ABSTRACT

Microprobe analysis of detrital alkali feldspar and plagioclase is useful in provenance studies. Detrital grains of alkali feldspar more sodic than 50 weight percent albite molecule are derived almost exclusively from volcanic sources. Alkali feldspar grains more potassic than 87 percent orthoclase molecule are derived from metamorphic and plutonic rocks. For the more sodic varieties of detrital plagioclase, it is possible to distinguish on the basis of potassium content between volcanic and non-volcanic sources.

Non-volcanic albite and volcanic and non-volcanic oligoclase and andesine are abundant forms of detrital plagioclase in Tertiary sandstones from the southern Colorado Plateau area. Labradorite and bytownite are present as detrital grains in some deposits from this region. Most of the detrital alkali feldspar in these rocks is highly potassic, containing more than 90 percent Or molecule. Intermediate alkali feldspar is common in sandstones of volcanic and mixed provenance.

Detrital plagioclase from several sandstones of mixed provenance contains between 0 to 0.7 percent Fe and 0 to 0.5 percent Sr. Alkali feldspar from the same samples contains between 0 and 0.5 percent Fe and 0 and 1.5 percent Ba. Detrital plagioclase interpreted as volcanic in origin generally contains more Fe and Sr than non-volcanic plagioclase.

Other approaches to the study of detrital feldspar with the microprobe are useful in provenance analysis. Chemical zoning of plagioclase, established previously as a provenance tool, can be studied more accurately and in more detail with a microprobe than with a petrographic microscope. Also, sand-sized volcanic lithic fragments can be classified as mafic, intermediate, or silicic in composition by analyzing their feldspar microphenocrysts.