Abstract
A Quaternary volcanic field at Fort Portal, SW Uganda, contains approximately 50 vents that erupted only carbonatite. The vents are marked by monogenetic tuff cones defining two ENE-trending belts. Lava from a fissure at the west end of the northern belt formed a flow 0.3 km2 in area and 1–5 m thick. The lava is vesicular throughout with a scoriaceous top, and probably formed by agglutination of spatter from lava fountains. “Phenocrysts” are olivine, clinopyroxene, phlogopite, and titanomagnetite enclosing blebs of pyrrhotite. Rims of monticellite, gehlenite, and reinhardbraunsite surround olivine, clinopyroxene and phlogopite, and magnetite is rimmed by spinel. The reaction relations suggest that these “phenocryst” phases are actually xenocrysts, perhaps from a source similar to that which supplied phlogopite clinopyroxenite xenoliths in the Katwe-Kikorongo volcanic field 75 km SW of Fort Portal. The groundmass of fresh carbonatite lava consists of tabular calcite, spurrite, periclase, hydroxylapatite, perovskite, spinel, pyrrhotite, and barite. The lava was readily altered; where meteoric water had access, spurrite and periclase are lacking, and some calcite is recrystallized. Vesicles in lava and rare dike rocks are partly filled with calcite, followed by jennite and thaumasite. Pyroclastic deposits cover 142 km2 and are far more voluminous than lava. Carbonatite ejecta were identical to lava in primary mineralogy, but are much more contaminated by crustal rock fragments and xenocrysts. At Fort Portal, eruption of a CaO-MgO-CO2-SiO2-P2O5-SO2-H2O-F liquid was unaccompanied by that of a more silica-rich or alkali-rich liquid. Alkali-rich carbonatite lavas and pyroclastic deposits have been documented elsewhere in East Africa, and calcite-rich volcanic carbonatites have been attributed to replacement of magmatic alkali carbonates by calcite. However, the alkali-poor volcanic carbonatites at Fort Portal were not formed by leaching of alkalis in meteoric water; tabular calcite is not pseudomorphous after alkali carbonates such as nyerereite. The Fort Portal magma was low in alkalis at the time of eruption.
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References
Andersen T (1988) Evolution of peralkaline calcite carbonatite magma in the Fen complex, southwest Norway. Lithos 27:99–112
Barker DS (1989) Field relations of carbonatites. In: Bell K (ed) Carbonatites-Genesis and Evolution, Unwin Hyman, London (in press)
Beckett MF, Gittins J (1989) The origin of periclase-bearing carbonatites. Am Mineral (in press)
Bell K, Blenkinsop J (1987) Nd and Sr isotopic compositions of East African carbonatites: Implications for mantle heterogeneity. Geology 15:99–102
Bell K, Powell JL (1969) Strontium isotopic studies of alkalic rocks: the potassium-rich lavas of the Birunga and Toro-Ankole regions, east and central equatorial Africa. J Petrology 10:536–572
Clarke MGC, Roberts B (1986) Carbonated melilitites and calcitized alkalicarbonatites from Homa Mountain, western Kenya: a reinterpretation. Geol Mag 123:683–692
Combe AD (1938) The Kasekere Volcanic Area, North Western Toro. Ann Rept Geol Surv Uganda, 14–17
Dawson JB, Garson MS, Roberts B (1987) Altered former alkalic carbonatite lava from Oldoinyo Lengai, Tanzania: inferences for calcite carbonatite lavas. Geology 15:765–768
Deans T, Roberts B (1984) Carbonatite tuffs and lava clasts of the Tinderet foothills, western Kenya: a study of calcified natrocarbonatites. J Geol Soc London 141:563–580
Deer WA, Howie RA, Zussman J (1982) Rock forming minerals, Vol 1A, Orthosilicates (2nd ed). Longmans, London pp 919
Deines P, Gold DP (1973) The isotopic composition of carbonatite and kimberlite carbonates and their bearing on the isotopic composition of deep-seated carbon. Geochim Cosmochim Acta 37:1709–1733
