ON THE AGE DATING OF THE DYKE SWARMS OF SOUTHWESTERN SINAI, EGYPT

The dyke swarms in Sinai are important in understanding late Precambrian activity in the northern Arabian-Nubian Shield. Fieldwork and K-Ar age dating indicate that these dyke swarms can be described in terms of two major members. (1) The old (mafic) dykes, mainly consisting of basalts and andesites, which, with minor lamprophyres and plagiophyres, range in age from 586-563 Ma giving a mean age of 574 Ma. They are distinguishable from older 586-577 Ma basalts with a mean of 581 Ma; and younger 570-563 Ma andesites with a mean of 567 Ma. The old dykes crosscut the basement rocks except the Younger granites. (2) The young (felsic) dykes, comprising dacites-to-rhyolites and quartz porphyries and rang in age from 545-536 Ma with a mean of 541 Ma. They crosscut almost all the basement rocks, the exception being the Younger Granites phase-III. All the former basement rocks, together with the old and young dyke swarms, are cut by 517-502 Ma (509 Ma mean) late basalt dykes. These data represent the age of old dykes emplacement and cooling, and place a minimum age of the young dykes probably due to the loss of argon attributable to the influence of the post-extrusion deformation.