Textural and compositional records of crystallization processes in a rapidly cooled pegmatite dike from northeastern Wisconsin, USA

Unidirectional and skeletal textures in the inward-crystallizing Animikie Red Ace (ARA) thin pegmatite dike suggest rapid, disequilibrium crystallization of the outer zones, formed when the pegmatite magma came in contact with much colder country rocks. The observed textures support our previous fluid/melt inclusion study and conductive cooling modeling which has estimated that the dike cooled and solidified in less than 50 days, in conditions of strong to moderate undercooling. Primary, magmatic disequilibrium textures are accompanied by a suite of replacement textures resulted from the aggressive reaction of first generation mineral assemblages deposited on the colder walls with a liquid derived from the hotter inner zones. Apatite habit, frequency, cathodoluminescence, and chemical composition fluctuate systematically within the ARA pegmatite from the outer to the inner zones. Apatite of distinctive Mn, Cl, and Fe contents occurs in the border zone, wall zone, and core-margin zone, while the intervening intermediate zones and main core zones do not contain any apatite. Mn concentrations alternate several times from very low to high, reaching up to 12.7% wt. MnO (manganoan fluoroapatite) in the wall zone. The jigsaw-like pattern of apatite occurrence and compositions is interpreted as a consequence of rhythmical evolution of magma composition in boundary layers formed ahead of solidified layers during the rapid, inward crystallization of the dike. Keywords: Animikie Red Ace pegmatite, LCT pegmatite, apatite, undercooling, disequilibrium crystallization, boundary layers, cathodoluminescence.