PROTOLITH DETERMINATION OF SUBDUCTION-RELATED METAMORPHIC ROCKS: COMPARING BULK-ROCK AND ZIRCON-BASED APPROACHES

The composition of the subducting oceanic plate affects deep element and volatile cycling, as well as the location of key dehydration reactions, which impact where subduction-related volcanism and seismicity occur. Therefore, accurately determining the composition of the subducted oceanic plate is important for understanding what and where dehydration reactions occur, and the nature of the materials being recycled into the mantle. One of the ways to study the composition of the subducted oceanic plate is by studying high-pressure/low-temperature (HP/LT) metamorphic rocks, which represent remnants of the subducted oceanic plate that have been exposed on Earth’s surface. The methods most commonly used to determine the original composition (protolith) of the subducted oceanic plate involve measuring the bulk rock composition of HP/LT rocks and plotting the concentrations of largely immobile trace elements on tectonic discrimination diagrams and/or comparing the compositions of the HP/LT rocks to the compositions of potential igneous protoliths using normalized trace element diagrams. However, the composition of HP/LT rocks can be modified during seafloor alteration and/or subsequent subduction metamorphism, potentially leading to inaccurate protolith determinations. To overcome these challenges, this research tests the utility of a zircon-based approach, as zircon can potentially preserve an unaltered record of the igneous protolith composition in its trace element signature. In some cases, the protolith predicted by the bulk-rock and zircon methodologies agrees. For example, some Sivrihisar samples show affinities to OIB in both their bulk-rock and zircon trace element compositions. However, in other complexes the protolith predicted by the two methodologies disagrees. For example, bulk-rock geochemical analyses of samples from the Franciscan and Rio San Juan paleosubduction zones suggests MORB and/or volcanic arc basalt protoliths, whereas zircon geochemistry suggests continental arc protoliths. Regardless of locality, all samples have bulk-rock geochemical signatures that are consistent with some degree of metasomatism through either seafloor alteration and/or metamorphic enrichments/losses which may, in part, explain the mismatch in protoliths predicted by the two methods.