SPATIAL AND TEMPORAL VARIATIONS IN THE COMPOSITIONS OF UPPER MIOCENE TO RECENT BASIC LAVAS IN THE NORTHERN MAIN ETHIOPIAN RIFT - IMPLICATIONS FOR THE CAUSES OF CENOZOIC MAGMATISM IN ETHIOPIA

The rate of lithospheric extension has previously been suggested as th e most important factor governing the compositions of magmas generated in the Cenozoic Ethiopian volcanic province (CEVP). However, the dist ribution and chemistry of volcanic rocks extending from the western pl ateau margin at Addis Ababa to the rift floor in Nazret, northern sect or of the main Ethiopian rift (MER), suggest that transitional magmati sm in the region may have been triggered by an increase in the amount of lithospheric extension in the Early Pliocene. The rocks occur acros s an area of variable crustal thickness and show a general age progres sion from Upper Miocene (less than or equal to 9 Ma) to Recent toward the rift. Alkalic basalts are extensive in the western part of the rif t and along its margin but are found only locally within the rift, whe reas transitional basalts are found within the rift only. Both types o f basalts appear to have been derived from a common mantle source. In contrast, alkalic and transitional basalts on the Ethiopian plateau ar e mutually exclusive in terms of their spatial distribution, but exhib it a compositional contiguity which suggests that transitional magmas on the plateau formed at the expense of alkalic magmas, i.e. by equili bration of alkalic magmas at relatively shallow depth. The alkalic bas alts bear clear record of a decrease in the degree of partial melting with time, suggesting that magmatism on the plateau was possibly trigg ered by a transient thermal anomaly.