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Cretaceous diamond-bearing olivine lamproite occurrences of southwestern Arkansas straddle the geologic and physiographic boundary between the Gulf Coastal Plain and the Ouachita Mountains (Fig. 1). The Gulf Coastal Plain here is characterized by gently south-dipping Cretaceous sedimentary rocks that onlap and lie unconformably on intensely folded and faulted, east-west-trending, Paleozoic sedimentary rocks of the Ouachita Mountains. Rocks of this region are widely believed to lie near the southern margin of the midcontinent craton. XENOLITH PETROLOGYApproximately 2 kg of crustal xenoliths, 1-3 cm in size, were recovered from heavy media concentrates with a density of >2.82 g/cm 3 . Representative xenoliths of each group were selected for thin section analysis based on their abundance and lack of visible alteration.Approximately half of the crustal xenoliths are composed of nearsurface sedimentary rocks, which include conglomerate, sandstone, and chert. These rocks are similar in appearance to Cretaceous and Paleozoic Ouachita facies sedimentary rocks, which are known to exist in the area. Two pink oolitic dolomite xenoliths with pellets and intraclasts are anomalous with respect to local lithologies.Crystalline xenoliths are ~50% amphibolite, consisting dominantly of amphibole and lesser amounts of plagioclase with accessory biotite, chlorite, microcline, quartz, titanite, ilmenite, and apatite. The amphibolite specifi c gravity as measured in water ranges from 3.01 to 3.16, with an average value of ~3.08. Felsic composition xenoliths, consisting of granitic and rhyolitic rocks, are relatively rare, making up only ~5% of the deeper crystalline xenoliths. The average specifi c gravity of the felsic xenoliths is ~2.90. This high density is attributed to an abundance of iron oxides and epidote alteration, making them more susceptible to heavy media concentration.Approximately 40% of the recovered deep crystalline xenoliths are classifi ed as epidote-rich rocks. The specifi c gravity of epidote-rich rocks is variable, but it typically ranges from 3.1 to 3.2. Some fi ne-grained felsic xenoliths have been almost completely replaced by epidote. Epidote alteration of amphibolite xenoliths is observed but is less common. Relatively rare (~5%) dark-colored xenoliths consist almost exclusively of coarse-grained (~7 mm), weakly aligned micas defi ning a foliation. Specifi c gravities of the micaceous xenoliths average ~3.10. Selected crustal xenolith data are presented in Table 1.

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