Growth of megaspherulites in a rhyolitic vitrophyre

Megaspherulites occur in a rhyolitic vitrophyre, near the base of a thick r hyolitic vitrophyre that occupies a late Eocene to early Oligocene volcanic -tectonic basin near Silver Cliff, Colorado. Diameters of the megaspherulit es range from 0.3 to over 3.66 m, including an alteration layer. The megasp herulites are compound spherulites. They are holocrystalline, very fine- to fine-grained, com posed of sanidine and quartz, and surrounded by a thin s anidine-quartz rind, and an alteration layer containing mordenite and 15 An gstrom montmorillonite. Megaspherulite crystallization began soon after the vitrophyre was emplaced as the result of sparse heterogeneous nucleation, under highly nonequilibr ium conditions. Spherulite growth proceeded in a diffusion-controlled manne r under lowered viscosity as a result of a high water content (5 to 7 wt%), which also contributed to sparse nucleation. When nucleation did occur, it was at a large DeltaT (245-350 degreesC) on near critical-sized nuclei. On ce the spherulites began to grow, continued nucleation was on existing grow th cones and new, independent spherulites did not form. Sanidine crystals g rew with a fibrous habit, which is favored by a large DeltaT, restricting f ibril lengths and diameters between 10-30 and 3-7 mum, respectively. During crystallization, these growth cones impinged upon each other, resulting in fibril cone-free areas. These cone-free (interconal) areas consist of coar ser, fine-grained phases, dominated by quartz, which crystallized from the melt as it accumulated between the crystallizing sanidine fibrils of the co nes. The anhydrous nature of the sanidine and quartz suggests that their cr ystallization concentrated a water-rich residual fluid, enriched in Ca, Mg, and Fe, and depleted in Si, K, and Na, at the megaspherulite-vitrophyre in terface. This fluid phase enclosed the megaspherulites resulting in the hyd rothermal alteration of the vitrophyre, forming a hydrothermal alteration l ayer, as indicated by the presence of the minerals mordenite and montmorill onite.