THE SIGNIFICANCE OF LAWSONITE + CHLORITE ROCKS FOR TRACE ELEMENT AND WATER CYCLING IN SUBDUCTION ZONES: INSIGHTS FROM THE TAVSANLI ZONE, TURKEY

Hybrid rocks, which have bulk rock compositions that do not correspond to any known sedimentary or magmatic protolith, are found in exhumed subduction complexes worldwide, where they have been interpreted as forming as a result of tectonic mixing and/or metasomatic interactions between mafic, ultramafic, and metasedimentary 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 chlorite (± talc) and accessory phases. In the Tavsanli Zone, a Late Cretaceous subduction complex exposed in NW Turkey, chlorite-rich hybrid rocks formed at the contact between antigorite serpentinite lenses and metamafic rocks contain a significant proportion of lawsonite (~30 – 50%), a trace element (Sr, Pb, Th, U, REE) and water-rich (11.5 wt.% H₂O) mineral that is stable at pressures exceeding 2.5 GPa. The incorporation of trace elements in lawsonite (as opposed to accessory phases) in hybrid rocks formed in cold thermal regimes and/or in bulk compositions with sufficient CaO may affect the depths at which trace elements and volatiles are released in subduction zones. To understand the origins and significance of these lawsonite-bearing hybrid rocks, we conducted a petrologic and geochemical study of representative lawsonite + chlorite rocks from across the Tavsanli Zone. Lawsonite + chlorite rocks sampled adjacent to the serpentinite lens consist of chlorite + lawsonite, with minor amounts of epidote, and accessory titanite, whereas those sampled farthe­r away from the serpentinite consist of chlorite + lawsonite + epidote + garnet + clinopyroxene, with accessory titanite and Fe-oxides. Bulk rock geochemical analyses show that lawsonite + chlorite rocks share geochemical affinities with both metamafic rocks and serpentinites, with SiO₂ (35 – 40 wt.%), K₂O (0 – 0.02 wt.%), and Na₂O (0.08 – 0.37 wt.%) concentrations similar to serpentinites and CaO (4 – 9 wt.%) concentrations similar to metamafic rocks. Magnesium concentrations (10 – 22 wt.%) are intermediate between those of metamafic rocks and serpentinites. Mineral compositions and zoning patterns, coupled with thermodynamic modeling, will be used to elucidate the relative timing and P-T conditions at which these rocks form and dehydrate.