Petrographic and geochemical characteristics of the Kiejo, Rungwe and Ngozi volcanic rocks, southern Tanzania: Implication for tectonic activities and magmatic differentiation

Rock samples were collected from the Kiejo, Rungwe and Ngozi volcanoes, southern Tanzania. Results show that, in the Ngozi and Rungwe volcanoes, magma cooled relatively fast. After large part of the magma had crystallized, nucleation of crystals at a relatively slow rate crystallized from interstitial fluids of the primary magma that had already crystallized fine grained minerals resulting into large grains of minerals. In the Kiejo volcano, both zoned and unzonned olivine crystals exist; unzoned crystals being primary and the zoned ones are secondary. Such observations also indicate that the magma responsible for these rocks had interstitial fluids that crystallized into olivine of different chemical and grain sizes.The lack of olivine, and the presence of orthopyroxene, the high amount of SiO2 wt% (~ 63) as well as (Na2O + K2O) wt% ~ 9.38 in the Ngozi volcano, indicate that the volcanic rocks in the Ngozi volcano are a result of a relatively silica saturated magma source. Otherwise, the magma could have originated from deeper in the crust such that it became contaminated with continental crust materials as it was erupting.Based on the Alikali-Silica diagram of Le Bas et al. (1986), the Ngozi and Rungwe magma contents fall in the field of trachy-dacite, and those from the Kiejo volcano fall in the field of basaltic trachy-andesite and basaltic magma. Such a pattern highlights close genetical and ?structural relationship of the magma that formed the three volcanoes.Keywords: Kiejo, Rungwe, Ngozi, volcanic rocks, Southern Tanzani

The Ituwa Surge deposits of the Holocene Ngozi caldera, Mbeya Region, Tanzania

The Ituwa Surge deposits belong to the Holocene Ngozi volcano (volcano number 222164 of the Smithsonian Institute Global Volcanism Program; Siebert et al. 2010) of the Rungwe Volcanic Province (RVP; Harkin 1960; Lenhardt and Oppenheimer 2014). They are named after their type locality and place of best exposure, the village of Ituwa in the Mbeya Region of southwestern Tanzania. Volcanism within the RVP started approximately 9 Ma ago and can be divided into a Late Miocene (*9.2–5.4 Ma), a Late Pliocene-Early Pleistocene (*3–1.6 Ma) and a Mid-Pleistocene-recent (\*0.6 Ma) phase (Ebinger et al. 1989, 1993; Ivanov et al. 1999).http://link.springer.com/journal/5312016-04-30hb201

Strain accommodation by slow slip and dyking in a youthful continental rift, East Africa

Continental rifts begin and develop through repeated episodes of faulting and magmatism, but strain partitioning between faulting and magmatism during discrete rifting episodes remains poorly documented. In highly evolved rifts, tensile stresses from far-field plate motions accumulate over decades before being released during relatively short time intervals by faulting and magmatic intrusions1-3. These rifting crises are rarely observed in thick lithosphere during the initial stages of rifting. Here we show that most of the strain during the July–August 2007 seismic crisis in the weakly extended Natron rift, Tanzania, was released aseismically. Deformation was achieved by slow slip on a normal fault that promoted subsequent dyke intrusion by stress unclamping. This event provides compelling evidence for strain accommodation by magma intrusion, in addition to slip along normal faults, during the initial stages of continental rifting and before significant crustal thinning