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Lower crustal and upper mantle xenoliths along the Cheyenne belt and vicinity

Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309, U.S.A.

G. Lang Farmer

Cooperative Institute for Research in the Environmental Sciences (CIRES) and Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309, U.S.A.

Lower-crustal and upper-mantle xenoliths entrained in Phanerozoic igneous rocks in the northern Southern Rocky Mountains provide the only direct means of studying the petrologic and geochemical nature of the deep-continental lithosphere in this region. Petrographic and geochemical features of peridotite xenoliths from the State Line kimberlite district, near the Cheyenne belt, suggest that during Devonian kimberlite emplacement, infertile, cryptically metasomatized, Archean mantle dipped south beneath a distinctly more fertile, and veined, lithospheric mantle associated with the Proterozoic continental crust of the Colorado province. Lower-crustal xenoliths recovered from the State Line kimberlites and late Cenozoic ultrapotassic igneous rocks at Leucite Hills (north of the Cheyenne belt) are dominated by mafic granulites. This indicates that both the Archean and Proterozoic lower-continental crust was, and may remain today, mafic in composition.

Existing major-element data support a tholeiitic basalt precursor for the mafic granulites associated with the Colorado province. Unfortunately, despite its importance for constraining models of lithospheric structure in the northern Rocky Mountains, detailed information regarding the age, trace element and isotopic compositions, or physical attributes of the mafic xenoliths is presently not available. In order to evaluate the age and composition of continental lithosphere in this region and to assess the possible role of mafic lower-crustal rock types as sources for Laramide magmatism we outline a plan (as part of the Rocky Mountain Continental Dynamics project) to obtain geochemical and geochronological information from xenoliths recovered at the State Line kimberlite district and Leucite Hills.

Key Words: continental lithosphere • xenoliths • lower crust • granulites • kimberlite • Cheyenne belt

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