Franciscan subduction off to a slow start: evidence from high-precision Lu–Hf garnet ages on high grade-blocks

Lu–Hf analyses of garnet from metabasic amphibolite, glaucophane schist and eclogite facies blocks from the Franciscan complex give highly precise ages that allow us to place new constraints on the early thermal history of the Franciscan subduction zone. Garnets yield 176Lu/177Hf ratios ranging from 1.5 to 28 with the highest ratios from garnets with high spessartine/pyrope ratio. Sulphuric acid leaching (SAL) of garnets revealed the presence of inclusions with significantly higher Lu/Hf ratios than those of garnet itself (most likely apatite). Their removal by SAL brings the 176Lu/177Hf ratios in garnets down by as much as 40%. This suggests that 176Lu/177Hf ratios of apparently pure garnets can be greatly overestimated due to the presence of such inclusions. Sm–Nd garnet analyses were dominated by inclusions (mainly sphene), and failed to provide precise and accurate age information. dest Lu–Hf ages are 168.7±0.8 and 162.5±0.5 Ma on plagioclase-bearing garnet amphibolite from Panoche Pass and the Berkeley Hills, respectively, which suggests initiation of the subduction zone at about 169 Ma, coeval with the formation of the tectonically overlying Coast Range Ophiolite. Relatively high temperature conditions persisted for about 14 Ma as indicated by 153.4±0.8 Ma garnet growth recorded in epidote amphibolite and 157.9±0.7 in eclogite from Ring Mountain and Jenner, respectively. A 146.7±0.7 Ma age was obtained from garnet glacuophane schist, metamorphosed at around 400 °C. The sequence of ages from central and northern California shows a younging trend with decreasing metamorphic grade, which supports previous suggestions that the high-grade metamorphic blocks and slices resulted from progressive underthrusting and underplating in a cooling subduction system. Combining geothermometry with geochronological data allow us to estimate cooling rate along the subduction zone interface from amphibolite to blueschist facies conditions as ca. 15 °C/Ma. The thermal history requires high initial geothermal gradients within both the footwall and the hangingwall of the subduction zone and a relatively slow subduction rate of the order of 10 km/Ma during the initial stages of Franciscan subduction. Such conditions are consistent with initiation of the subduction zone at or close to an oceanic spreading centre. The data also suggest slow exhumation rates and significant residence time at depth of the earliest Franciscan rocks. younger age of 114.5±0.6 Ma on garnet hornblendite from Santa Catalina Island confirms significantly younger initiation of the subduction zone in Southern California.