LATE CRETACEOUS TECTONIC DISPLACEMENT OF SUB-CONTINENTAL MANTLE LITHOSPHERE BENEATH THE SW U.S. CORDILLERA: MANTLE XENOLITH CONSTRAINTS FROM THE COLORADO PLATEAU TRANSITION ZONE (CENTRAL ARIZONA)

Mantle xenolith and seismic studies provide evidence for removal of sub-continental mantle lithosphere (SCML) from beneath the Southern California Batholith (SCB), likely during a well-documented Late Cretaceous shallow subduction event. Petrologic studies of mantle xenoliths hosted further east in the Colorado Plateau transition zone (central Arizona) suggest that these nodules may be remnants of the missing SCML of the SCB. Understanding the processes that control the removal and reconstruction of the SCML is integral for studying the physical properties of the Earth. By studying these mantle xenoliths, we endeavor to answer two questions: 1) Do the xenoliths erupted from beneath the Colorado Plateau transition zone represent displaced SCML removed from the SCB? and 2) If so, what are the processes controlling the removal and reconstruction of the SCML? To address these questions, we studied mantle xenoliths from Chino Valley and Camp Creek localities in central Arizona. The xenoliths occur as inclusions within ca. 25 Ma latite and include garnet-pyroxene-amphibole rocks, garnet granulites, supracrustal rocks, and subordinate peridotites. Petrographic work and geochemical screening revealed accessory zircon in garnet clinopyroxenite and garnet amphibolite nodules from Camp Creek and Chino Valley. Zircon from garnet clinopyroxenite xenoliths yield a spread of concordant Late Cretaceous U-Pb ages ranging from ca. 95 to 65 Ma. Garnet amphibolite xenoliths exhibit ages along concordia from ca. 170 to 70 Ma, perhaps corresponding to well-documented Jurassic and Late Cretaceous pulses of magmatism in the Sierra Nevada batholith. The age dispersion in each sample likely has geological relevance, and may record protracted development of the sub-SCB root over a ~100 Myr interval. Titanite extracted from one garnet clinopyroxenite xenolith yields a lower intercept U-Pb age of 47 ± 13 Ma. This age is tentatively interpreted to constrain the timing of cooling through ~650-700 ̊C following displacement of sub-SCB SCML and also suggests that U-Pb systematics in this sample were not significantly disturbed by ca. 25 Ma latite emplacement. Overall, the zircon and preliminary titanite data match the thermal history of the SCB, suggesting that the studied xenoliths represent displaced SCML of Southern California.