TECTONIC TRAUMA AND HEALING OF SUB-CONTINENTAL MANTLE LITHOSPHERE BENEATH THE SW NORTH AMERICAN CORDILLERA: INSIGHTS FROM U-PB ZIRCON AND SM-ND GARNET AGES FROM COLORADO PLATEAU TRANSITION ZONE XENOLITHS (Invited Presentation)

Xenolith and seismic studies provide evidence for removal of lower crust (LC) and sub-continental mantle lithosphere (SCML) from beneath the southern California batholith (SCB), likely during Laramide shallow-angle subduction. Xenoliths located further east in the Colorado Plateau Transition Zone may represent fragments of the displaced materials. If so, Colorado Plateau Transition Zone xenoliths provide an opportunity to study the processes by which LC-SCML may heal following removal. If Colorado Plateau Transition Zone LC-SCML originated from southern California, we expect xenoliths to yield U-Pb zircon ages of 100 ± 20 Ma and garnet Sm-Nd ages of 90 ± 20 Ma. Alternatively, these nodules may represent a previously undocumented autochthonous mafic root beneath central Arizona that formed during Laramide construction of, and magmatism within, the Nevadaplano. In this case, we expect the xenoliths to yield U-Pb zircon ages of 60 ± 20 Ma and garnet Sm-Nd ages of 50 ± 20 Ma. To distinguish between these end-members, we studied garnet-pyroxene-amphibole xenoliths extracted from ca. 25 Ma latite host rocks from Chino Valley and Camp Creek localities in central Arizona. Both garnet-whole rock Sm-Nd and U-Pb zircon geochronology yield a spread of ages. Five samples record Sm-Nd ages ranging from 40 to 33 Ma, suggesting partial re-equilibration with the host latite during eruption. One sample yields an Sm-Nd age of ca. 165 Ma as well as an initial εNd value of +8, suggesting derivation from depleted mantle associated with Jurassic magmatism in the SW U.S. The remaining samples yield Sm-Nd ages ranging from ca. 60 to 47 Ma. These ages, combined with zircon U-Pb ages ranging from ca. 170 to 50 Ma, indicate that LC-SCML beneath the SCB were at or above U-Pb zircon and Sm-Nd garnet-whole rock closure temperatures for a protracted interval (several 10s of Ma). The thermal histories of the studied xenoliths partially overlap those of both the SCB and the Nevadaplano, suggesting that these xenoliths may represent LC-SCML displaced from the SCB and reaffixed inboard to become sub-Nevadaplano root materials. Partial Oligocene re-equilibration of xenoliths occurred during the initial stages of Nevadaplano collapse and core complex formation.