Potassic Granophyre Associated with Precambrian Diabase, Sierra Ancha, Central Arizona

Lenses of granitic and syenitic granophyre unusually rich in potassium crop out at and near the roof of a Precambrian sill complex of olivine diabase intruded into the Apache Group. The lenses locally are 45 m thick; the diabase is 210 to 240 m thick in the same area. Granophyre contains 7 to 11 wt percent K₂O, whereas intruded rocks of the Dripping Spring Quartzite contain 9 to 14 wt percent K₂O. The sedimentary rocks were enriched in potassium before emplacement of diabase, probably during diagenesis while saturated with saline water.

The granophyre apparently was mostly liquid when emplaced. Chemical data, together with arguments based on mineralogy, textures, and field relations, indicate that most of the granophyre was derived from Dripping Spring Quartzite. The granophyre, however, contains more Na, Ca, and P and less K than the sedimentary rocks. Rocks representing a differentiated fraction of diabase magma are concentrated below the granophyre at the roof of the complex. The unusual crystallization profile may reflect an influx of water from the overlying sedimentary rocks into the magma. The influx of water may have been responsible for the extensive interaction between magma and country rock. Country rock fused during the interaction appears to have migrated to local structural highs.

Because the evidence that the granophyre formed largely from country-rock material is unusually clearcut, this granophyre may serve as a petrogenic model for some other silica-rich igneous rocks associated with intrusions of basaltic magma.