CRYSTAL SIZE DISTRIUBTION ANALYSIS OF A QUARTZ SYENITE SILL IN THE FRANKLIN MOUNTAINS, TEXAS

Crystal size distribution (CSD) analysis of a syenitic to granitic sill in the Franklin Mountains, Texas can be used to understand the nucleation and growth rates. The Grenville age sill intrudes the Castner Marble and Lanoria Quartzite. Field observations and chemical data show an abrupt change 15 to 20ft from the contact with an alkali feldspar granite margin and a quartz syenite interior. This change shows a decrease from 74.74 to 64.56% in SiO2 concentration and an increase in Al2O3, Fe2O3, TiO2, and P2O5. Trace elements show increases in Ba, V, Zr, Sc, Sr and decreases in Rb and Be. Samples were collected at 10ft intervals from the contact to 100ft and then 50ft to a thickness of 800ft into the interior of the sill.

The contact samples consist of quartz (19.7%) and alkali feldspar (24.4%) phenocrysts that constitute 44.1% of the rock. The groundmass is made of quartz, feldspar, and devitrified glass. The alkali feldspar phenocrysts are 2-16mm in diameter subhedral to euhedral and elongated. The quartz phenocrysts reach 2mm in diameter and subhedral to euhedral. The center of the sill consists of alkali feldspar (61.4%) and quartz (6.2%) phenocrysts set in a groundmass of quartz, feldspar, zircon, and mafic minerals. The alkali feldspar phenocrysts are 30mm in diameter and anhedral and the quartz phenocrysts reach 1mm in diameter and anhedral.

Crystal size distributions are plots of the natural logarithm of the population density (Ln(n)) versus crystal size (L) where the population density is the number of crystals per size per unit volume. Phenocrysts of alkali feldspar in slab and thin section images were outlined, exported as black and white images, and converted into binary images. Measurements of the length, width, and area of each crystal were determined and then analyzed using the CSDcorrection program (Higgins, 2000). Crystal size distributions are linear for large crystals possible indicating a continuous nucleation and growth rate. The small crystal size population patterns are concave down, suggesting possible change in the nucleation and growth rate, texture coarsening and/or assimilation. The marginal crystal size distributions produce a steep slope which decreases in plots toward the center of the sill producing a fanning relationship.