Quartz-in-garnet barometry constraints on formation pressures of eclogites from the Franciscan Complex, California

Determining pressure and temperature variations between high-pressure/low-temperature (HP-LT) eclogite blocks is crucial for constraining end-member exhumation models; however, it has historically been challenging to constrain eclogite pressures due to the high variance associated with this bulk-rock composition. In this work, we utilize quartz-in-garnet elastic barometry to constrain formation pressures of eclogites from the northern (Junction School, Ring Mountain, Jenner Beach) and southern Franciscan Complex (Santa Catalina Island). Multiple eclogite blocks from Jenner Beach are analyzed, and a single eclogite from the other localities. By comparing garnet growth conditions from within a single outcrop and between distinct outcrops, we evaluate the local and regional spatial distribution of P conditions recorded by eclogites. We compare the mean, median, and max pressures between different garnet zones and eclogites. Pressures sometimes exhibit systematic changes across garnet zones; however, some eclogites exhibit no systematic pressure variations across garnet zones. Pressures from northern Franciscan eclogites range from similar to 1.4-1.8 GPa, at an estimated temperature of 500 degrees C; pressures from the Catalina eclogite range from similar to 1.2-1.5 GPa, at an estimated temperature of 650 degrees C. Mean and maximum pressures of different eclogites from the northern Franciscan exhibit negligible differences (< 0.1 GPa). The results are inconsistent with models that propose exhumation of metamorphic blocks from different structural levels, and suggest that now exposed HP-LT eclogites from the northern Franciscan Complex may represent rocks that were coherently underplated, and exhumed from similar structural levels.

Automated summary

By using quartz-in-garnet elastic barometry, we have constrained the formation pressures of eclogites from different locations, suggesting that these rocks may have originated from similar structural levels.