The Occurrence and Significance of Xenocrysts of Apatite, Ilmenite, and Na-Fe-Ti Oxide in Ultrapotassic Lavas from the Leucite Hills, Wyoming

Michael Barton
Department of Geology and Mineralogy, The Ohio State University, Columbus, Ohio 43210, USA

Abstract: The occurrence and mineral chemistry of apatite, magnesian ilmenite, and an Na-Fe-Ti oxide in lavas from the Leucite Hills are reported. Magnesian ilmenite and apatite occur as xenocrysts and as crystals in amphibole-mica-pyroxenite xenoliths. Na-Fe-Ti oxide and also rutile occur as inclusions in ilmenite. The latter mineral contains up to 7.2% MgO and shows evidence of oxidation by, and reaction with, the host magma. The apatite differs from that which occurs as phenocrysts and microphenocrysts inasmuch as REE were not detected. The occurrence of these minerals, which are important repositories for REE and High Field Strength elements, together with phlogopite in the upper mantle source regions of ultrapotassic lavas, is important and may explain some unusual aspects of the geochemistry of such lavas (low K/Rb, P2O5/Ce, Ti/Zr, high Ti/V, Zr/Nb). The source regions must be grossly heterogenous and a two-component model is suggested for the source. This model is similar to that suggested for the source regions of other alkaline magmas and is capable of explaining the unusual Nd-Sr isotopic characteristics of the Leucite Hills lavas.

Keywords: xenocrysts • apatite • ilmenite • ultrapotassic lavas • Leucite Hills • Wyoming

Mineralogical Magazine; June 1987 v. 51; no. 360; p. 265-270; DOI: 10.1180/minmag.1987.051.360.08
© 1987, The Mineralogical Society
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