DEVELOPMENT OF THE YEMENI CENOZOIC VOLCANIC GROUP IN RELATION TO RIFT FORMATION

Rift formation is an important problem in global tectonics. The Yemen rift-related area is one of these important provinces, which is connected to the rifting processes of the Gulf of Aden, Red Sea and Afar triple junction. The Yemen Volcanic Group can be divided into the late Oligocene-early Miocene (31.6-15Ma) Yemen Trap Series (YTS), separated by an unconformity from the late Miocene-Recent (10-0Ma) Yemen Volcanic Series (YVS). The former was associated with the synrift phase, and consists of thick volcanic outcrops, including alkaline to mildly alkaline basalts and peralkaline rhyolites. The majority of the acidic rocks are of peralkaline, comenditic character. The “Yemen Volcanic Series” (YVS) was formed after rifting (postrift phase) and includes three phases of volcanic activity, outcrops along the Gulf of Aden coast, at the inland areas, and the southern Red Sea islands group, among others. Except for minor differentiates, and restricted acidic lavas, this series is basaltic. The YTS is classified as a product of high volcanicity rift. In support of this claim, YTS has clear bimodal character with an acidic/basic ratio in access of 0.5, and can be compared to the Ethiopian Rift, eastern branch of the Kenyan Rift and also to some sectors of the Rio Grande Rift. Based on heat flow history and trace element distribution, rhyolites are classified as products of fractional crystallization of basic magma, and it can be inferred, based on Sr isotopes and resorption of feldspar and pyroxene, that these parent magmas had assimilated some crustal materials. REE distributions of the basic and felsic volcanic rock types, all have approximately parallel trends. The relatively high REE content in the acidic rocks and the systematic variation of the LREE in basalt and rhyolites from the YTS indicate that these rock types are genetically related. The continental within-plate basalt (WPB) character is recognized for the YTS and YVS, but the younger is more alkaline. Degrees of partial melting were calculated and found to be 10-25% for the average alkali olivine basalt of the older YTS, and between 3 and 8% for the alkali olivine basalt of the younger YVS range. This model calls for emplacement of high-level magma chamber during development of the YTS to account for the large amounts of rhyolite observed.