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PETROGRAPHY AND MINERAL CHEMISTRY OF ULTRAMAFIC AND RELATED INCLUSIONS FROM THE ORAPA A/K-1 KIMBERLITE PIPE, BOTSWANA
Abstract
The Orapa A/K-1 kimberlite pipe occurs within a cluster of more than thirty pipes in east-central Botswana and is noted for preserving a nearly complete stratigraphic section of the original diatreme. Deep-seated nodules from the Orapa pipe include (1) an oxide suite comprised of ilmenite, rutile, and ilmenite-rutile intergrowths, (2) a silicate suite comprised of garnet, clinopyroxene, and various silicate-oxide intergrowths, and (3) a lithic suite comprised primarily of ecologites and various crustal rocks. The compositions of individual ilmenite nodules are typical of those in kimberlites with MgO values varying between 7 and 15 wt. % and Cr(,2)O(,3) contents averaging approximately 2.5 wt. %. Marked peripheral zoning in magnesium content is probably indicative of reaction with the transporting kimberlitic magma. Discrete nodules of rutile and rutile host containing ilmenite lamellae recovered from this pipe are first reported in the literature and are characterized by rutile with up to 13.5 wt. % Nb(,2)O(,5) and 8 wt. % Cr(,2)O(,3). Results from experiments involving controlled conditions of temeperature and oxygen fugacity (1 atm) suggest the importance of TI('3+)-bearing chromian armalcolite as a possible high pressure precursor to the ilmenite-rutile intergrowths. The clinopyroxene and garnet discrete nodules are diopsidic and pyropic, respectively, in composition. Large differences in corresponding Cr(,2)O(,3) contents indicate that these discrete nodules did not originate by disaggregation of the associated eclogites. The eclogites from Orapa include foliated and non-foliated textural types and have been reported to contain both diamond and graphite. Compositions of the garnet and clinopyroxene coexisting in these eclogites suggest temperatures of equilibration consistent with upper mantle conditions when plotted on model geotherms determined for neighboring areas in southern Africa. The petrographic and mineral chemical data presented in this study demonstrate an overall close similarity between the Orapa nodule suite and those representative of other, more thoroughly investigated districts in Africa, Siberia, and the United States. The Orapa suite is distinctive, however, in its wide variety of oxide discrete nodules, the presence of certain complex silicate/oxide intergrowths, and the previously reported coexistence of diamond and graphite in eclogite.
Subject Area
Mineralogy
Recommended Citation
TOLLO, RICHARD PAUL, "PETROGRAPHY AND MINERAL CHEMISTRY OF ULTRAMAFIC AND RELATED INCLUSIONS FROM THE ORAPA A/K-1 KIMBERLITE PIPE, BOTSWANA" (1982). Doctoral Dissertations Available from Proquest. AAI8229619.
https://scholarworks.umass.edu/dissertations/AAI8229619