Browsing by Author "Fontana, G."

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    Eruption of kimberlite magmas: physical volcanology, geomorphology and age of the youngest kimberlitic volcanoes known on earth (the Upper Pleistocene/Holocene Igwisi Hills volcanoes, Tanzania)
    (Springer-Verlag, 2012-09-01) Brown, R. J.; Manya, Shukrani; Buisman, I.; Fontana, G.; Field, M.; Mac Niocaill, C.; Sparks, R. S. J.; Stuart, F. M.
    The Igwisi Hills volcanoes (IHV), Tanzania, are unique and important in preserving extra-crater lavas and pyroclastic edifices. They provide critical insights into the eruptive behaviour of kimberlite magmas that are not available at other known kimberlite volcanoes. Cosmogenic 3He dating of olivine crystals from IHV lavas and palaeomagnetic analyses indicates that they are Upper Pleistocene to Holocene in age. This makes them the youngest known kimberlite bodies on Earth by >30 Ma and may indicate a new phase of kimberlite volcanism on the Tanzania craton. Geological mapping, Global Positioning System surveying and field investigations reveal that each volcano comprises partially eroded pyroclastic edifices, craters and lavas. The volcanoes stand <40 m above the surrounding ground and are comparable in size to small monogenetic basaltic volcanoes. Pyroclastic cones consist of diffusely layered pyroclastic fall deposits comprising scoriaceous, pelletal and dense juvenile pyroclasts. Pyroclasts are similar to those documented in many ancient kimberlite pipes, indicating overlap in magma fragmentation dynamics between the Igwisi eruptions and other kimberlite eruptions. Characteristics of the pyroclastic cone deposits, including an absence of ballistic clasts and dominantly poorly vesicular scoria lapillistones and lapilli tuffs, indicate relatively weak explosive activity. Lava flow features indicate unexpectedly high viscosities (estimated at >102 to 106 Pa s) for kimberlite, attributed to degassing and in-vent cooling. Each volcano is inferred to be the result of a small-volume, short-lived (days to weeks) monogenetic eruption. The eruptive processes of each Igwisi volcano were broadly similar and developed through three phases: (1) fallout of lithic-bearing pyroclastic rocks during explosive excavation of craters and conduits; (2) fallout of juvenile lapilli from unsteady eruption columns and the construction of pyroclastic edifices around the vent; and (3) effusion of degassed viscous magma as lava flows. These processes are similar to those observed for other small-volume monogenetic eruptions (e.g. of basaltic magma).
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    Physical volcanology, geomorphology, and Cosmogenic
    (2012) Brown, R. J.; Manya, Shukrani; Buisman, I.; Sparks, R. S. J.; Field, M.; Stuart, F. M.; Fontana, G.
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    Physical Volcanology, Geomorphology, and Cosmogenic He Dating Of the Youngest Kimberlite Volcanoes On Earth (The Holocene Igwisi Hills Volcanoes, Tanzania
    (2012) Brown, R. J.; Manya, Shukrani; Buisman, I.; Sparks, R. S. J.; Field, M.; Stuart, F. M.; Fontana, G.
    The Igwisi Hills kimberlite volcanoes (IHV;Sampson 1953; Reid et al. 1975; Dawson 1994) are thought to be very young (Quaternary) but no reliable radiometric dates have yet been published. The next youngest kimberlite rocks are Eocene- Oligocene in age. Because of their youth, the IHV still have surface rocks and volcanic constructs. Kimberlite pyroclastic rocks emplaced on the Earth’s surface, as opposed to within vents, have eat potential to provide insight into eruptive dynamics, much as they have for other volcanic systems. The IHV are located on the western side of the Archaean Tanzanian craton, several kilometres NW of the village of Igwisi. They were emplaced through granitic gneiss basement. The IHV were first recognised by the Geological Survey of Tanganyika in the early 1950s (Sampson 1953). Subsequent studies focussed mainly on the petrology of the lava flow from the NE volcano and its mantle xenoliths (Reid et al. 1975; Dawson 1994). Dawson (1994) concluded that the mineralogy, major element and isotope chemistry of the Igwisi Hills rocks have strong affinities with calcite-rich kimberlites (e.g. Benfontein sills, South Africa). Little is known about the physical volcanology of the volcanoes, except that they comprise three volcanic centres formed of cones and craters of pyroclastic rocks and lavas. The IHV consist of three small volcanic centres (NE, Central and SW volcanoes) comprised of pyroclastic cones, craters and lavas. Together they constitute >3.2 × 106 m3 of erupted products preserved above the Earth’s surface and cover >2.7 × 105 m2. The relative ages of the IHV cannot be fully constrained, although field relationships indicate that the Central volcano postdates the NE volcano. They are aligned NE–SW and sit upon a broad, low NE–SW-oriented ridge, 500 m wide that is probably comprised of pyroclastic material (poorly exposed) from early stages of the eruptions. The volcanoes have been partially buried by younger sedimentary rocks and soils, and are presently covered in grassland and low-density forest. They exhibit similar pyroclastic lithofacies and all have small volume extrusions of lava.