Chapter 7 The Neogene-Recent volcanic rocks

Abstract

The late Tertiary–Recent volcanoes are largely confined to the area of the late Tertiary depression, the exceptions being the most southerly volcanoes of Hanang and Kwaraha, and the Basotu explosion craters of the Mbulu Plateau (Dawson 2008, fig. 3.1). The first systematic attempt to classify the volcanoes was that of Guest (1953) who, mainly on the basis of rock type, divided them into the Older Extrusives (volcanoes characterized by extrusion of basalt) and the Younger Extrusives (formed by more alkaline rock types). Guest's classification is broadly correct, but modifications to the scheme have resulted from later regional mapping by the Tanzania Geological Survey, together with radiometric dating of rocks from individual centres. This has resulted in a more refined stratigraphy, particularly with respect to the timing of the volcanicity relative to the regional tectonics. For example, some peralkaline volcanoes, formerly included with the Younger Extrusives, are older than, or contemporaneous with, some Older Extrusive basaltic centres. The term ‘peralkaline volcano’ is used here to indicate that the structure is formed dominantly of peralkaline rocks i.e. those for which whole-rock chemical analyses show (Na + K)/Al>1. Following the faulting that formed the late Tertiary tectonic depression, a group of major volcanoes erupted within the depression, the lavas largely infilling the depression and eventually overstepping the boundary fault-scarps at several points. In the SW, volcanoes erupting in the Crater Highlands infilled the northeastern end of the Eyasi half graben, and in the east, lavas from the Kilimanjaro massif filled the depression to the north of the Pangani graben. The earliest date of eruption is poorly constrained since, even in the major fault escarpments, the first extrusive rocks of most of the volcanoes are not exposed. The only exposure of volcanic rocks in contact with the underlying basement rocks within the depression …