Hydrothermal Blueschist Facies Zircons from the Franciscan Complex, California

Zircons from garnet hornblendite and eclogite blocks hosted by the Franciscan Complex display oscillatory zoning in CL but have oxygen isotope ratios and ages inconsistent with an igneous origin as part of the ocean- floor protolith. Elongate zircons, 100-200 μm long, from two high-grade metamorphic blocks were analyzed for U-Pb isotopes, δ18O, and trace elements by ion microprobe. Additional δ18O data were obtained from matrix mineral separates by laser fluorination. A garnet hornblendite block from Panoche Pass contains slightly rounded oscillatory-zoned zircons. Despite CL patterns that typify igneous zircon, low Th/U ratios (0.02-0.13) and a concordant U-Pb age (147 ± 4 Ma) that post-dates existing Lu-Hf garnet and Ar-Ar hornblende ages by ~20 My suggest that the zircons formed after amphibolite facies metamorphism. The elevated δ18O of zircon (8.2 ± 0.5‰ VSMOW, 1SD, n=22) is inconsistent with igneous formation from a mantle source. Zircons are not in oxygen isotopic equilibrium with the primary metamorphic assemblage of garnet (9.96‰) + hornblende (6.12‰), and do not have the flat HREE pattern characteristic of growth in equilibrium with garnet. These features can be explained by zircon formation as a result of garnet breakdown or metasomatic introduction of Zr during fluid-rich blueschist-facies retrogression. The observed fractionation of 3.5‰ between zircon and blueschist-facies sphene (4.71‰) could be a result of the low temperatures of formation (< 300° C), or may represent disequilibrium. The Junction School eclogite contains zircons with oscillatory-zoned rims and dark or patchy zoned cores in CL. Core and rim ages are indistinguishable and yield a pooled age of 144 ± 10 Ma. Zircons are zoned in δ18O and Th/U with cores 4.6 ± 0.4‰ (1SD, n=6), Th/U = 0.3-0.6 and rims 6.5 ± 0.2‰ (1SD, n=10), Th/U = 0.01 0.02. Zircon cores are in oxygen isotopic equilibrium with matrix garnet (4.77‰), but zircon cores and rims do not have flat HREE patterns. Matrix omphacite (5.78‰) is not in equilibrium with garnet and likely recrystallized during blueschist-facies retrogression. Low δ18O, high Th/U zircon cores may be igneous relicts from a mantle source; their young age may be the result of Pb loss during metamorphism. Garnets in this rock display dramatic cation zonation and evidence of resorption and blueschist facies regrowth. Zircon rims must have formed after prograde garnet growth, during or after garnet resorption and hydrothermal alteration of the eclogite, but not contemporaneously with renewed garnet growth. If zircon rims are in equilibrium with late sphene (5.34‰), their compositions yield a temperature of 550° C. Combination of in situ U-Pb, oxygen isotope and trace element analysis of zircon from metamorphic rocks with complex fluid histories has begun to shed light on these events. Direct analysis of δ18O of zoned matrix minerals in thin section will further elucidate these complex, and perhaps highly channelized fluid events.