ANOMALOUS TUFFITE: BUNYARUGURU EXPLOSION VOLCANO ZONE, UGANDAN WESTERN AFRICAN RIFT VALLEY (UWARV)

Bunyaruguru volcanic deposits are an example of UWARV deposits that are not consistent with expected melilitic or carbonatitic ash thought to dominate the region. X-ray diffraction and X-ray fluorescence analyses of a thin tuff bed (~ 7 cm) collected at a Katunguru Road cut near Kichwamba, Uganda suggest that magmas may be more compositionally diverse than previously thought. All volcanic rocks in the Toro-Ankole region between Lakes Albert and Edward have been previously reported to be carbonatite. But, this sample contains at most 40% combined CaCO₃ plus (Ca,Mg)CO₃ content calculated from XRF data, below the 50% definition. The approximately 1% XRF P₂O₅ content, as apatite, would further reduce that by using some available Ca. The carbonates could be carbonatitic, xenolithic, or both, as there are small, unidentified, angular xenoliths. XRF showed Na below detection limits; therefore, Na minerals are not present. Melilite ((Ca,Na)₂(Al,Mg,Fe++)(Si,Al)₂O₇) was previously reported as common in the five explosion volcano zones but is missing from this sample, as are several others known to commonly occur in the region, such as akermanite (Ca₂Mg(Si₂O₇)), gehlenite (Ca₂Al(SiAlO₇)), and monticellite (Ca(Mg,Fe)SiO₄). In fact, melilitic basalt was reported to be the primary component of Bunyaruguru tuffs. Results suggest that some regional, parent magmas are not melilitic, though they might be carbonatitic. Alternatively, weathering or post-depositional alteration might have changed the original composition during the 50 to 4 ky period of regional deposition (dating reported by others). To constrain geochemical variability in petrologic studies, volcanics in the region should therefore be considered on a site-by-site, (possibly bed-by-bed) basis.