PRESSURE-TEMPERATURE EVOLUTION OF LOWER CRUSTAL-UPPER MANTLE XENOLITHS FROM THE COLORADO PLATEAU TRANSITION ZONE

The Late Cretaceous-early Paleogene Laramide orogeny is widely regarded to have resulted from shallow-angle subduction of the Farallon plate beneath North America. This event is hypothesized to have led to "delamination" of the sub-continental mantle lithosphere (SCML) from under the southern California batholith and "relamination" of this SCML fragment ~500 km to the east, underneath the Colorado Plateau transition zone. Material detached from beneath the hot batholith and displaced below the colder craton interior at the same structural level should record isobaric cooling. In light of this prediction, we test the above hypothesis by constraining the P-T-t evolution of a suite of eclogitic xenoliths collected from the Colorado Plateau transition zone (and inferred to represent displaced SCML).

All studied xenoliths consist of almandine garnet + diopsidic clinopyroxene + ilmenite + rutile + clinoamphibole (secondary phase). Pressure-Temperature psuedosections, constructed via thermodynamic modeling, suggest that the mineral assemblage is stable over a wide range of pressures (from 12 to to 80 kbar). Peak P-T conditions of the xenoliths are more tightly constrained to 650°C - 850°C and 15 to 30 kbar based on Ti-in-zircon and Zr-in-rutile thermometry and existing garnet-opx-cpx thermobarometry for rocks from the same region. X-Ray maps of garnet grains reveal a monotonic decrease in Mg# (Mg / (Mg + Fe)) from core to rim, which, along with garnet isopleth modeling, suggest garnet growth during cooling. Zircon U-Pb geochronology and trace element geochemistry suggests that these rocks formed under eclogite facies conditions over more than 100 Myr, from ~170 Ma to ~60 Ma. Also, the presence of amphibole replacing clinopyroxene indicates a late hydration (addition of ~2.0 wt. % H₂O) of these rocks. The results of this study are consistent with the hypothesized relamination of the SCML fragment beneath the Colorado Plateau. We suggest that the texturally late partial replacement of clinopyroxene by amphibole reflects hydration of studied xenolith suites during shallow subduction of Laramide slab.