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APATITE TEXTURES AND COMPOSITIONS AS RECORDS OF CRYSTALLIZATION PROCESSES IN THE ANIMIKIE RED ACE PEGMATITE DIKE, WISCONSIN, USA

Mona-Liza C. Sirbescu^1,, Mark A. Leatherman^1,¶, James J. Student¹ and Alison R. Beehr²

¹ Department of Geology, Central Michigan University, 314 Brooks Hall, Mount Pleasant, Michigan 48859, USA
² Department of Earth and Planetary Sciences, Washington University, Campus Box 1169, 1 Brookings Drive, Saint Louis, Missouri 63130-4862, USA

E-mail address: mona.sirbescu{at}cmich.edu

Unidirectional and skeletal textures in the thin inward-crystallizing Animikie Red Ace (ARA) pegmatite dike, in northeastern Wisconsin, suggest rapid, disequilibrium crystallization of the outer zones as the pegmatite-forming magma came in contact with much cooler country-rocks. The rock textures support the cooling and solidification of the dike in less than 50 days in conditions of strong to moderate undercooling, as estimated in a previous fluid- and melt-inclusion study and modeling of the conductive cooling. The magmatic disequilibrium textures are accompanied by a suite of replacement textures resulting from the aggressive reaction of assemblages of first-generation minerals 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; the intervening intermediate zones and main core zones do not contain any apatite. Manganese concentrations alternate several times from very low in apatite-(CaF) to as much as 12.7 wt% MnO in manganoan apatite-(CaF). The jigsaw-like pattern of apatite occurrence and chemical composition is interpreted as a consequence of rhythmic evolution of melt composition in boundary layers formed ahead of solidifying fronts during the rapid, inward crystallization of the dike.

Keywords: LCT pegmatite, apatite, undercooling, disequilibrium crystallization, boundary layers, cathodoluminescence, Animikie Red Ace, Wisconsin.