DATING SUBDUCTION ZONE METAMORPHISM WITH COMBINED LAWSONITE AND GARNET LU-HF GEOCHRONOLOGY

Studies of ancient and active subduction zones are critically important to understanding processes of interplate coupling, crust-mantle recycling, and arc magmatism. Dating subduction metamorphism along prograde and retrograde paths in order to constrain such processes, however, has proven extremely difficult. Lu-Hf dating of lawsonite, a critical index mineral of high-pressure, low-temperature metamorphism associated with subduction zones, provides a potentially powerful new tool for constraining subduction zone processes in a pressure-temperature window where few successful geochronometers exist.

We are working to address the role of the metamorphic path on the applicability of lawsonite Lu-Hf geochronology within the Franciscan Complex of California. The Franciscan Complex preserves mafic high-grade exotic blocks in mélange that underwent a counterclockwise pressure-temperature path wherein garnet, which strongly partitions heavy rare-earth elements, formed prior to lawsonite. Coherent mafic rocks within the Franciscan Complex, however, underwent a clockwise pressure-temperature path and lawsonite growth occurred prior to garnet.

Ages of garnet and lawsonite from exotic blocks of garnet-hornblendite, garnet-epidote amphibolite, garnet-epidote blueschist, and lawsonite blueschist span ~171-130 Ma. Complex multi-stage garnet provides both core and rim ages within a single sample. In addition, lawsonite and garnet separates from the same rock are separated by ~15 Ma. While the ages from retrograde lawsonite within exotic blocks are similar to existing ages from the Franciscan Complex, no meaningful lawsonite ages were obtained from prograde coherent blocks of Ward Creek. The lack of ages from these samples may be due several factors such as non-equilibrium at very low metamorphic grades, the presence of micro-zircon within lawsonite and matrix minerals, and low bulk rock Lu contents.