Sector collapse events at volcanoes in the North Tanzanian divergence zone and their implications for regional tectonics

Delcamp, Audray; Delvaux, Damien; Kwelwa, Shimba; Macheyeki, Athanas S.; Kervyn, Matthieu

doi:10.1130/b31119.1

Cited by 14 publications

(11 citation statements)

References 79 publications

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“…Geological data and K-Ar dating of phlogopite from tuffs of Loolmurwak and Kisete craters suggest volcanic 4 activity between 1.2 and 0.4 Ma (Dawson and Powell, 1969;Macintyre et al, 1974). Kerimasi is highly vegetated with limited outcrops in erosion gullies on the volcano slopes (Church, 1995) and in the upper parts of a debris avalanche deposit (Kervyn et al, 2008;Delcamp et al, 2016).…”

Section: Kerimasi Volcanomentioning

confidence: 99%

Oscillatory- and sector-zoned pyrochlore from carbonatites of the Kerimasi volcano, Gregory rift, Tanzania

Zaitsev

Spratt

Shtukenberg

et al. 2020

MinMag

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The Quaternary carbonatite–nephelinite Kerimasi volcano is located within the Gregory rift in northern Tanzania. It is composed of nephelinitic and carbonatitic pyroclastic rocks, tuffs, tuff breccias and pyroclastic breccias, which contain blocks of different plutonic (predominantly ijolite) and volcanic (predominantly nephelinite) rocks including carbonatites. The plutonic and volcanic carbonatites both contain calcite as the major mineral with variable amounts of magnetite or magnesioferrite, apatite and forsterite. Carbonatites also contain accessory baddeleyite, kerimasite, pyrochlore and calzirtite. Zr and Nb minerals are rarely observed in rock samples, though they are abundant in eluvial deposits of carbonatite tuff/pyroclastic breccias in the Loluni and Kisete craters. Pyrochlore, ideally (CaNa)Nb2O6F, occurs as octahedral and cubo-octahedral crystals up to 300 μm in size. Compositionally, pyrochlore from Loluni and Kisete differs. The former is enriched in U (up to 19.4 wt.% UO2), light rare earth elements (up to 8.3 wt.% LREE2O3) and Zr (up to 14.4 wt.% ZrO2), and the latter contains elevated Ti (up to 7.3 wt.% TiO2). All the crystals investigated were crystalline, including those with high U content (a = 10.4152(1) Å for Loluni and a = 10.3763(1) Å for Kisete crystals). They have little or no subsolidus alteration nor low-temperature cation exchange (A-site vacancy up to 1.5% of the site), and are suitable for single-crystal X-ray diffraction analysis (R1 = 0.0206 and 0.0290; for all independent reflections for Loluni and Kisete crystals, respectively). Observed variations in the pyrochlore composition, particularly Zr content, from the Loluni and Kisete craters suggest crystallisation from compositionally different carbonatitic melts. The majority of pyrochlore crystals studied exhibit exceptionally well-preserved oscillatory- and sometimes sector-type zoning. The preferential incorporation of smaller and higher charged elements into more geometrically constrained sites on the growing surfaces explains the formation of the sector zoning. The oscillatory zoning can be rationalised by considering convectional instabilities of carbonatite magmas during their emplacement.

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Section: Kerimasi Volcanomentioning

confidence: 99%

Oscillatory- and sector-zoned pyrochlore from carbonatites of the Kerimasi volcano, Gregory rift, Tanzania

Zaitsev

Spratt

Shtukenberg

et al. 2020

MinMag

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“…A sub‐E‐W trending volcanic chain forms the northern margin of the Manyara basin, and it represents a broadly distributed transfer fault zone linking the Natron extensional basin to the Pangani rift zone to the east [ Foster et al ., ; Muirhead et al ., ; Delcamp et al ., ] (Figure ). Although magmatism started between 6 and 4.7 Ma along the flanks of the Manyara basin [ Mollel et al ., ], the present morphology developed after the widespread outpouring of largely basaltic lavas at 1 Ma [ Foster et al ., ].…”

Section: Introductionmentioning

confidence: 99%

Fault‐magma interactions during early continental rifting: Seismicity of the Magadi‐Natron‐Manyara basins, Africa

