BULK ROCK AND SINGLE-CRYSTAL GEOCHEMISTRY OF CORUNDUM-BEARING ROCKS AT CASCADE CANYON, CA

Outcrops of corundum (Cor)-bearing metasedimentary rocks in Cascade Canyon, northeast of Claremont, CA, have been visited for decades by collectors seeking the 1-5 mm rubies that they contain. This deposit is part of the San Antonio Terrane which experienced upper amphibolite facies metamorphism in the Late Cretaceous; however, the precise timing and controls on ruby growth have not been studied in detail. This study uses field relations; petrography (optical and scanning electron microscopy); whole rock chemistry (XRF); oxygen isotopes (laser fluorination); plus trace elements and U-Pb geochronology (LA-ICP-MS) to assess a number of attributes of the Cor as well as the host rock and potential protoliths. The host rocks are typically alkali feldspar and sillimanite rich with subordinate rutile, late stage Cr-muscovite, and trace monazite. Sulfides (pyrite and pyrrhotite) appear as inclusions in Cor, but are weathered from the bulk rock. XRF analysis shows that the rocks are conspicuously high in Al₂O₃ (24.1 wt.%) and alkali-rich (10.4 wt.% Na₂O+K₂O). V, Cr, Ni, and Cu are elevated over crustal values, but Zr is not enriched as is expected for a deeply weathered protolith (e.g., bauxite). Values of δ¹⁸O are 11.7–12.1‰, consistent with a protolith that was formed at low temperatures in a supracrustal environment. Preliminary pseudo-section models of the bulk composition show equilibrium mineral assemblages in excess of 650°C, consistent with observed sillimanite and absence of primary muscovite. Cathodoluminescence imaging of the Cor shows chaotic to sector growth zoning, implying at least two stages of crystal growth. Trace element profiles were collected perpendicular to the c-axes of a number of crystals. These profiles correspond to CL-identified growth domains, wherein Cr and Ti concentrations vary between 200–800 ppm and 200–500 ppm, respectively. Notably, the Cr/Ti ratio varies greatly between the interior and exterior of the crystals, being higher in cores. Because the Cor rocks appear barren of zircon, an adjacent granite was U-Pb zircon dated and yields an age of 72±1.5 Ma (2.S.E.), a minimum age. Other rock domains associated with the deposit are being examined for zircon and alternate geochronometers (e.g., titanite, monazite) are being investigated.