The results of mineralogical, geochemical and geochronological studies carried out on dyke swarms outcropping between the coastal plain (Tihama) and the western border of the highland in the Yemen Arab Republic (YAR) are reported. On the basis of field relations the dyke swarms have been grouped into a younger association (TDS) and an older one (ATD). The younger Tihama dyke swarm (TDS) represents the southern prolongation of the Tertiary Tihama-Asir Complex, which is well developed in the Jizan Region of Saudi Arabia (Coleman et al., 1979). K/ Ar ages obtained on TDS samples range from 20 to 23 m.y.; considerably higher ages (54-75 m.y.) were established for the ATD samples. Chemical and mineralogical data indicate that the Miocene TDS are transitional in nature and similar to basaltic dykes related to the Trap Series (PBD). A MORB-type magmatism can be excluded on the basis of the distribution of hygromagmatophile elements. Geochemical variations within the TDS and PBD samples can be related to different degrees of partial melting of a garnet-bearing mantle source. Our data agree well with the interpretation of the TDS magmatism as related to a continental rifting episode (Arno et al., 1980) closely following the Plateau magmatism, better than an early oceanization event in the evolution of the Red Sea system.
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