Weinstein

Oliva

Ebinger

et al. 2017

Geochem Geophys Geosyst

100

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Although magmatism may occur during the earliest stages of continental rifting, its role in strain accommodation remains weakly constrained by largely 2‐D studies. We analyze seismicity data from a 13 month, 39‐station broadband seismic array to determine the role of magma intrusion on state‐of‐stress and strain localization, and their along‐strike variations. Precise earthquake locations using cluster analyses and a new 3‐D velocity model reveal lower crustal earthquakes beneath the central basins and along projections of steep border faults that degas CO2. Seismicity forms several disks interpreted as sills at 6–10 km below a monogenetic cone field. The sills overlie a lower crustal magma chamber that may feed eruptions at Oldoinyo Lengai volcano. After determining a new ML scaling relation, we determine a b‐value of 0.87 ± 0.03. Focal mechanisms for 65 earthquakes, and 13 from a catalogue prior to our array reveal an along‐axis stress rotation of ∼60° in the magmatically active zone. New and prior mechanisms show predominantly normal slip along steep nodal planes, with extension directions ∼N90°E north and south of an active volcanic chain consistent with geodetic data, and ∼N150°E in the volcanic chain. The stress rotation facilitates strain transfer from border fault systems, the locus of early‐stage deformation, to the zone of magma intrusion in the central rift. Our seismic, structural, and geochemistry results indicate that frequent lower crustal earthquakes are promoted by elevated pore pressures from volatile degassing along border faults, and hydraulic fracture around the margins of magma bodies. Results indicate that earthquakes are largely driven by stress state around inflating magma bodies.

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“…The major avalanche deposit formed by collapse of the south-eastern flank of Kerimasi ( Fig. 1) has been described by Hay (1983), Kervyn et al (2008) and Delcamp et al (2016). This deposit extends from a scarp at an elevation of ~ 2000m approximately 8.9 km in a south-easterly direction to the surrounding Engaruku Basin plains (~ 800m).…”

Section: Kerimasi Debris Flowmentioning

confidence: 99%

Mineralogy of volcanic calciocarbonatites from the Trig Point Hill debris flow, Kerimasi volcano, Tanzania: implications for the altered natrocarbonatite hypothesis

Mitchell

Dawson

2020

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A major debris flow, the Trig Point Hill flow, originating from Kerimasi volcano (Tanzania) contains numerous blocks of extrusive/pyroclastic carbonatites similar to those exposed at the rim of the currently inactive crater. The blocks of calcite carbonatite consist of: (1) large clasts of corroded and altered coarse grained calcite; (2) primary prismatic inclusion bearing phenocrystal calcite; and (3) a matrix consisting primarily of fine-grained prismatic calcite. The large clasts are inclusion free and exhibit a ‘corduroy-like’ texture resulting from solution along cleavage planes. The resulting voids are filled by brown Fe–Mn hydroxides/oxides and secondary calcite. The prismatic or lath-shaped phenocrystal calcite is not altered and contains melt inclusions consisting principally of primary Na–Ca carbonates which contain earlier-formed crystals of monticellite, periclase, apatite, Mn–Mg-magnetite, Mn–Fe-sphalerite and Nb-perovskite. Individual Na–Ca carbonate inclusions are of uniform composition, and the overall range of all inclusions analysed (wt.%) is from 28.7 to 35.9 CaO; 16.7–23.6 Na2O; 0.5–2.8 K2O, with minor SO3 (1.1–2.2) and SrO (0.34–1.0). The Na–Ca carbonate compositions are similar to that of shortite, although this phase is not present. The Na–Ca carbonates are considered to be primary deuteric phases and not secondary minerals formed after nyerereite. Monticellite shows limited compositional variation and contains 2–4 wt.% MnO and 12 wt.% FeO and is Mn-poor relative to monticellite in Oldoinyo Lengai natrocarbonatite. Periclase is Fe-bearing with up to 13 wt.% FeO. Spinels are Cr-free, Mn-poor and belong to the magnetite–magnesioferrite series in contrast to Mn-rich spinels of the magnetite–jacobsite series occurring in Oldoinyo Lengai natrocarbonatite. The matrix in which the ‘corduroy’ clasts and phenocrystal calcite are set consists of closely packed small prisms of calcite lacking melt inclusions, with interstitial fine-grained apatite, baryte, strontianite and minor fluorite. Pore spaces are filled with secondary Mn–Fe hydroxides/oxides, anhydrite and gypsum. The hypothesis that flow-aligned calcite in volcanic calciocarbonatites from Kerimasi, Tinderet, Homa and Catanda is altered nyerereite is discussed and it is considered that these calcite are either primary phases or altered melilite. The nyerereite alteration hypothesis is discussed with respect to the volumetric and compositional aspects of pseudomorphism by dissolution–precipitation replacement mechanisms. This study concludes that none of the volcanic calciocarbonatites containing flow-aligned calcite phenocrysts are altered natrocarbonatite.

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Sector collapse events at volcanoes in the North Tanzanian divergence zone and their implications for regional tectonics

Cited by 14 publications

References 79 publications

Oscillatory- and sector-zoned pyrochlore from carbonatites of the Kerimasi volcano, Gregory rift, Tanzania

Oscillatory- and sector-zoned pyrochlore from carbonatites of the Kerimasi volcano, Gregory rift, Tanzania

Fault‐magma interactions during early continental rifting: Seismicity of the Magadi‐Natron‐Manyara basins, Africa

Mineralogy of volcanic calciocarbonatites from the Trig Point Hill debris flow, Kerimasi volcano, Tanzania: implications for the altered natrocarbonatite hypothesis

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