LOWER CRUSTAL AND MANTLE XENOLITHS OF NAVAJO DIATREMES – WITNESSES OF ACCRETIONARY TECTONICS IN THE COLORADO PLATEAU REGION

While lower crustal xenoliths from serpentinized ultramafic microbreccia diatremes in the Four Corners area of the Colorado Plateau (CP) have been used to infer details of the tectonic accretion between the Proterozoic Yavapai and Mazatzal provinces in the CP area, mantle-derived lawsonite- and coesite-bearing eclogites have been interpreted as fragments of the Mesozoic Farallon plate subducted under, and accreted to, the Proterozoic subcontinental lithosphere beneath the CP during the Laramide orogeny. Although this model has been disputed on the grounds that certain isotopic signatures suggest Protorezoic protolith ages for the eclogites, a strong case can now be made that not only the ages of metamorphism for the progressively metamorphosed eclogites are young (ca. 80 Ma to 28 Ma, as seen from zircon and new monazite dates), but that protolith ages may also be young and compatible with the Laramide subduction model. In addition to the metabasic eclogites of the Farallon slab, the Navajo diatreme xenolith suite includes pyrope-bearing peridotites that were hydrated by slab-derived fluids in both the chlorite-eclogite and chlorite-pargasite facies, suggesting that at ca. 30 Ma a Proterozoic mantle wedge under the Four Corners extended from the Moho down to at least 75 km. The closest surface evidence for Laramide low-angle northeastward subduction toward the CP are Orocopia Schists at Cemetery Ridge, in Yuma County of southwestern Arizona, considered correlative with parts of the Franciscan subduction complex (Haxel et al., 2014). Metamorphosed at ca. 8 to 10 kb, they are exposed below Proterozoic crustal rocks in structural windows of the lower plate of the Orocopia Mountains detachment fault (Jacobson et al., 2007). This implies that a subcontinental mantle wedge was tectonically eroded and that the hanging wall of the Laramide subduction system dipped from a depth of about 30 km at Cemetery Ridge to about 75 to 80 km under the Four Corners area of the CP, ca. 600 km to the NE.