PETROLOGY, GEOCHEMISTRY AND 40AR/39AR DATING OF OLDEANI VOLCANO, NORTHERN TANZANIA

Oldeani Volcano in northern Tanzania is the southernmost edifice of the Ngorongoro Highland Volcanic Complex (NHVC), situated at the southern bifurcation of the Gregory rift. Oldeani is a monstrous volcano towering about 3000 meters above sea level. Lava from Oldeani has been inferred to be the source for stone tools recovered from the hominid bearing Laetoli Ngaloba Beds of the adjacent western situated lowlands.

Lava samples collected from the eastern and western flanks of Oldeani are mostly fine-grained phaneritic to aphanitic and occasionally scoriaceous. Silica content ranges from 46.44 - 54.07 wt % whereas total alkali (Na₂O + K₂O) varies from 4.49 - 6.41 wt %. Based on total alkali verses silica content, Oldeani is primarily composed of basalt, trachybasalt and basaltic trachyandesite. Mineral assemblages consist of plagioclase, augite, olivine and titanomagnetite; amphibole is common and biotite is rare. Major and trace elements plot in a narrow range that can be modeled by fractional crystallization of a single parent magma. Mg number (100[Mg]/[Fe+Mg]) ranging from 15 - 28.5 and Cr values (2 - 47 ppm) suggest that Oldeani lavas are evolved. Compared to ordinary chondrites, the Oldeani samples are enriched in Rare Earth Elements (REEs). Oceanic Island Basalt (OIB) normalized trace element ratios are close to 1 implying an OIB source.

⁸⁷Sr/⁸⁶Sr isotope data from four representative lava samples are tightly constrained between 0.70406 and 0.70476 indicating that unlike Ngorongoro Crater that has yielded values ranging from 0.70405 to 0.70801, Oldeani appears to have suffered limited crustal contamination during magma eruption, a common issue to other East African Rift volcanics that have erupted through Archean and Proterozoic basement rocks.

Four representative lava samples were dated using ⁴⁰Ar/³⁹Ar incremental-heating techniques, resulting in well-behaved plateau ages ranging from 1.59 ± 0.02 Ma to 1.55 ± 0.02 Ma, and a combined isochron age of 1.57 ± 0.02 Ma, indistinguishable from the plateau ages. These ages indicate that volcanic activity that produced Oldeani was very brief in duration lasting < 5ka. An age of about 1.6 Ma makes Oldeani the youngest basaltic center of the NHVC. The age and location of Oldeani are inconsistent with the general northward younging trend of the volcanic centers previously noted for the NHVC.