Geochronologic data from xenoliths near the Stillwater Complex, Montana

The Archean Stillwater Complex, a layered ultramafic-to-mafic intrusion, is exposed along the northern side of the Beartooth Mountains in Montana. To the north of the Stillwater outcrops, a gravity anomaly is located where Phanerozoic rocks may unconformably overlie the complex. These Phanerozoic rocks are intruded by Cretaceous plutons that have gabbroid inclusions that look like metamorphosed rocks of the Stillwater Complex. To determine the age of the inclusions, U-Pb isotopic data were collected on zircon in xenoliths from the Sliderock Mountain and Susie Peak plutons, Montana, using the sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG) at Stanford University.