PETROGRAPHY AND GEOCHEMISTRY OF BASALTIC AND RHYODACITIC ROCKS FROM LAKE TANA AND THE GIMJABET-KOSOBER AREAS (NORTH CENTRAL ETHIOPIA)

Lake Tana, the largest lake in Ethiopia, is a circular basin 70 km in diameter, set in Quaternary volcanic rocks. At least two types of volc anic rocks cover the area south of Lake Tana, including the Bahir Dar, Tiss Abay, and Gimjabet-Kosober regions. The rocks east of Lake Tana are subalkaline rhyodacites displaying evidence of chemical differenti ation, whereas those on Dek Island (within Lake Tana) and in the Gimja bet and Kosober areas are alkali basalts. The distinct ranges in mg nu mbers agree with the major and trace element classifications: Lake Tir ba and Lake Zengena alkali basalts have mg numbers of 60-70; Dek Islan d alkali basalts have 50-60; and the rhyodacites have less than 40. Th e alkali basalts of Gimjabet-Kosober and Dek Island have incompatible trace and rare earth element (REE) patterns that are similar to each o ther, while the rhyodacites have different patterns, indicating a diff erent source or crustal contamination. The more differentiated Dek Isl and and the more primitive Gimjabet-Kosober alkali basalts are likely to be the result of fractional crystallisation of the same basaltic ma gma. In general, the compositions of all analysed basalts are typical of those of within-plate basalts, with REE and incompatible element pa tterns also being similar to those of the post-rift volcanics of the M ain Ethiopian Rift, the Afar Depression, the Red Sea Coast, and recent volcanics of the southeastern Ethiopian Plateau. The temporal, minera logical, and chemical similarity of these rocks in the Lake Tana area to the recent regional alkaline volcanics suggests a common source of magma centred in the Main Ethiopian Rift or the Afar Depression. (C) 1 998 Elsevier Science Limited.