Evolution and Geology of Eburru-Badlands Geothermal Prospect-Central Kenyan Rift

摘要

Eburru-Badlands Geothermal Field is characterized by Trachyte, Pantellerites, Basalts and Pyroclastics. The lavas are classified into pre- and post- faulting events estimated to be aged between 1.2 Ma and 100 years BP. The older formations are affected by a major faulting event dated between 0.8 Ma and 0.4 Ma (Clarke et al., 1990). The younger lavas include Trachyte (Et2), basalt (Eb2), Pyroclastic (Ep2) and Pantellerite (Ep2). They are not faulted and occur along the younger N-S faults. Thirteen rock samples from surface outcrops were analyzed and identified as Trachyte (Et1), Trachyte (Et2), Basalt (Eb2), Pyroclastic (Ep2) and Pantellerite (Ep2). The Trachyte (Et1) is folded and fractured, and hence belongs to the pre-faulting stratigraphic sequences. The rocks are porphyritic with quartz, sanidine, pyroxenes, amphiboles, plagioclase and olivine of varying amounts. Classification of these rock units indicates that SiO_2 weight percent increases from basalts (45-50%) through Trachyte (65%) and Pantellerites (69-74%) to Pyroclastics (78%). The Pyroclastic rocks (Ep2) are mainly obsidian flows which are devitrified and appear brecciated. High (Na_2O+K_2O) weight percent content in Trachytes and Pantellerites makes them strongly peralkaline. The silicic rocks are characterized by low Al_2O_3 and Fe_2O_3. Basalts are associated with an increase in Al_2O_3 and with a decrease in SiO_2. Analyzed basaltic and Pyroclastics samples are subalkaline. Geochemical models of basalts and silicic peralkaline lavas show that the magmas are ultimately mantle-derived and that the evolution was dominated by fractional crystallization processes. This classification shows an evolution trend from basalts through Trachyte to Pantellerite.