TEXTURAL AND COMPOSITIONAL VARIATIONS IN RAPIDLY COOLED PEGMATITE SHEETS

Fluid and melt inclusion studies and conductive cooling simulations suggest that thin pegmatite sheets may cool and solidify in only a few weeks in conditions of strong to moderate undercooling. We aim to document disequilibrium textures and mineral compositions that may have been a consequence of rapid, inward crystallization, under strong temperature gradients in the Li-Cs-Ta Animikie Red Ace (ARA) pegmatite, Northeastern Wisconsin. The post-tectonic, < 2.5 m thick sheet of superheated magma was emplaced at ~700°C as indicated by melt inclusion homogenization, intruding country rocks colder by as much as 500°C. Unidirectional and skeletal textures including comb-texture tourmaline, fibrous apatite and tourmaline, and skeletal quartz-mica intergrowths characterize the highly-undercooled outer zones of the ARA pegmatite. The magmatic textures are accompanied by a suite of replacement features resulted from the aggressive reaction of first generation mineral assemblages with a liquid derived from the hotter inner zones. The ARA pegmatite also includes recurrent tourmaline ± apatite layers that may represent local enrichments in slowly diffusing elements P, B, Al etc. Apatite occurrence, cathodoluminescence, and chemical composition fluctuate in a jigsaw-like pattern from the outer to the inner zones. Apatite generations of distinctive Mn, Cl, and Fe contents occur 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 in apatite-(CaF) to as much as 12.7 wt% MnO in manganoan apatite-(CaF). These variations are interpreted as a consequence of rhythmical evolution of melt composition in boundary layers formed ahead of solidifying fronts during the rapid, inward crystallization of the dike.