USING QUARTZ-GARNET OXYGEN ISOTOPE THERMOMETRY TO RECOVER PEAK METAMORPHIC CONDITIONS OF THE CUCAMONGA TERRANE

The Cucamonga Terrane, in the eastern San Gabriel Mountains, was metamorphosed to granulite facies conditions during Cretaceous arc magmatism. Understanding temperature and isotopic values during the peak metamorphic conditions in these rocks provides insight into the preservation of whole-rock isotopic values during metamorphism in the lower crust, and is important for evaluating thermal and mass transfer during arc magmatism. Among the lithologically diverse array of metasedimentary rocks in the Cucamonga Terrane, which includes metapelites, marbles, and calc silicates, garnet-bearing quartzites are ideal for quartz-garnet oxygen isotope thermometry. The refractory nature of garnet (Grt) to resetting of δ¹⁸O and its occurrence in a large reservoir of quartz (Qtz), ensures preservation of peak metamorphic fractionations (δ¹⁸O) between the two minerals. Quartzites from Deer, San Savine, and Cucamonga Canyons were analyzed for δ¹⁸O by laser fluorination. Mineral pairs were analyzed as single grains or clusters of grains that were in mutual contact. Values of δ¹⁸O(Qtz) range from 12.4% to 13.8%; δ¹⁸O(Grt) is 10.0% to 11.3%. The resulting Δ¹⁸O values among quartz-garnet pairs yield temperatures of 772–824°C and average 807±25°C (n = 4). Rocks in one outcrop in Upper Deer Canyon have high δ¹⁸O values (δ¹⁸O(Grt) = 14%, δ¹⁸O(Qtz) = 15.6%, yielding apparent temperatures of ~1000° C, which are unreasonable based on phase equilibria estimates of temperature. With the exception of the Upper Deer canyon, temperatures are in good agreement with the 700-800°C (avg. = 783°C) garnet exchange temperatures reported by Barth and May (1992, J. Met Geol), although slightly higher and suggestive of some resetting of cation exchange thermometers. Overall, the results show that quartz-garnet oxygen isotope thermometry is a useful technique for recovering peak temperatures in young granulite facies terranes.