TESTING EVIDENCE FOR SUBDUCTION EROSION IN A PALEOSUBDUCTION ZONE (SIVRIHISAR, TURKEY)

Subduction erosion, which involves the removal of crustal material from the overriding plate during subduction, occurs at all modern convergent margins and plays an important role in arc magma genesis, the balance of crustal growth and destruction, and the long-term geochemical evolution of the crust and mantle. Despite its importance, studying the process of subduction erosion is challenging because it cannot be directly observed or measured. Instead, subduction erosion is commonly invoked to explain a variety of geophysical, geochemical, and geological observations from modern and ancient subduction zones that cannot be explained by other geologic processes. However, paleosubduction zones that preserve high-pressure/low-temperature (HP/LT) blocks with geochemical affinities to both the upper and lower plate may represent natural laboratories in which to investigate the process of subduction erosion.

One place in the geologic record where subduction erosion has been proposed to occur is in western Anatolia. To test whether subduction erosion occurred in this region, a combined bulk-rock geochemical and zircon age and trace element study of lawsonite blueschists and eclogites from the Sivrihisar Massif, a paleosubduction complex exposed in western Anatolia, is being conducted. The bulk-rock geochemical and zircon core age and trace element data will be used to assess the source (upper plate vs. lower plate) of the HP/LT metamorphic rocks, and the metamorphic zircon rim ages will be used to evaluate when (if at all) material from the upper plate was incorporated into the subduction zone. Lawsonite eclogite zircons have rims with ages of ~92 Ma and cores with ages of ~220 Ma, which are interpreted as reflecting the timing of subduction metamorphism and protolith formation, respectively. Zircon cores from this sample have trace element characteristics consistent with derivation from a mid-ocean ridge environment. Lawsonite blueschist zircons have ~105 Ma metamorphic rims and ~175 Ma igneous cores. The trace element signature of these zircon cores suggests derivation from a continental arc environment. These preliminary results suggest that during the Early to Late Cretaceous material with both MORB and arc affinities was being subducted, which may be indicative of a period of subduction erosion.