Fanelli MF, Cava N, Wyllie PJ (1986) Calcite and dolomite without portlandite at a new eutectic in CaO-MgO-CO2-H2O, with applications to carbonatites. In: Bonev I, Kanazirski M (eds) Morphology and phase equilibria of minerals. Proc 13th General Meeting, Int Mineral Assoc, vol 2. Sofia, Bulgaria, pp 313–322
Gard JA, Taylor HFW, Cliff G, Lorimer GW (1977) A reexamination of jennite. Am Mineral 62:365–368
Gaspar JC, Wyllie PJ (1983) Magnetite in the carbonatites from the Jacupiranga complex, Brazil. Am Mineral 68:195–213
Gaspar JC, Wyllie PJ (1987) The phlogopites from the Jacupiranga carbonatite intrusions. Mineral Petrol 36:121–134
Giampaolo C (1986) Dehydration kinetics of thaumasite at ambient pressure. Neues Jahrb Mineral Mh 1986:126–134
Gittins J (1989) The origin and evolution of carbonatite magmas. In: Bell K (ed) Carbonatites — Genesis and evolution. Unwin Hyman, London (in press)
Gittins J, McKie D (1980) Alkalic carbonatite magmas: Oldoinyo Lengai and its wider applicability. Lithos 13:213–215
Hamm H-M, Hentschel G (1983) Reinhardbraunsite, Ca5-(SiO4)2(OH,F)2, a new mineral — the natural equivalent of synthetic “calcio-chondrodite”. Neues Jahrb Mineral Mh 1983:119–129
Hay RL (1978) Melilitite-carbonatite tuffs in the Laetolil beds of Tanzania. Contrib Mineral Petrology 67:357–367
Hay RL (1983) Natrocarbonatite tephra of Kerimasi volcano, Tanzania. Geology 11:599–602
Hay RL (1986) Role of tephra in the preservation of fossils in Cenozoic deposits of East Africa. In: Frostick LE, Renaut RW, Reid I, Tiercelin JJ (eds) Sedimentation in the African Rifts. Geol Soc London Spec Publ 24, pp 339–344
Hay RL (1989) Holocene carbonatite — nephelinite tephra deposits of Oldoinyo Lengai, Tanzania. J Volc Geotherm Res (in press)
Hay RL, O'Neil JR (1983) Carbonatite tuffs in the Laetolil Beds of Tanzania and the Kaiserstuhl in Germany. Contrib Mineral Petrology 82:403–406
Holmes A, Harwood HF (1932) Petrology of the volcanic fields east and south-east of Ruwenzori, Uganda. Q J Geol Soc London 88:370–442
Hornung G (1969) Volcanic rocks around Fort Portal, western Uganda. 13th Ann Rep Inst African Geology, Univ Leeds 26–27
Huang W-L, Wyllie PJ, Nehru CE (1980) Subsolidus and liquidus phase relationships in the system CaO-SiO2-CO2 to 30 kbar with geological applications. Am Mineral 65:285–301
Hubberten H-W, Katz-Lehnert K, Keller J (1988) Carbon and oxygen isotope investigations in Carbonatites and related rocks from the Kaiserstuhl, Germany. Chem Geol 70:257–274
Kapustin Yu L, Polyakov AI (1985) Carbonatite volcanos of East Africa and the genesis of Carbonatites. Int Geol Rev 27:434–448
Keller J (1981) Carbonatite volcanism in the Kaiserstuhl alkaline complex: evidence for highly fluid carbonatitic melts at the earth's surface. J Vole Geotherm Res 9:423–431
Keller J (1989) Extrusive Carbonatites and their significance. In: Bell K (ed) Carbonatites: genesis and evolution. Unwin Hyman, London (in press)
von Knorring O (1963) Report on mineralogical research. 7th Ann Report Inst African Geology, Univ Leeds 33–35
von Knorring O (1967) Carbonatitic rocks from the volcanic field of western Uganda, 11th Ann Report Inst African Geology, Univ Leeds 30–32
von Knorring O, Du Bois CGB (1961) Carbonatitic lava from Fort Portal area in Western Uganda. Nature 192:1064–1065
Le Bas MJ (1987) Nephelinites and Carbonatites. p 53–83. In Fitton JG, Upton BGJ (eds) Alkaline Igneous Rocks. Geol Soc London Spec Publ 30
Lloyd FE (1981) Upper-mantle metasomatism beneath a continental rift: clinopyroxenes in alkalic mafic lava and nodules from South West Uganda. Mineral Mag 44:315–323
