Aksum-Adwa (Axum-Adua)

In an area east and west of Aksum and Adwa and extending for some 230 km about 20 occurrences of peralkaline trachytic lavas are found, several of which are more than 10 km in diameter. In the earlier literature some of these rocks were considered to be intrusive but according to Mohr (1962a) they are all lavas. Specimens described by Prior (1899) were collected by Blanford (1870) on an expedition in 1868 but, unfortunately, few details of field occurrence are available. The rocks described came from Kishyat, Fokada and Akub Teriki. Rocks from the first two localities consist of sparse phenocrysts of alkali feldspar in a fine-grained matrix of felted alkali feldspar laths, a little quartz and moss-like sub-ophitic patches of riebeckite. The rock from Akub Teriki is coarser and also contains phenocrysts of alkali feldspar, in which are embedded crystals of zircon, in a felted feldspathic groundmass. A brown amphibole is intergrown with aegirine the whole being fringed by tufts of acicular riebeckite.A larger collection described by Prior (1900), collected by Sadebeck, is accompanied by a series of profiles which imply that these grorudites, solvsbergites, trachytes and tinguaites derive from intrusions that form prominent and often substantial hills, but according to Mohr (1962a) these are extrusive rocks. They were collected in the immediate vicinities of Aksum and Adwa. The grorudites all consist of alkali feldspar, quartz and aegirine. A variety referred to as paisanite by Prior (1900), is characterised by the presence of moss-like riebeckite. The solvsbergites are alkali feldspar-aegirine rocks with quartz sparse or absent. Obelisks at Aksum are made from this rock. The tinguaites, according to Prior's profiles, come from Amba Gollo, which is a substantial hill northwest of Aksum, and a smaller nearby occurrence, with two other areas referred to in the text. They are described petrographically in some detail by Prior (1900) and generally comprise alkali feldspar, aegirine, nepheline and, in some rocks, a riebeckitic amphibole. Analyses of grorudite, solvsbergite and tinguaite are given by Prior (1900) and Minucci (Merla and Minucci, 1938, p. 320) reviews the alkaline rocks of the Aksum-Adwa area, collecting together rock analyses from various sources. Le Bas and Mohr (1968) have also described ijolite from this area. General views of the Adwa Mountains are given in Merla et al. (1979).