DIOPSIDE PHENOCRYSTS FROM NEPHELINITE LAVAS, NAPAK VOLCANO, EASTERN UGANDA: EVIDENCE FOR MAGMA MIXING
ANTONIO SIMONETTI*
Ottawa-Carleton Geoscience Centre, Department of Earth Sciences, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario K1S 5B6
MARK SHORE
Ottawa-Carleton Geoscience Centre, Department of Geology, University of Ottawa, Ottawa, Ontario K1N 6N5
KEITH BELL
Ottawa-Carleton Geoscience Centre, Department of Earth Sciences, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario K1S 5B6
* Present address: Max-Planck-Institut für Chemie, Abteilung Geochemie, Postfach 3060, D-55020 Mainz, Germany.
Abstract
Samples of olivine nephelinite from Napak volcano, eastern Uganda, contain two populations of clinopyroxene phenocrysts, one chromian and the
other titanium-bearing aluminian diopside. The chromian diopside crystals are among the most magnesian to occur in nephelinite. The diopside
phenocrysts show normal, reverse and oscillatory zoning, and some have an embayed texture suggestive of resorption. Disequilibrium in Nd,
Pb, and Sr isotopic ratios for both populations of clinopyroxene and their host rocks is interpreted in terms of crystallization in an open
system. Populations of chromian and Ti-bearing aluminian diopside from the same lavas have distinct isotopic ratios, indicative of
derivation and crystallization from different melts. We attribute the zoning, textural features, and bimodality of the Napak diopsides to
the mixing of at least two nephelinitic magmas at a shallow crustal level.