CONSTRAINTS ON THE NATURE OF THE LOWER CRUST ACROSS THE CHEYENNE BELT: PETROLOGIC EVIDENCE FROM LOWER CRUSTAL XENOLITHS

Xenoliths from the Paleozoic State Line Kimberlite District, Yavapai Province, and from the Tertiary lavas of Leucite Hills, Wyoming Craton, allow direct study and comparison of lower crust across the Proterozoic Cheyenne Belt suture, southern Wyoming. State Line xenoliths include mafic granulites with the assemblage plg + cpx ± opx ± grt ± hbl. Many display strongly annealed textures; others contain garnet coronae on orthopyroxene, ilmenite, and plagioclase, and also garnet foam or lamellae along cleavage planes in plagioclase. Mineral textures and compositions suggest the reaction opx + Ca-plg ± cpx=grt + cpx + Na-plg. Evidence for late garnet growth and diminishing diffusion suggest this reaction was crossed during (isobaric?) cooling. P-T estimates (grt-cpx-opx-plg) are 1.1-1.4 GPa and 625-750C. U-Pb zircon dates are Paleo/Mesoproterozoic, suggesting that State Line xenoliths preserve Proterozoic lower crustal metamorphic conditions and textures.

Leucite Hills xenoliths include mafic granulites with the assemblage plg ± amph + opx + cpx ± bt. These granulites differ from State Line xenoliths in their lack of garnet, abundance of hydrous minerals (amphibole, biotite), and comparatively strong deformational fabric. Preliminary P-T estimates (amph-px-plg) are 1.0-1.4 GPa and 800-1050C, similar to State Line estimates. U-Pb zircon dates are Archean, but microprobe monazite analyses suggest Mesoproterozoic metamorphism.

Reconstruction of crustal thicknesses before Paleozoic exhumation and during the time of sampling suggests that State Line and Leucite Hills xenoliths represent Precambrian lower crust. If these samples are typical of their respective crusts, the lower crust of the Wyoming Craton was (is?) more hydrous and anisotropic than, and lacks the garnet of, the lower crust of the Yavapai Province. These petrologic differences may characterize the high (south) to low (north) velocity transition imaged across the suture. Crustal thickness reconstructions also allow that State Line xenoliths may have sampled a roughly 10 kilometer-thick high-velocity layer, which may represent a Precambrian mafic underplate, at the base of the Yavapai crust.