ZR-IN-RUTILE THERMOMETRY OF A RUBY-BEARING METAMORPHIC ROCK, CASCADE CANYON, SAN BERNARDINO COUNTY, CA

A metamorphic rock in Cascade Canyon, a tributary of San Antonio Canyon on the south side of Mount Baldy in Southern California, contains pink-purple corundum (ruby) crystals up to 5 mm in length. This is a popular mineral collector’s site because the corundum crystals can be easily retrieved from landslide deposits at the bottom of the canyon. However, little work has been done to understand the complete mineral assemblage and petrogenesis of this rock. Imaging and EDS analyses were performed using the Leo 1430VP SEM facility at 25 kV at James Madison University and the FEI NOVA nanoSEM600 FEG Variable Pressure Scanning Electron Microscope at 15 kV at the Smithsonian Institution. The rock is comprised of 90-95% sodium and potassium feldspar, 5-10% corundum, and minor muscovite mica, monazite, sillimanite, pyrite, and rutile. The corundum crystals contain inclusions of rutile (up to 3% by volume) as well as at least two generations of fluid inclusions. Quantitative chemical analyses of matrix rutile and rutile inclusions in corundum were obtained using the JEOL JXA-8900R WD/EDS microanalyzer at 20 kV and 200 nA with a focused beam at the Smithsonian Institution. Zr concentrations of matrix rutile (1627±134 ppm), and rutile inclusions in corundum (1724±231 ppm) are the same within error. Maximum peak metamorphic temperatures were calculated as 796±9°C for the matrix rutile and 802±16°C for the rutile inclusions using Zr-in-rutile thermometry (Watson et al., 2006, Contrib Mineral Petrol 151: 413–433; Ferry and Watson, 2007, Contrib Mineral Petrol, 154:429–437) and assuming corundum-sillimanite equilibrium. The similarity in temperatures obtained from the inclusions and matrix suggests there was a single overprinting metamorphic event. This peak metamorphic temperature is consistent with the presence of sillimanite and implies the rubies were formed in a contact metamorphism event during emplacement of nearby Cretaceous granitic intrusions.