NEW U-PB AGES FROM ZIRCON INCLUSIONS IN GARNET SHOW EVIDENCE FOR MULTI-STAGE METAMORPHISM OF AMPHIBOLITE IN FRANCISCAN MÉLANGE, RING MOUNTAIN, CALIFORNIA

The oldest ages of high-grade exotic blocks in the Franciscan complex, California, are synchronous with the age of the Coast Range ophiolite and are commonly considered to mark the regional onset of subduction. However, the best of these ages are based on Lu-Hf and the poorly known closure-temperature for this system in garnet. Zircon geochronology may provide a better estimate of the metamorphic ages of high-grade exotic blocks. Our studies of a garnet amphibolite from Ring Mountain show that zircon inclusions in garnet porphyroblasts are ~15 m.y. older than zircon within the fine-grained matrix. The garnet porphyroblasts are strongly zoned in major elements and occur in a matrix of Amp + Chl + Ep + Phg. Zircon is included within each of these phases. Mineral inclusions in garnet are often concentrated in the interior of the grains, but the zircon grains are not restricted to the interior. To maintain petrological context, the zircon inclusions in garnet were analyzed in situ within sectioned garnet grains, while the matrix zircon was separated from crushed garnet-free sample material. The Stanford/USGS SHRIMP-RG was used to analyze U and Pb with a ~16 µm O₂ analytical spot. Zircon ranged in size from <15 to ~40 µm. No significant U or Pb was detected in the garnet, suggesting that beam overlap between the smallest zircon and the surrounding garnet did not affect the overall isotopic data. We calculated a weighted mean age of 159 ± 4 Ma (1σ, n = 19, MSWD = 1.8) for zircon separated from the matrix and 174 ± 3 Ma (1σ, n = 23, MSWD = 1.05) for zircon inclusions in garnet. This 15 m.y. difference in age suggests that the zircon inclusions in garnet are associated with an earlier stage of metamorphism than the younger matrix grains. The existence of two distinct zircon populations is also supported by much higher U, Th/U, and lower Hf in matrix zircon. Comparison with the generally accepted ~159-169 Ma age for the onset of Franciscan subduction suggests that the zircon inclusions in garnet are likely to be coincident with the early prograde path of this high-grade block and suggest an earlier start to Franciscan subduction than previously recognized. These data demonstrate that in situ dating of inclusions is a useful method for investigating the prograde path of metamorphism, and possibly extend the recorded metamorphic history of the Franciscan Complex.