It has long been apparent that the major-, minor-, trace-, and rare-earth-element geochemistry of igneous rocks may be related to the tectonic environment in which these rocks form. Numerous studies have attempted to establish such correlations and to use them to determine the tectonic environment of suites of ancient rocks, the setting of which has been obscured by subsequent geological events (e.g. Pearce and Cann, 1973; Pearce and Norry, 1979; Thompson et al. 1982; Thompson et al. 1983; Pearce et al. 1984; Hammond, 1986; Francis, 1987). A primary purpose of the present study is to apply these techniques to a suite of rocks collected from a series of diabase sills which crop out in east-central Arizona between the Mogollon Rim and a point approximately 50 km southeast of Tucson, Arizona (Figure 1; note that the Mogollon Rim in the area of interest is represented by the boundary between the Plateau and Mountain Regions as located on the map.). Constraints provided by the geochemistry will be used to characterize the source regions and crystallization history of the diabase.
In order to accomplish this task, it will be necessary to describe the geochemistry of these rocks in detail as well as to evaluate the usefulness and reliability of their chilled border facies as indicators of liquid compositions. In the absence of any generally accepted name for the widespread intrusive series of east-central Arizona, and in the interest of brevity, the name "Central Arizona Sill Complex" (CASC) is proposed for it and will be used henceforth in this work. The name is derived from the fact that the majority of the outcrop area is located between the twin cities of Globe and Miami to the south, and the Mogollon Rim to the north (Figure 2). The previously named Sierra Ancha Sill Complex of Nehru (1961) and Nehru and Prinz (1970) constitutes a single complex sill within the larger Central Arizona Sill Complex, with which it should not be confused. That portion of the sample collection which represents the Central Arizona Sill Complex was obtained by the author from sills visited throughout the area of outcrop.
As a final note, all of the previous workers in the region used the English system of measures (fps) to describe distances, elevations, thicknesses of units, and so forth. To avoid confusion, and to facilitate the efforts of those who may wish to consult original sources, I have used this system whenever such data from previous work are under discussion or when similar data collected in the course of this project are presented for comparison. In all cases except in tables, where it would be unreasonably cumbersome, and elevations taken from topographic maps, the metric (cgs) equivalent measure is given in parenthesis immediately following the fps value.
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