Mineralogy and petrology of an olivine diabase sill complex and associated unusually potassic granophyres, Sierra Ancha, central Arizona

Citation

Smith, Douglas (1969) Mineralogy and petrology of an olivine diabase sill complex and associated unusually potassic granophyres, Sierra Ancha, central Arizona. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/JRYS-CR97. https://resolver.caltech.edu/CaltechTHESIS:10072010-111049115

Abstract

The Precambrian Sierra Ancha sill complex, 700 to 800 feet thick, was intruded into flat-lying sedimentary rocks of the Apache Group in central Arizona. The bulk of the complex consists of a central layer of feldspathic olivine-rich diabase and upper and lower layers of olivine diabase. Diabasic rocks present in relatively minor quantity in the upper part of the complex include diabase pegmatite, albite diabase, and quartz diabase. Potassic granophyres locally form lenses up to two hundred feet thick near and at the roof of the complex. The intrusion was studied in the field and in the laboratory to determine the origins of the rock types and the conditions under which they formed. Extensive electron microprobe studies of mineral compositions and zoning are presented together with nineteen new whole rock chemical analyses. The Sierra Ancha olivine diabase has a high-alumina olivine basalt composition. Olivine diabase and olivine-rich diabase display a differentiation pattern characterized by moderate iron enrichment. Diabase pegmatite is relatively enriched in alkalis. The principle primary minerals in feldspathic olivine-rich diabase and olivine diabase include: plagioclase (An_(72)-An_(16)); augite (Wo_(43)En_(44)Fs_(13) to Wo_(40)En_(38)Fs_(22)); olivine (Fo_(74)-Fo_(54)); orthopyroxene (En_(77)-En_(44)); magnetite_ss (Mgt_(66)Usp_(34)-Mgt_(89)Usp_(11)); and ilmenite_(ss) (Ilm_(86)Hem_(14)-Ilm_(96)Hem_4). All of the orthopyroxene is primary. Fe- Mg fractionations between mafic mineral pairs increase with iron enrichment and declining crystallization temperatures. Ilmenite which formed by reaction-exsolution from magnetite was found to be consistently different in composition from primary ilmenite. The late-crystallizing diabase pegmatites contain an assemblage including iron-rich chlorite together with calcic pyroxene; from textural evidence the two phses appear primary. The calcic pyroxene has a compositional range from Wo_(49)En_(28)Fs_(23) to Wo_(49) En_(14)Fs_(37); its compositions define an iron-enriched trend in the pyroxene quadrilateral more calcic (i.e., closer to the diopsidehedenburgite join) than other iron-enriched igneous pyroxene trends described in the literature. Most diabasic rocks in the sill display some deuteric alteration. The mineral assemblage seemingly stable in the most-altered rocks includes albite (An_(2-0)), prehnite, calcic pyroxene (saute), chlorite, sphene, and apatite. Albite diabase contains this assemblage and apparently formed by recrystallization of normal diabase under deuteric conditions. The alteration assemblages are similar to those found in spilites. They provide an important example of the development of a spilitic assemblage by autometamorphism. The massive granophyres at and near the top of the sill appear to be igneous. The larger lenses occur at local high points in the roof of the complex near discordant contacts. The granophyres consist primarily of alkali feldspar with subordinate calcic pyroxene, iron-rich hornblende, biotite, and quartz and minor plagioclase and other phases. They have no relict sedimentary textural features, and they contain miarolitic cavities and rotated and displaced sedimentary rock inclusions. Locally, they occur as masses truncating overlying strata and as dikes in the overlying sedimentary rocks. Some of the dikes have apparent chilled contacts against the sedimentary rocks, suggesting that they were emplaced largely as melts. The granophyres formed as a result of the interaction of diabase magma with stratified rocks of the overlying Dripping Spring Quartzite. The massive granophyres are generally similar in composition to the overlying sedimentary rocks; both rock types have very unusual and distinctive high potassium contents. Contact metamorphism by the diabase has produced layered metasedimentary rocks with granophyric textures and mineral assemblages comparable to those in some massive granophyres. Consistent compositional differences between granophyres and sedimentary rocks may have been caused by metasomatic processes or by mixing of diabase magma with the sedimentary rock material which constitutes most of the granophyres. The interaction of diabase and sedimentary rocks may have occurred because magma in the upper part of the intrusion absorbed water from the overlying sedimentary rocks and solidified after magma in the central part of the intrusive. If this happened, the sedimentary rocks over the sill might have been melted to form the granophyres. No chilled facies of diabase occurs at the sill roof where granophyres are present. Compositional trends in mineral series indicate that the diabase magma in the upper part of the sill solidified towards the roof in at least one locality. Normal processes of magmatic differentiation produced feldspathic olivine-rich diabase, olivine diabase, and diabase pegmatite in the Sierra Ancha complex. The processes which produced the granophyres include recrystallization and fusion of rocks overlying the intrusion. The Sierra Ancha granophyres offer a superb opportunity to study these processes and others which may have produced many of the granitic rocks in the crust of the earth.

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