UNDERCOOLING IN DEEP PEGMATITES: AN EXAMPLE FROM SPRUCE PINE, NORTH CAROLINA

Mining of various minerals, including mica, feldspar and ultra high purity quartz, from the granitoid rocks of the Pine Plutonic Suite (SPPS) started with Native Americans and continues today. A long bibliography of papers, including work by Jahns during WWII, documents the field relations of the 100's of individual intrusions in the Spruce Pine Mining District. The SPPS is composed of small plutons, dikes and sills of peraluminous granodiorite in the Blue Ridge of western NC. Amphibolite-grade schists and gneisses host the SPPS. Contacts between the graniotid and host rocks lack evidence of contact metamorphism and are generally sharp and concordant. Jahns collected bulk samples of the SPPS and used these samples in various experimental studies with Burnham. The same Spruce Pine samples were used in various experimental studies by graduate students at Penn State and Stanford.

The SPPS granodiorites contain very coarse-grained phenocrysts of Ksp and plag in a matrix of coarse-grained quartz, feldspar, muscovite, and garnet. Gradual coarsening of graniotid texture produces local pods of pegmatite with the same mineralogy as the host granodiorite. Other pegmatites form dikes within the granodiorite. About half the pegmatites show a simple zoning from a coarse-grained outer shell to a core of quartz and Kspar. Skeletal crystals, graphic granite and unidirectional solidification textures occur in the pegmatites indicating some undercooling during crystallization. Late stage fracture-filling and very fine-grained (aplitic textures) accompanied the loss of fluid from the crystallizing pegmatites.

Combining the results of experimental studies and geobarometry results in an estimate of the crystallization pressure of the SPPS of 5-10 GPA. Retention of water during crystallization of the Spruce Pine magmas increased the fluid component in the magma resulting in a reduction in crystal nucleation and growth rates and resulting feldspar phenocrysts. Local fluid enrichment produced a delay in crystal nucleation. When crystals did nucleate, it was often from an undercooled magma resulting in skeletal or unidirectional crystal growth. Crystallization from undercooled melts is an important process even in deep-seated pegmatitic systems.