Miocene to Oligocene diatremes of the Colorado Plateau (southwestern USA) contain garnet xenocrysts derived from peridotite as well as inclusions of spinel and garnet peridotite. The inclusions and xenocrysts originated from depths of 60 km or less for spinel peridotites to 150 km for some garnet peridotites peridotites (Smith et al., 1991; Smith, 1995). The crust in the region is early Proterozoic in age and model ages from some spinel peridotite inclusions also suggest formation of the mantle lithosphere in the Proterozoic (Roden et al., 1990; Alilbert, 1994). Thus these inclusions are samples of Proterozoic lithosphere. Lithologies are variable but coarse, porphyroclastic and even mylonitized olivine-rich peridotites containing forsteritic (Fo88-92) olivine, enstatite, diopside and spinel or garnet with or without hydrous phases are common (Roden et al., 1990; Smith and Levy, 1976; Ehrenberg, 1982a). Published major element analyses (Smith and Levy, 1976; Ehrenberg, 1982a) are similar to spinel peridotites from alkali basalts; none have the relatively high SiO2 contents typical of some coarse kimberlite xenoliths (Boyd, 1989). Garnets have CaO contents indicating saturation with clinopyroxene (Smith et al., 1991; Ehrenberg, 1982a; Hunter and Smith, 1981).
We used the ion microprobe to characterize mineral chemistry of garnets and clinopyroxenes from these inclusions and xenocrysts.
Clinopyroxenes from spinel peridotites are predominantly LREE depleted and relatively rich in the HREE (Roden and Shimizu, 1993). Ce/Yb ratios vary widely and the clinopyroxenes with the lowest Ce/Yb have very low Sr (<10 ppm) and Ti contents (<1000 ppm) and are similar to clinoyroxenes from abyssal peridotites. These peridotite inclusions are residues from partial melting in which the process approached that of ideal fractional melting; moreover, it is likely that the melt extraction coincided with crust formation in the Proterozoic. Many spinel peridotite inclusons have been closed systems with respect to the REE since that time.
Most garnet xenocrysts from peridotite located at intermediate depths in the lithosphere have convex upward REE patterns with nearly flat profiles from Dy to Yb at 5-15 x chondrites. These patterns are similar to those of most garnets in garnet peridotites from The Thumb diatreme (Ehrenberg, 1982b) and suggest that the region sampled by the xenocrysts is composed mainly of garnet peridotite with flat to LREE depleted patterns. "Sinusoidal" REE patterns are present in several garnet xenocrysts including a garnet that contained inclusons of magnesite, chlorite and amphibole (Smith, 1987).
Garnets and clinopyroxenes from garnet peridotites derived from 120-150 km depth are compositionally diverse and some garnets are zoned as noted previously (Smith et al., 1991). Chondrite-normalized trace element patterns of garnets can be divided into two types: garnets rich in Zr and Ti and having convex-upward REE patterns similar to those of most of the garnet xenocrysts described above and garnets with low Ti contents and sinusoidal REE patterns. One sample records a transition from the latter pattern to the former: garnet is zoned with a core having a sinusoidal REE pattern and a rim having a convex-upward REE pattern. As for garnets from South African kimberlites (Hoal et al., 1994), garnets with relatively low FeO contents have the sinusoidal pattern; however these garnets appear to be relatively rare in the lithosphere beneath the Colorado Plateau.
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