Dating subduction‐zone metamorphism with combined garnet and lawsonite 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 proved to be extremely difficult. The complex thermal history of subduction systems poses significant challenges to low-T geochronometers and subduction-zone assemblages commonly lack suitable minerals for higher temperature geochronology. Garnet and lawsonite, however, are critical index minerals of high- and low-T subduction-zone metamorphism and are well suited to Lu–Hf geochronology. In addition, the closure temperature for Lu–Hf in garnet and lawsonite should be sufficiently high that an age will date mineral crystallization and therefore the timing of subduction zone metamorphism. The relative stability and timing of garnet and lawsonite formation will be controlled by the bulk composition, peak metamorphic conditions and shape of the P–T path experienced by a particular sample. To test the influence of metamorphic conditions and P–T path on Lu–Hf geochronology, garnet and lawsonite-bearing samples were dated from rocks where lawsonite formed after garnet along a retrograde path, stabilized prior to garnet along a prograde path and formed contemporaneously with garnet. In the Franciscan Complex, the ages of garnet–epidote amphibolite, garnet–epidote blueschist, garnet–lawsonite blueschist and lawsonite blueschist range from c. 166–130 Ma and generally decrease with decreasing metamorphic grade, consistent with previous studies. Garnet–lawsonite blueschist/eclogite formed along an apparent prograde path at Ward Creek records an apparent age of c. 152 Ma. Lower temperature lawsonite blueschist at Ward Creek, however, failed to provide a geologically significant date and likely reflects isotopic disequilibrium at low temperatures. The apparent temperature–time history from Franciscan Complex Lu–Hf ages most likely reflects samples derived from various portions of the subduction zone or that were subducted and metamorphosed at different times in the thermal evolution of the subduction zone. In the Sivrihisar Massif, lawsonite eclogite and garnet–lawsonite blueschist record distinctly different ages of 91.1 ± 1.3 Ma and 83.3 ± 1.8 Ma. The different ages date the timing of high-P metamorphism within each protolith and suggest that garnet–lawsonite eclogite metamorphism pre-dated garnet–lawsonite blueschist metamorphism in these samples by c. 8 Ma. The age of lawsonite eclogite metamorphism extends the timing of high-P metamorphism and requires subduction initiation prior to 91 Ma. The results indicate that the Lu–Hf system provides a reliable tool for dating the wide range in P–T conditions of subduction-zone metamorphism. Lawsonite dating, in particular, provides a reliable method by which to date low-T retrograde and prograde metamorphism in the absence of garnet. Lawsonite may not be ideal for geochronology if sufficient garnet coexists in the mineral assemblage, the lawsonite has very low-T stability, or if extremely fine-grained Hf phases such as zircon are present in lawsonite. In poly-metamorphic assemblages where the pressure and temperature can be estimated for separate garnet and lawsonite sub-assemblages, the age discrepancy between garnet and lawsonite may provide the ability to quantify the rates of heating or cooling and subduction or exhumation.