Lloyd FE (1985) Experimental melting and crystallization of glassy olivine melilitites. Contrib Mineral Petrology 90:236–243
Lloyd FE, Bailey DK (1975) Light element metasomatism of the continental mantle: the evidence and the consequences. Phys Chem Earth 9:389–416
Lloyd FE, Nixon PH, Hornung G, Condliffe E (1987) Regional K-metasomatism in the mantle beneath the west branch of the East African Rift: alkali clinopyroxenite xenoliths in highly potassic magmas, p 641–659. In: Nixon PH (ed) Mantle xenoliths. Wiley, Chichester
Mariano AN (1988) Some further geological applications of cathodoluminescence. p 94–123. In: Marshall DJ (ed) Cathodoluminescence of geological materials. Unwin Hyman, Boston, pp 146
Mariano AN, Roeder PL (1983) Kerimasi: a neglected carbonatite volcano. J Geology 91:449–455
Nixon PH (1987) Kimberlitic xenoliths and their cratonic setting. In: Nixon PH (ed) Mantle xenoliths Wiley Chichester, pp 215–239
Nixon PH, Hornung G (1973) The carbonatite lavas and tuffs near Fort Portal. Western Uganda. Overseas Geol Mineral Res 41:168–179
Nixon PH, Morton WH, von Knorring O (1971) Tychite and northupite from Lake Katwe, Uganda. Geol Soc Finland Bull 43:125–130
Robie RA, Hemingway BS, Fisher JR (1978) Thermodynamic properties of minerals and related substances at 298.15 K and 1 bar (105 pascals) pressure and at higher temperatures. US Geol Survey Bull 1452
Romanchev BP, Sokolov SV (1980) Liquation in the production and geochemistry of the rocks in carbonatite complexes. Geochem Internat 16:125–135
Sharp ZD, Essene EJ, Anovitz LM, Metz GW, Westrum EF Jr, Hemingway BS, Valley JW (1986) The heat capacity of a natural monticellite and phase equilibria in the system CaO-MgO-SiO2-CO2. Geochim Cosmochim Acta 50:1475–1484
Sheridan MF, Wohletz KH (1983) Hydrovolcanism: basic considerations and review. J Vole Geotherm Res 17:1–29
Simkin T, Smith JV (1970) Minor-element distribution in olivine. J Geol 78:304–325
Sommerauer J, Katz-Lehnert K (1985) A new partial substitution mechanism of CO 2−3 /CO3OH3− and SiO 4−4 for the PO 3−4 group in hydroxyapatite from the Kaiserstuhl alkaline complex (SW Germany): Contrib Mineral Petrology 91:360–368
Thomas CW, Nixon PH (1987) Lower crustal granulite xenoliths in carbonatite volcanoes of the Western Rift of East Africa. Mineral Mag 51:621–633
Treiman AH, Essene EJ (1983) Phase equilibria in the system CaO-SiO2-CO2. Am J Sci 283-A:97–120
Twyman JD, Gittins J (1987) Alkalic carbonatite magmas: parental or derivative? In: Fitton JG, Upton BGJ (eds) Alkaline igneous rocks. Geol Soc London Spec Publ 30, pp 85–94
Vinogradov VI, Krasnov AA, Kuleshov VN, Sulerzhitskiy LD (1978) 13C/12C, 18O/16O, and 14C concentrations in the carbonatites of the Kalyango Volcano (East Africa) (in Russian). Izv Akad Nauk SSSR, Ser Geol 6:33–41
Warner RD (1973) Liquidus relations in the system CaO-MgO-SiO2-H2O at 10 kb PH2O and their petrologic significance. Am J Sci 273:925–946
Wyllie PJ, Biggar GM (1966) Fractional crystallization in the carbonatite systems CaO-MgO-CO2-H2O and CaO-CaF2-P2O5-CO2-H2O. Proc 4th General Meeting Int Mineral Assoc, Mineral Soc India IMA Vol pp 92–102
Zharikov VA, Shmulovich KI (1969) High temperature mineral equilibria in the system CaO-SiO2-CO2, Geochem Internat 6:853–869
Zimanowski B, Lorenz V, Frohlich G (1986) Experiments on phreatomagmatic explosions with silicate and carbonatitic melts. J Volc Geotherm Res 30:149–153
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Barker, D.S., Nixon, P.H. High-Ca, low-alkali carbonatite volcanism at Fort Portal, Uganda. Contr. Mineral. and Petrol. 103, 166–177 (1989). https://doi.org/10.1007/BF00378502
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DOI: https://doi.org/10.1007/BF00378502