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The Igwisi Hills extrusive 'kimberlites'The petrography and mineral chemistry of volcanic rocks from the Igwisi Hills in Tanzania are discussed. There is considerable evidence to suggest that the Igwisi rocks are extrusive kimberlites: a two-component nature with high P-T minerals in a low P-T matrix; the presence of chrome pyrope, Al enstatite, chrome diopside, chromite and olivine; a highly oxidized, volatile-rich matrix with serpentine, calcite, magnetite, perovskite; high Sr, Zr, and Nb contents; occurrence in a narrow isolated vent within a stable shield area. The Igwisi rocks differ from kimberlite in the lack of magnesian ilmenite, the scarcity of matrix phlogopite, and the overall low alkali content. They apparently contain material from phlogopite-bearing garnet peridotites with a primary mineral assemblage indicative of equilibrium at upper mantle temperatures and pressures. This primary assemblage was brought rapidly to the surface in a gas-charged, carbonate-rich fluid. Rapid upward transport, extrusion, and rapid cooling have tended to prevent reaction between inclusions and the carbonate-rich matrix that might otherwise have yielded a more typical kimberlite.
Document ID
19760048657
Acquisition Source
Legacy CDMS
Document Type
Other - Collected Works
Authors
Reid, A. M.
(NASA Johnson Space Center Houston, Tex., United States)
Donaldson, C. H.
(Lunar Science Institute Houston, Tex., United States)
Dawson, J. B.
(St. Andrews, University St. Andrews, United Kingdom)
Brown, R. W.
(Lockheed Electronics Co., Inc. Houston, Tex., United States)
Ridley, W. I.
(Lamont-Doherty Geological Observatory Palisades, N.Y., United States)
Date Acquired
August 8, 2013
Publication Date
January 1, 1975
Subject Category
Earth Resources And Remote Sensing
Accession Number
76A31623
Funding Number(s)
CONTRACT_GRANT: NSR-09-051-001
Distribution Limits
Public
Copyright
Other

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