ring complexes, zircon and sphene ages are concordant with K/Ar and Rb/Sr emplacement ages, signifying rapid cooling following intrusion. Apatite ages, except for Abu Khruq, are significantly younger than the emplacement ages and are interpreted as uplift ages. Annealing of the apatite clocks is attributed to a combination of cauldron subsidence, sedimentary burial, and an increase in regional heat flow. Renewed operation of the apatite clocks by cooling on uplift occured in mid-Jurassic, Late Cretaceous, and Early Oligocene times. Apatite age for Abu Khruq is near-concordant with the emplacement age. Concerning Precambrian plutonics, sphene ages are in the range 424-543 m.y. It is suggested that the sphenes were annealed during the Pan-African orogeny and the clocks began to operate anew as a result of cooling on uplift recorded in a pronounced regional unconformity. Metamict zircons recovered were not amenable to dating. Apatite ages are in the range 280-\u3c4 m.y., and clearly represent uplift-related cooling ages. In the area south of 25(DEGREES)N latitude the onset of uplift was in the Late Cretaceous and operated continuously or intermittently to the Pliocene. North of this latitude, in addition to the Late Cretaceous-Pliocene uplift period, older events are recorded. Combining ages on the ring complexes and the granites suggests that, block faulting was important in the dynamics of the uplift history since early Late Paleozoic time. Four major blocks have been identified in southeastern Egypt. Uplift along major faults affected blocks of regional dimensions. On the basis of the apatite ages it appears that, the updoming preceding the rifting stage of the Red Sea took place in two stages: the earlier and more widespread phase occurred during Late Cretaceous, the later in Early Oligocene time. Total duration of the two stages prior to the initiation of the opening of the Red Sea was about 50 m.y. A time-table for events related to the rifting stage in the Eastern Desert is also presented