Mineral chemistry and petrogenesis of carbonatite intrusions, Perry and Conway counties, Arkansas

Xenocrystic alvikite (C₂) breccia containing fragmental xenoliths of magnetite-apatite rock, phlogopitite, and argillite together with xenoliths of granite, syenite, and two varieties of sovite (C₁) all exhibiting reaction rims are described from Morrilton, Oppelo dump, Perryville, and Brazil Branch, Arkansas. The texture indicates that the bodies were explosively emplaced, and anorthoclase and aegirine in syenite xenoliths indicate that feniti-zation preceded one or more of the carbonatite intrusive phases.

The mica chemistry indicates that green phlogopite represents remnants of the earliest (C₁) sovite intrusive phase, and golden yellow, reversed pleochroic, phlogopite represents the metasomatized or “dolomitized” phase of the C₁ sovite. The metasomatism introduced Si, Mg, and Fe³⁺ and removed Fe²⁺, Al, and Ti from the magma.

Zoned pyroxene and amphibole phenocrysts and xenocrysts from the C₁ sovite indicate that the C₁ magma was being enriched in Mg and losing Fe; there is also a slight increase in Ti and Al. All ferromagnesian xenocrysts show magnetite-bearing reaction rims where they are in contact with the C₂ alvikite, which suggests oxidation of the alvikite magma.

Eastonite, which is present as overgrowths around reaction rims on a number of phlogopite xenocrysts, contains Ba and also the highest percentage of Al and Mg of all the micas. This eastonite may represent a C₄ alvikite phase, but in any case crystallized after the formation of the Fe-Ti reaction rim.