XENOLITHS FROM HIGH VELOCITY LOWER CRUST, MONTANA: CONSTRAINTS ON TIMING AND MECHANISMS OF HIGH-VELOCITY LOWER CRUST FORMATION

Over the last two decades, the integration of seismic, geochemical and geochronologic data has allowed the development of models for the formation, evolution and present day structure of the continental lithosphere. Using combination of thermobarometry and thermochronology on lower crustal xenoliths, constructed T-P-t paths allow us to constrain the evolution of the deep lithosphere. In the Rocky Mountain region of the western US, seismic experiments reveal an anomalously thick (up to 25 km) and high seismic velocity (Vp >7.0 km/s) lower crustal layer, called the 7.x layer.

We present mineralogical and geochemical analyses, geochronology, and bulk seismic velocity calculations for xenoliths from within and above the 7.x layer, providing an opportunity to investigate modes of high velocity lower crust formation. The xenoliths were exhumed from the Great Falls Tectonic Zone by Eocene minettes at Robinson Ranch and Little Sand Creek, Montana. The suite includes mafic garnet granulites (Grt+Cpx+Pl±Qz±Kfs±Amph±Opx±Bt±Spl±Ilm±Rt±Ttn), mafic eclogite (Grt+Omph+Amph+Kfs+Bt+Ilm+Rt+Ttn), and felsic granulites (Grt+Pl+Kfs+Qz+Bt±Ilm±Rt). Peak pressures of 0.8-1.5+ GPa indicate derivation from depths of 28-55 km. Multiple samples preserve evidence for prograde burial and some xenoliths are polymetamorphic. For example, in one mafic granulite, a low-P, amphibolite-facies event is preserved by zincian spinel in garnet cores. An eclogite sample contains early plagioclase in omphacite and retrograde decompression reaction textures. Both samples record prograde burial interpreted as thickening during collision on the Great Falls Tectonic Zone. Subsequent eclogite decompression could represent post-orogenic collapse. Samples from within the seismically defined 7.x layer have calculated bulk P-wave velocities of 6.5 to 7.5 km/s, suggesting heterogeneity in the layer. Monazite total U-Th-Pb and zircon, titanite, and rutile U-Pb data indicate metamorphism at ca 2.1 Ga and 1.8 Ga. Additionally, a period of monazite growth occurred at ca 1.3 Ga. We propose a model for incremental assembly of the 7.x layer in this region that includes magmatic underplating/intraplating and eclogitization associated with multiple tectonic events from the late Archean to the Paleoproterozoic.