TRACKING THE CHEMICAL EVOLUTION OF SUBDUCTION ZONE ROCKS

The compositions of rocks exhumed from subduction zones can be used to understand the element fluxes that occur during subduction metamorphism. An important step in understanding the chemical record of subduction is to determine the protoliths of the metamorphic rocks. This can be challenging because oceanic crust can undergo many geochemical changes after it is generated at the mid-ocean ridge: seafloor alteration can cause the liberation and addition of elements, and subduction metamorphism/exhumation may further modify composition via interaction with sediments and fluids. To track the chemical evolution of subduction rocks from protolith through different stages of metamorphism, we have determined the major and trace element composition of blueschist and eclogite from the Sivrihisar Massif (Turkey) and applied geochemical modeling techniques. In the Sivrihisar Massif, eclogite and blueschist are interlayered with metasediment. These associations and major element compositions suggest an affinity with NMORB, but trace element data suggest 2 protoliths. Enrichment in the LILE, HFSE, and LREE as compared to chondrite and NMORB, and high ratios of relatively immobile elements (Th/Nb, Ti/Zr, Zr/Y) suggests that eclogite has a composition similar to OIB. Blueschist shows a depletion of those elements, suggesting a composition similar to NMORB. However, both rock types may have undergone considerable mixing and alteration. To evaluate the proportions of potential source materials, we modeled EMORB, NMORB, and OIB compositions mixed with GLOSS, metasediment, and veins in eclogite. Models using GLOSS, EMORB, and OIB did not match the observed compositions, but NMORB + veins models the signatures of blueschist and eclogite. Element analyses alone did not show these results, indicating that the compositions may be highly altered by metamorphism, and therefore the use of mixing models could be an important tool in tracking the chemical evolution of subducted rocks.