Previous research on the rocks of the CASC can be divided into two major categories, the first having to do with geochronology, and the second having to do with mineralogy, composition, and petrogenesis.
To date, geochronological work on the CASC has been done either on a single sill, the Sierra Ancha Sill Complex of Nehru (1961) and Nehru and Prinz (1970), or cannot presently be related to any particular suite of rocks. As will be shown below, there are at least three distinct suites in the CASC identifiable on geochemical grounds, but only the age of this particular sill has been established with certainty. By various means, Silver (1960, 1963) arrived at a minimum age for this sill of 1.075 Ga with an error of plus or minus 0.05 Ga, and thought that the actual age is 1.15 Ga. Other workers have made contributions as well: Damon et al. (1962) report a K-Ar age of 1.140 Ga for the diabase, and Banks et al. (1972) report 1.040 Ga plus or minus 0.03 Ga; but neither of these workers indicate the geochemistry of the rocks for which they obtained dates.
Prior to this radiometric analysis, there had been a continuing controversy regarding the age of the sills based on the interpretation of field relations. A group including Ransome (1903, 1904a, 1904b, 1912, 1919, 1923), Lindgren (1905), Peterson et al. (1951), and Peterson (1954, 1962), thought they were Mesozoic in age, while another, including Darton (1925), Short et al. (1943), Silver (1960, 1963), Shride (1967), Peterson (1969), Granger and Raup (1956, 1969), and Cornwall and Krieger (1978), held for a Precambrian age. The former group's claims can commonly be explained as resulting from misidentification of "the diabase," or to an incorrect interpretation of structural relations. For example: There is a great deal of impure limestone in the Mesozoic stratigraphic column into which the diabase is believed to have been intruded. That there were active groundwater convection regimes established around the intrusions is indicated by ubiquitous, though variable, alteration in the diabase itself, and is suggested by the existence of uranium orebodies associated with carbon-bearing host rocks immediately adjacent to some of the sills. Under such conditions, diabasic magma injected into a carbonate host would be expected to engender very significant contact metamorphic effects in that host, yet such effects are absent in these rocks save for a tendency (emphasis mine) towards a little coarser crystalline texture which extends a few inches from the contact (Ransome, 1903). It is very unlikely that this could describe an intrusive contact involving massive diabase and limestone.
In carrying out the field work necessary for this research, the author found two instances, one at Chrysotile and the other in the area west of Young (Figure 2), in which selveges, one or two centimeters in thickness, of fine-grained basalt apparently had been intruded along the contact between the diabase and the host rocks. On superficial inspection, these appeared to be chilled margins of the diabase, but careful examination and geochemical analysis established that this was not the case, and that the chilled nature of these selvages resulted from their being emplaced while the adjacent sill and country rock were relatively cool. It is possible that such occurrences could explain some of the interpretations of a Mesozoic age for the diabase. The small volume of magma involved would not have constituted anything like the reservoir of heat contained by one of the sills, and contact metamorphic effects associated with it might be expected to be small to negligible.
Not all the claims for a "young" diabase can be dismissed, however. Creasey (1965), mapping in the San Manuel area, found diabase of three distinct ages. Two were similar in age, he thought Precambrian, with the third younger, he thought perhaps Cretaceous. From his map the interpretation appears sound, so in what follows it must be remembered that more than one episode of diabasic intrusion may have occurred in central Arizona, perhaps widely separated in time. It is clear that some at least, if not most, of the CASC igneous province is approximately 1.1 Ga in age, but it is not necessarily true that all of it is.
Other research conducted on the CASC includes that of Neuerburg and Granger (1956, 1960), who described the diabase and documented its association with uranium deposits, as noted above, though the diabase was certainly not the ore-bringer as they suggest. Descriptions of the mineralogy of the diabase are to be found in Ransome (1903), Peterson et al. (1951), Nehru and Prinz (1970), Smith and Silver (1975), Smith (1969, 1970), and Krieger (1974). A general description from Granger and Raup (1956), which is representative of these, is: 45-70% plagioclase, 20-30% augite, with as much as 20% biotite and 20% olivine. Accessory ilmenite-magnetite, apatite needles, and zircon comprise less than 10% of the rock. The plagioclase is generally labradoritic, zoned, with cores of An75 and rims of An35. Alteration, thought to be deuteric, is common, with plagioclase altered to sericite, pyroxene altered to aggregates of fibrous hornblende or chlorite, illite and other amphiboles, and olivine altered to magnetite and fine-grained aggregates of "epidote- and serpentine-like minerals." Granger and Raup (1956) also describe "pegmatoid" and "aplitic" phases of the diabase.
Relations with the host rocks are detailed by Granger and Raup (1956), and Short et al. (1943) indicate that many sills in fact represent multiple intrusions. Shride (1967) provides a careful and detailed description of the younger Precambrian of Arizona, including excellent descriptions of the mineralogy and habit of the diabase, the stratigraphic column in which it occurrs, and the structure of the region. The investigator wishing to obtain a comprehensive and accurate overview of the diabase, its host rocks, and their relations, is referred particularly to this work. Nehru (1961), Nehru and Prinz (1970), Smith (1969, 1970), and Smith and Silver (1975) have studied in considerable detail that part of the CASC which the former pair of authors name the Sierra Ancha Sill Complex. Nehru (1961) and Nehru and Prinz (1970) provide modal and petrological analyses of this sill at Pocket Creek, and Smith (1969, 1970) does similar work in the nearby Reynolds Creek area. Smith and Silver (1975) examine the granophyre associated with the diabase at Reynolds Creek; it is considered to be an anatectic melt of the host Dripping Spring Quartzite, and will not be considered further.
Fouts (1974) studied four individual sills, located in the Roosevelt dam area, in the vicinity of Winkleman, in the Santa Catalina Mountains, and in the Salt River Canyon; all are Arizona localities. His is the earliest, and until this the only, primarily geochemical approach to the study of these rocks, dealing for the most part with their major element chemistry. Some of his findings figure prominently in this research and are therefore worthwhile to summarize here: First, the sills in the Roosevelt dam area and in the Salt River Canyon are the result of multiple injection of magma, while the others are not. Second, there are two distinct suites of rocks, one having high silica and one having low, the former located generally south of the Globe-Miami area and the latter to the north of it (Figure 2). Low-silica content he correlated with elevated alumina, and vice versa, and found that the two suites are distinct petrographically. Fouts (1974) believed that the low-silica suite differentiated at a depth of approximately 30 km, while the high-silica suite did so at much shallower depths and was derived from the same parental liquid as, but not directly from, the low-silica magma. These findings are explicitly reviewed in the Conclusions of this work.
Hammond (1983, 1984, 1986) has studied rocks in the Death Valley region of California which are very similar in age, habit, petrology, and geochemistry to those of the CASC. Burke and Dewey (1973) document the widespread occurrence of Belt-age sediments in western North America in a series of troughs oriented generally east-west, which they interpret as the failed arms of a series of RRR triple junctions. The north-south trending arms appear to have connected as the proto-Pacific basin opened at about 1.2 Ga. Burke and Dewey (1973) indicate that basaltic sills and dikes of ages similar to those under study here are found in many occurrences of these Belt-age rocks.
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