DEEP CRUSTAL XENOLITHS FROM THE GREAT FALLS TECTONIC ZONE, MONTANA: INVESTIGATING THE TIMING AND MECHANISMS OF HIGH-VELOCITY LOWER CRUST FORMATION

The Rocky Mountain region of the western U.S. preserves a rich and protracted history of continental assembly, crustal growth and lithospheric stabilization. The role of lower crust is central to this evolution, yet it is perhaps the part of the lithospheric column for which we know the least. One of the most important questions concerns the nature and age of an anomalously thick (20-25 km) and high seismic velocity (Vp >7.0 km/s) lower crustal layer in Montana and Wyoming. Xenoliths represent a window into the composition, age, and structure of the deep crust, providing a link between surface exposures and geophysical observations. We use xenoliths from the ca 1.8 Ga Great Falls Tectonic Zone and the adjacent Medicine Hat and Wyoming cratons to investigate the timing and mechanisms of high velocity lower crustal growth. Calculated peak metamorphic pressures (0.8-1.3 GPa) and bulk P-wave velocities, ranging from 6.3 to 7.3 km/s, are consistent with sample derivation from within and just above the 7.x layer.

A particularly useful suite of middle to lower crustal xenoliths was exhumed by Eocene minettes at Robinson Ranch, located within the Great Falls Tectonic Zone. The suite contains a range of mafic to felsic garnet granulites with combinations of the assemblage grt+cpx+pl+kfs+bt+sp+qtz+ilm+rt+ttn. Textural and mineralogical data indicate that some xenoliths are polymetamorphic. In one sample, an early relatively low-P, amphibolite-facies event is preserved by a zincian spinel-bearing assemblage included in garnet cores. This event was followed by a higher-P event at conditions of 0.8 GPa and 700 °C. In situ monazite EMP data in at least two Robinson Ranch xenoliths indicate early metamorphism at ca 2.1 Ga, which is similar to both a zircon TIMS U-Pb age from a mafic xenolith from the Homestead kimberlite in the northern Wyoming craton and a reported age of a mafic dike swarm exposed in SW Montana. The younger peak metamorphic event, recorded by monazite and zircon in these samples, is ca 1.8 Ga and correlates with known magmatic and collisional events in the Great Falls Tectonic Zone. We propose that the 7.x layer in this region did not form in a single event, but instead formed incrementally including an event at ca 2.1 Ga. This event may represent underplating or intraplating of mafic material and associated host rock metamorphism.