ZIRCON, APATITE, AND GARNET: RECORDERS OF METAMORPHISM AND METASOMATISM ON THE SUBDUCTION INTERFACE

Subduction zone metamorphism and metasomatism occurring on the deep subduction interface contribute to changes in the density of the sinking slab and the generation of fluids. Dating the timing of metamorphism and fluid-rock interactions, as well as understanding their geochemical signatures can constrain the rates of subduction and release of fluids during dehydration. Exhumed rocks from subduction complexes record these reactions and can provide insights into processes taking place below the seismogenic zone at decoupling depths where episodic tremor and slow-slip are prevalent. This study focuses on the Nevado-Filabride Complex subduction complex in the Betic Cordillera in Southern Spain, where previously, by using zircon and apatite U/Pb geochronology, two metamorphic events were identified in Paleocene/Eocene and Miocene.

Here, by integrating trace and major element analyses on zircon, apatite, and garnet, we further investigate their geochemical signatures and how they record the different stages of metamorphism on the subduction interface during subduction and exhumation. We show that Eocene apatite and zircon growth exhibit TE compositions similar to their initial magmatic geochemistry, supporting High Pressure/Low Temperature metamorphism with short diffusion scales accommodated mainly by recrystallization-precipitation mechanisms. The subsequent Miocene metamorphic event is related to exhumation and metasomatic fluids in which these rocks experienced a prolonged metamorphic reheating event characterized by elevated temperatures (~550-600 C). The presence of metamorphic fluids during Miocene metamorphism are recorded by zircon with increased Hf values and depleted MREE, apatite with dispersed LREE and increased Sr/Y, garnet with flat REE, and new rutile growth. Such fluids that could mobilize HFSE as well as Ti are most likely rich in fluorine and derived from the dehydration of sedimentary rocks since subducting sediments have been shown to have the highest fluorine concentrations among potential metamorphic reservoirs. We conclude that more intensely sheared regions may serve as weak permeable zones to preferentially accommodate fluid flow pathways during the exhumation of high-pressure rocks escaping subduction.