Petrological Significance of Mineral Chemistry in the Agathla Peak and the Thumb Minettes, Navajo Volcanic Field

Abstract

The Agathla Peak and The Thumb minettes from the Navajo province, southwestern USA, are characterized by macrocrysts of phlogopite and diopside plus rare olivine () set in a crystalline to glassy groundmass containing diopside, phlogopite, sanidine, apatite, Ti-magnetite and analcime. The diopside has low Al, Ti, and Na like late-stage diopside in kimberlite groundmass. Ti-rich phlogopite is zoned with increasing Fe, Ti and Ba from a Cr, Ni-bearing core. High Sr in the minettes is expressed in groundmass sanidine (up to 1.2 wt % SrO), apatite (0.5-0.9) and diopside (0.03-0.11). Xenocrystal apatite is chemically distinct from phenocrystal apatite in The Thumb minette in having low SrO (< 0.04 wt %) and a distinct REE distribution which matches apatites in garaet-granulite xenoliths. All chemical properties of the minerals are consistent with derivation of the minette magmas from a region of the upper mantle containing olivine, clinopyroxene, phlogopite and ilmenite and enriched in K, Ti, Ba, Sr, P, REE, H, and halogens, as proposed by earlier workers.