The Seven Springs Formation of the Barrilla Mountains, western Texas, consists of 4 tuff units, 3 ignimbrite units, and 1 basalt unit. The ignimbrites consist of anorthoclase (Or35–40) phenocrysts in a matrix of glass, alkali feldspar, and quartz. The percentage of anorthoclase phenocrysts, the degree of welding of the matrix, and the grain size of the matrix all increase from the oldest to the youngest ignimbrites. The ignimbrites can be genetically related to the underlying Star Mountain Rhyolite flow in terms of sequential eruption of all units from a single magma chamber in which anorthoclase phenocrysts were crystallizing, settling, and being partially resorbed. All ignimbrites underwent posteruptive loss of Na2O.

First Page Preview

First page PDF preview
You do not currently have access to this article.