Geochemistry of metavolcanics and granitic intrusive from western margin of northern afar depression, Dallol, Northern Ethiopia

The western margin of northern Afar Depression constitutes various rock units of Neoproterozoic basement complex. Major oxide data of metavolcanic and granitoid rock samples from basement complex indicate significant variation in the concentration of major oxides (e.g. Fe 2 O 3 , TiO 2 , CaO, MgO). Metavolcanic rocks of Dallol demonstrate heterogeneous nature of magma source. The mafic metavolcanics indicate two distinct groups, (i) CaO-enriched and (ii) tholeiitic composition. Another group of magmatic series, the felsic metavolcanic group, shows a co-genetic relationship with the tholeiitic metavolcanics. Granitoids on the other hand have a wider range of SiO 2 (61.5 – 76.65 wt %) and are peraluminous to metaluminous in nature. Keywords : Geochemistry, Metavolcanics, Granitioids, Dallol, Northern Afar Depression, Ethiopia.

world's unique terrains consisting of highlands standing well beyond 2500m above sea level and deep water-free basin subsiding to 146 m below sea level. The plateaus are built on the Neoproterozoic basement complexes with daily temperature ranging from 20-35°C, and average annual rainfall of 500 mm/year. This environment is a home of many wild animals and birds.
On the other hand, the lowland area is filled with more than 1400 m thick encrusted evaporite deposits. It is thought as one of the deepest and hottest spots in Africa with an average daily temperature of 37°C; during summer (June-August) the temperatures reaches above 52°C on its floor.

Geology of the Dallol Area
Although the northern Afar Depression has been one of few places on earth that imprint a complete record of the volcano-tectonic evolution of a young ocean basin; it, however, lacks extensive geological investigations due to extremely difficult field conditions that have prevented access to the region. Despite the difficulties, few authors (e.g., Abbate et al., 1995;Barberi and Varet 1970, 1977Barberi et al., 1972aBarberi et al., &b, 1975Barberi et al., ,1980Beyene andAbdeselam, 2005, Hagos 2011 and others) have tried to constrain the volcano-tectonic evolution of northern Afar and to lesser extent the geochemical and isotopic signatures of the rift volcanics; but surprisingly, none of the authors addressed the nature of the basement complexes and sedimentary successions of the marginal areas of Dallol Depression.
Most of the geological makeup of northern Ethiopian is represented from the old to the young by the low-grade metavolcano-sedimentary sequences, Paleo-Mesozoic sedimentary successions and Cenozoic volcano-sedimentary sequences. However, these rocks are found spatially distributed/isolated throughout the regions. It is only in the western Afar margin where most of these successions found one overlying the other. The geology of the western Afar margins is of great interest because it may represent the complete sequence of rocks spanning from the Late Proterozoic to the present (Hagos 2011). The 3000 m-thick fault-scarps constitute the three genetically distinct and tectonically disturbed rock families. The upper part of the scarp constitutes the basement rocks. The lower part on the other hand is covered by the Mesozoic sedimentary rocks and in places the older basement rocks.
The geological formations of the western Afar margin and surrounding plateaus can be therefore, divided into two broad divisions (Barberi et al., 1972a): (1) Pre-rift complexes and (2) Syn-rift volcano-sedimentary rocks.

Pre-rift complexes
These complexes consist of the Neoproterozoic crystalline basement rocks (Arabian-Nubian Shield), and the Paleozoic-Mesozoic sedimentary sequences, which are exposed along the peripheries and plateaus surrounding the northern Afar Depression (Kazmin et al., 1978;Vail, 1985) (Fig 1). The Arabian-Nubian Shield (ANS) also called the 'low-grade basement complex'

Geochemistry of Metavolcanics and Granitoids
The general characteristics of the Dallol metavolcanic and granitoids are illustrated in the total alkali-silica (TAS) classifications of Le Bas et al. (1986) and Cox et al. (1979) (Fig 2). In this classification, the rocks with the exception of few samples are sub-alkaline in nature. All the analyzed rocks have SiO2 contents ranging from 36.9 to 76.65 wt% and also show a wide range in concentrations of other major and minor element oxides. Some of the Dallol mafic metavolcanics have exceptionally very high CaO (4.79 to 19.92 wt%) and low TiO2, MgO and Fe2O3 contents (Fig 3).
The Dallol granitoides on the other hand are subalkaline in nature and plotted on the granite and granodiorite fields (Fig 2b). They all display a bulk medium to high-K calc alkaline trend and strong affinity towards the metaluminous and to some extent peraluminous character with SiO2 contents ranging from 61.5 -76.65 wt% (Fig 4a &b Bas et al., 1986 andCox et al., 1979).   Though, many studies have already been conducted on ANS with integrated approaches of radiometric, isotopic and geochemical investigations, the nature and genesis of the juvenile crust of northeastern Africa is not still well constrained. Based on the geochemical results of samples collected from the western margin of northern Afar, two distinct magma series exist within the metavolcanic rocks of the area. The distinct geochemical and geotectonic signatures for the samples from Dallol area indicate that the petrogenetic processes of the provenance that formed these rock types were different (Fig 5). One of the major element binary plots that can show differences in the petrogenetic processes is the CaO/Al2O3 vs. SiO2 (Hagos et al., 2010).

CONCLUSIONS
In this study some genetic variations on the metavolcanic rocks of the western margin have been roughly constrained. Two well-defined metavolcanic rocks exist. The first group of metavolcanics have tendencies of mildly alkaline to tholeiitic nature and comprise differentiated rocks of maficfelsic compositions; whereas, the second group of metavolcanics have extremely higher contents of CaO and consequently higher values of loss-on-ignition (LOI) indicating the presence of carbonatitic lava as provenance. The presence of metaluminousperaluminous granite in the area indicated that the granitic magma producing these rocks might come from the melt of the volcano-sedimentary assemblages of volcanic arc terrains.

ACKNOWLEDGMENTS
The authors are very grateful to Dr. Barbara Cavalazzi, Bologna University, Italy, for XRF analyses. We are also too much indebted to the Mines and Energy office of Afar Regional state for financing the whole study and providing the necessary support during the entire fieldwork.