Mineralogy of Peralkaline Silicic Volcanics: Information from Kone Volcano, Ethiopian Rift Valley

*Dereje Ayalew and Bekele Abebe*

## **Abstract**

The presented paper described in detail the mineralogy of silicic peralkaline eruptives from Kone volcano within the Ethiopian rift system, which is formed predominantly by rhyolite with some small occurrence of trachyte. The majority of eruptive rocks in the Kone volcanic area are phenocryst-poor. The studied rocks contain alkali feldspars (anorthoclase and sanidine), quartz, clinopyroxene (hedenbergite), aenigmatite and olivine (fayalite), accompanied by rare Fe-To oxides (ilmenite) and apatite. All these minerals are described in detail. These data are very interesting for all researchers, who study similar eruptive rocks.

**Keywords:** comendite, Ethiopia, Kone, rift

## **1. Introduction**

The Ethiopian rift valley, forming a part of the East African rift system, is the largest active continental rift on Earth (**Figure 1**). This rift shows transitional character between continental rift and seafloor spreading [1] as it is underlain by thinned crust intruded by mafic dykes. The Ethiopian rift is characterized by numerous volcanic centres most of which are still active as evidenced by strong fumarolic activity and episodes of deformation, such as rapid uplift [2]. Kone is one of these silicic volcanic edifices.

Kone volcano (also previously known as Gariboldi volcano [3, 4]) is composed of a series of silicic cones with a summit caldera. It is located (8.8 N, 39.69 E) at the end of the Ethiopian rift valley, near the junction with Afar depression (**Figure 1**). Kone covers an area of 250 km2 . The caldera is elliptical in shape (5 × 7.5 km wide structure) trending E-W [5, 6]. The rim of the caldera rises about 100 m above the caldera floor. The summit of the volcano has an elevation of 1619 m above mean sea level. Roughly N-S-trending regional normal faults and fissures, forming a part of the Wonji fault belt, cut across the caldera and its flanks, especially the eastern side of the volcano.

Despite their petrological and volcanological interest, there is very little published data on the mineralogy of peralkaline silicic volcanics. Here, we present electron microprobe analyses for comendites of Kone volcano within the Ethiopian rift valley (**Figure 2**). The aim of this paper is to document in detail the phenocryst compositions of cemendites.

### **Figure 1.**

*Location of the active quaternary magmatic segments - zones of dense faulting and aligned eruptive centres which are the current locus of strain within the Ethiopian rift (southern Red Sea and main Ethiopian rift). Arrows show plate motions relative to stable Nubia. Danakil block is a microplate between the Nubian and Arabian plates. TGD is Tendaho-Goba'ad discontinuity marking the active and ancient boundary between the east African and the Red Sea rifts.*

### **Figure 2.**

*Simplified geological map of Kone volcano and its surroundings.*

*Mineralogy of Peralkaline Silicic Volcanics: Information from Kone Volcano, Ethiopian Rift Valley DOI: http://dx.doi.org/10.5772/intechopen.102677*
