Petrogenesis and Significance of Lawsonite-bearing Hybrid Rocks, Tavsanli Zone, NW Turkey

Hybrid rocks, which have bulk rock compositions that do not correspond to any known sedimentary or igneous protolith, are found in paleosubduction zones worldwide, where they have been interpreted as forming as a result of tectonic mixing and/or metasomatic interactions between mafic, ultramafic, and sedimentary rocks of the subducted slab. The formation and progressive metamorphism of these unique bulk compositions results in hydrous, low density, mechanically weak rocks that consist almost entirely of sheet silicates and trace element-rich accessory phases. As a result of these physical and chemical properties, hybrid rocks have been proposed to play an important role in subduction zone dynamics, including influencing the strength of the slab-mantle interface and contributing to arc magma genesis.

In most paleosubduction zones, chl ± tlc ± amph rocks are the most volumetrically significant hybrid rock. However, in the Tavsanli Zone (TZ), lawsonite (lws) + chl rocks are the most common hybrid rock and chl ± tlc ± amph rocks are less common. To understand the petrogenesis and metamorphic evolution of lws + chl rocks, which may represent the dominant hybrid rock in subduction zones with normal to cool geothermal gradients, we conducted a petrologic and geochemical study of lws-bearing and lws-free hybrid rocks from different structural contexts (lithologic contacts, pod rinds, shear zones) across the TZ. All hybrid rocks have similar bulk rock major element compositions, with low SiO₂, Na₂O, and K₂O, and high MgO. However, trace element concentrations vary depending on the structural context of the sample. Lws + chl rocks sampled at the contact between serpentinite and metamafic rocks have flat REE patterns and negative Ce anomalies, the latter indicating that the protolith may have experienced seafloor alteration. Mg zoning patterns in garnet are consistent with a later, HP Mg-metasomatic event. In contrast, hybrid rocks sampled from pod rinds and shear zones are enriched in LREE, lack Ce anomalies, and have compositions that are intermediate between metamafic rocks and serpentinite. These results indicate that there may be multiple mechanisms for generating lws + chl rocks, each of which can influence the mineralogy, trace element content, and spatiotemporal distribution of hybrid rocks during subduction.