Petrologic Study of the Sierra Ancha Sill Complex, Arizona

The Sierra Ancha sill complex is a major high-alumina basaltic intrusive of late Precambrian age in the central Arizona dolerite province. It consists of three zones: lower dolerite, 200 ft; microtroctolite (fine-grained plagioclase-olivine rock with minor coarse-grained poikilitic augite), 235 ft; upper dolerite, 275 ft. Dolerite zones crystallized from high-alumina basaltic magma and are taken to represent parent magma; chilled contacts have been deuterically altered, especially for potassium. The parent magma is similar to late Precambrian high-alumina basaltic magmas that crystallized anorthositic rocks and differs from high-alumina basalts from orogenic regions or major layered intrusions. Sierra Ancha rocks probably correlate with late Precambrian dolerite-sill areas in Grand Canyon, Arizona, Panamint Range, California, and other areas in the southwestern United States, and indicate a large high-alumina basaltic petrologic province. The three zones constitute separate and concordant sills. Microtroctolite is hypothesized to have formed at depth in the magma reservoir by undercooling, rapid nucleation, secondary enlargement, and diffusional interaction of interstitial and overlying magma; this material was intruded as a crystal mush. An alternative hypothesis is that liquid of troctolitic composition developed in the lower portion of the reservoir by liquid fractionation; it was then emplaced and crystallized after under-cooling. The upper dolerite sill differentiated upward from high-alumina basalt to lower alumina basalt by a process of crystallization and diffusion with overlying magma as in the process outlined by Hess (1960). Andesine anorthosite with excellent flow texture, locally found in the lower dolerite, probably formed as a crystal cumulate in the upper part of the reservoir by flotation. Dolerite pegmatite contains deuteric albite as its plagioclase; the deuteric process is analogous to spilitization.