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 bulk rock 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 will test 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. Protolith determinations using traditional bulk rock approaches and the proposed zircon-based approach will be conducted on 18 HP/LT rocks collected from five different paleosubduction complexes, some preserved as mélange and some preserved as structurally coherent terranes, each of which has experienced different degrees of alteration. A comparison of the results of the two methods will be used to determine if any bulk rock compositional alteration occurred, and if so, what elements were involved or mobilized during the alteration process. Results from this research will yield a better understanding of the protoliths of HP/LT rocks by providing a way to see through the effects of seafloor and/or subduction metamorphism that affect the bulk rock composition and result in incorrect protolith determinations.