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SUMMARY:Rifting of a Continent: the North Tanzanian Divergence Zone\, East
  African Rift System - Sara Mana (Rutgers University)
DTSTART:20120524T160000Z
DTEND:20120524T170000Z
UID:TALK38293@talks.cam.ac.uk
CONTACT:Yves Moussallam
DESCRIPTION:The role of magmatism in continental rift initiation and evolu
 tion is of much debate. Our research focuses on a section of the magmatic-
 rich eastern branch of the East African Rift in Northern Tanzania that dep
 icts the complex early stage of tectono-magmatic rift evolution. This area
 \, the North Tanzania Divergence (NTD)\, is currently volcanically active 
 with a magmatic history that initiated in the Miocene\, prior to documente
 d extension. Some of the NTD volcanoes are among Earth’s largest (Kilima
 njaro\, Ngorongoro)\, and have produced a diverse array of lavas from basa
 lt to rhyolite\, trachyte\, nephelinite to phonolite and carbonatite. Thei
 r distribution is widespread\, both N-S along the rift axis and E-W across
  the valley floor and onto the adjacent rift margins.\n\nThe oldest NTD ma
 gmatism is recorded at the centrally located Essimingor volcano. We report
  new 40Ar/39Ar ages\, major and trace element analyses and Sr-Nd-Pb radiog
 enic isotopic signatures on well-located lavas representing the observable
  variation in lithology and stratigraphy from the S and SW slopes of Essim
 ingor. Laser-incremental heating 40Ar/39Ar analyses of whole rock\, matrix
  and nepheline separates yield plateau ages ranging from 5.76±0.02 Ma to 
 5.91±0.01 Ma.\n\nEssimingor major element data define narrow compositiona
 l variations consistent with fractional crystallization. Open system proce
 sses of mixing or contamination are inferred from an increase in Sr isotop
 ic values with indices of fractionation. Ce/Pb varies over a large range (
 59 to 7)\, the lower end of which implies crustal assimilation that overpr
 ints the mantle signature. An FC versus AFC process has therefore been mod
 eled. The Sr-Nd-Pb isotopic values indicate the involvement of a HIMU-like
  component. Trace element abundances of the more primitive samples (MgO >9
  wt%) suggest partial melting of a metasomatized lithospheric mantle perid
 otite characterized by the presence of residual garnet and phlogopite comb
 ined with minor amphibole and apatite. The coexistence of garnet and phlog
 opite in the source suggests melting at ~80–150 km depth\, consistent wi
 th the base of the lithosphere in the eastern branch identified using Rayl
 eigh wave tomography (120-160 km\; Weeraratne et al. 2003) and indicating 
 that Essimingor represents the initial phase of lithospheric removal.\n\nO
 ngoing analysis of younger NTD volcanoes should help constrain the timing 
 and location of the progressive lithospheric thinning during early rifting
 .\n
LOCATION:Harker II\, Geology Department
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