"Hanang, and numerous minor car- bonatite tuff cones are known on the Mbulu plateau (Downie and Wilkinson, 1962) and in the Monduli area (Dawson, 1964). Alkalic carbonatite lava extrusions and tuff eruptions have taken place at Oldoinyo Lengai in modern times (summarised by Dawson, 1989). "
[Show abstract][Hide abstract] ABSTRACT: The Neogene tectonics and volcanism in the rift area of northern Tanzania are intimately related. A major phase of late Tertiary faulting, giving rise to a broad tectonic depression, was followed by extrusion of large amounts of basaltic to trachyte magmas from large shield volcanoes. This was separated by a second major phase of faulting at about 1.2 Ma from a Late Pleistocene-Recent phase of small volume, explosive nephelinite-phonolite-carbonatite volcanism that contrasts with the earlier phase in its volume, dominant magma type and eruptive style.In both its tectonic expression and contemporaneous magmatism, the northern Tanzania province contrasts with the southern Kenya sector of the Rift Valley. The area of tectonic disturbance is considerably broader in Tanzania where ultrabasic-basic magmatism predominates. The major episodes of basaltic magmatism. representing major thermal perturbations of the mantle, have moved southwards down the rift since the mid Tertiary.
"The MgCO 3 content varies between 15-38%. The carbonate rocks at Basotu are considered to have originated as carbonatite (Downie & Wilkinson, 1962; Dawson, 1964; Heinrich, 1966). Downie & Wilkinson (1962) propose that owing to local faulting the gas-charged magma exploded violently, causing eruptions, and the fragmentation of the carbonatitic magma gave rise to carbonatitic 'rain'. "
[Show abstract][Hide abstract] ABSTRACT: The biogeochemical evolution of solutes markedly alters the chemistry in the closed-basin maar lakes that comprise the Basotu Lake District (Tanzania, East Africa). Examination of 11 (out of 13) lakes in the Basotu Lake District identified two distinct evolutionary pathways: a gradual path and a rapid path. During the course of biogeochemical evolution these waters follow either the gradual path alone or a combination of the gradual and rapid paths. Solute evolution along the gradual path is determined by all of the biogeochemical processes that for these waters appear to be tightly coupled to evaporative concentration (e.g. mineral precipitation, sorption and ion exchange, C0 2 degassing, and sulfate reduction). Rapid evolution occurs when mixing events suddenly permit H 2 S to be lost to the atmosphere. The chemistry of waters undergoing rapid evolution is changed abruptly because loss of every equivalent of sulfide produces an equivalent permanent alkalinity. Peer Reviewed http://deepblue.lib.umich.edu/bitstream/2027.42/42881/1/10750_2004_Article_BF00026937.pdf
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