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Ising models of undercooled binary system crystallization: Comparison with experimental and pegmatite textures

The petrogenesis of granitic pegmatites may be one of the greatest enigmas of igneous petrology at the present time. Pegmatites are typically defined by the presence of large (centimeter scale) to extraordinarily large (several meters across) crystals. The presence of such large crystals suggests that pegmatites cooled slowly and that crystals grew from melts in which diffusion was rapid. However, pegmatites are small bodies, rarely more than 50 m long (Cameron et al. 1949), and commonly intrude cool upper crustal rocks at low temperatures. Consequently, cooling is rapid, requiring only hundreds to thousands of years, at most, to reach solidus temperatures (Chakoumakos and Lumpkin 1990; London 1992; Webber et al. 1997). Additionally, diffusion of silicate components in pegmatite melts is expected to be slow based upon their similarity to granitic compositions (Baker 1991, 1992) and their low liquidus temperatures, near 923 K (Chakoumakos and Lumpkin 1990). If one were to choose a melt composition and conditions of emplacement best suited to the production of small crystals in an intrusive rock, one would choose a pegmatite. Instead, we find that conditions thought to produce small crystals result in exactly the opposite. Most granitic pegmatite bodies are internally zoned, either symmetrically or asymmetrically, in mineral texture and element distribution. A general zoning pattern has been recognized in many pegmatites. This pattern consists of a lower zone (footwall) composed of a fine grained, albite-rich aplite (in ABSTRACT