A statistical study of phenocryst orientation fabrics in a dacite ignimbrite

Abstract

Preferred orientation of phenocrysts is common in oriented thin sections of Bonanza Tuff, an early Oligocene composite ignimbrite sheet of dacite composition in the San Juan Mountains, Colorado. The phenocryst fabric is considered the result of primary laminar flow during the final stages of ignimbrite emplacement. A detailed statistical analysis of eight samples of Bonanza Tuff shows the progressive development of preferred orientation with increasing sample size and suggests that approximately 100 measurements of the long axes of phenocrysts are required to yield a statistically significant fabric if it exists. Approximately 100 measurements are also needed to calculate a reproducible vector mean regardless of the strength of the fabric. Accumulation of a much larger population of data strengthens the evidence of preferred orientation, as shown by Tukey Chi-square and probability tests, but does not significantly alter the resulting vector mean. Samples which yield statistically random fabrics with 100 measurements remain random despite accumulation of a large data base.