HR: 1330h
AN: V13B-04    [Abstracts]
TI: Geochemistry and Geochronology of Ngorongoro Crater, Tanzania: Implication for Magma Evolution, Duration of Volcanic Activity and Age of the Ngorongoro N-R Geomagnetic Polarity Transition
AU: * Mollel, G F
EM: gmollel@rci.rutgers.edu
AF: Department of Geologicals Sciences, Rutgers University, Wright Labs,, 610 Taylor Rd., Piscataway, NJ 08854 United States
AU: Swisher, C C
AF: Department of Geologicals Sciences, Rutgers University, Wright Labs,, 610 Taylor Rd., Piscataway, NJ 08854 United States
AU: Feigenson, M D
AF: Department of Geologicals Sciences, Rutgers University, Wright Labs,, 610 Taylor Rd., Piscataway, NJ 08854 United States
AU: Carr, M J
AF: Department of Geologicals Sciences, Rutgers University, Wright Labs,, 610 Taylor Rd., Piscataway, NJ 08854 United States
AB: 40Ar/39Ar dates on volcanic rocks from the Ngorongoro Crater (NC) in northern Tanzania indicate that NC activity was very short in duration lasting approximately 120 ka. Laser incremental heating experiments on lava from the bottom and top of the NC crater-wall section gave ages of 2.08 +/- 0.04 and 1.96 +/- 0.02 Ma respectively. Lavas from the same section show a change in magnetic polarity from normal (N) at the lower part to reverse (R) polarity at the upper part (Gromm‚ et al. 1970). The new ages are about 400 ka younger than previously estimated by K-Ar technique. These new ages suggest correlation of the NC N-R polarity transition to the 2.1 Ma (N-R) Reunion-Matuyama boundary (Cande and Kent, 1995), instead of the Gauss-Matuyama boundary as proposed by Gromm‚ et al. (1970). 87Sr/86Sr measurements on lavas from the NC section vary widely from 0.70801 in the trachydacite at the base to 0.70405 in the basaltic lava near the top. The lower part of the section is more radiogenic varying from 0.70592 to 0.70801 whereas the upper part is constrained to 0.70405 to 0.70450. The more radiogenic lower part is likely to have interacted with crustal rocks. Two possible contaminants are the Tanzanian Archean Craton to the west and the late Proterozoic Mozambican belt in the east. The crater-wall section is composed of trachydacite at the bottom that becomes trachyandesite in mid-section. The top section is mainly basaltic. Major and trace elements show an inverted geochemical signature that is typical of stratified magma chambers characterized by a silicic top and basaltic bottom. Olivine basalt at the upper part of the section has the highest Mg&35; (56.60) and in general the upper section is more mafic than the lower section as inferred from Mg&35;. The upper part of the section is high in TiO2, MgO, FeOT, and CaO wt% whereas SiO2 and K2O wt% are higher in lower part of the section. No significant variations are observed in N2O, Al2O3, P2O5 and MnO wt% up-section. Highly incompatible elements e.g. Zr, Nb and Hf concentrations are higher in the lower section compared to the upper part probably indicating the extent of magma evolution.
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly