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NUMBER 19 19 80 60 40 20 20 40 60 80 Park, and Green Knobs shows again that most of the garnet in these kimberlites have a composition characteristic of garnets from garnet peridotite and were probably derived from disaggregated garnet peridotite xenoliths. The provenance of the eclogite xenoliths at Garnet Ridge presents an interesting problem. Since they are found in kimberlite pipes, they would fall in group A of Coleman, et al. (1965). Instead, as pointed out by Watson and Morton FIGURE 12.—Ca-Mg-Fe diagram. X garnets from Green Knobs A garnet inclusions in diamond FE (1969), the garnets in Garnet Ridge eclogites have a composition that places them in Coleman's group C, suggesting that the Garnet Ridge xenoliths came from a glaucophane schist facies in the basement rocks. Other factors, however, discussed in detail by Watson and Morton, lead them to conclude that the Garnet Ridge eclogite has come from the deep crust or upper mantle. O'Hara and Mercy (1966) concluded that the Garnet Ridge eclogite xenoliths are extremely sodic and not comparable to pre-
20 40 20 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES 60 40 20 40 60 80 FIGURE 13.—Ca-Mg-Fe diagram. (The analyses are given in Tables 3 and 8.) • eclogite xenoliths from Garnet Ridge O other garnetiferous rocks from Garnet Ridge • garnet peridotite xenoliths from South African kimberlite pipes viously described material from any source. The large garnet masses found at Garnet Ridge have a composition similar to garnet from the eclogite xenoliths, indicating that they came from the same source, perhaps from garnet cumulates. The Ca-Mg-Cr plot of Garnet Ridge xenocrysts (Figure 4) shows three analyses that lie on the border of the diamond inclusion field, but none within it. For Buell Park and Green Knobs there 80 60 20 are no analyses in or near the diamond inclusion field. It is not possible to draw any broad conclusions from these data for two reasons: (1) the number of xenocrysts analyzed from each locality is small, and (2) the true extent of the diamond inclusion field is not yet known because of the limited data available. From this study one could conclude that none of
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TABLE 1.—Chemical composition in
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