In-situ Rb-Sr geochronology of white mica in young metamafic and metasomatic rocks from Syros: testing the limits of LA-ICP-MS/MS mica dating using different anchoring approaches

Abstract. The recent development of LA-ICP-MS/MS has revolutionized Rb-Sr mica dating allowing to obtain isotopic data within their microstructural context. While effective for old and felsic materials, this method presents challenges for young metamafic and metasomatic rocks due to limited radiogenic ingrowth associated with low Rb/Sr and young ages. We quantitatively address these limitations by combining laser ablation ICP-MS/MS and MC-ICP-MS data for coexisiting white mica and epidote, respectively, for 10 Cenozoic metamorphic rocks from Syros Island (Greece). White mica analyses from metamafic and metasomatic rocks yield limited Rb/Sr spread, which typically does not exceed one order of magnitude (⁸⁷Rb/⁸⁶Sr = 14 to 231 for the combined dataset), and low radiogenic ⁸⁷Sr/⁸⁶Sr (generally <0.8), resulting in high age uncertainties of typically 10 to 50 % RSE, and thus hampering robust geological interpretations. Epidote ⁸⁷Sr/⁸⁶Sr values range between ~0.705 and 0.708. The former (lower-end) is expected for typical, unaltered metamafic materials, whereas the latter is interpreted to reflect fluid-rock interaction along shear zones, with fluids derived from or having interacted with  more radiogenic lithologies. These atypical values suggest that a commonly assumed value of 0.703 for mafic rocks may not always be representative. Anchoring white mica Rb/Sr to epidote ⁸⁷Sr/⁸⁶Sr data improves age accuracy and precision substantially (e.g., 29 ± 17 Ma vs 47.2 ± 4.4 Ma for sample SYGR36). The new ages obtained in this study are consistent with multiple events previously recorded in Syros and the Cyclades blueschists unit including: i) metasomatism at near-peak to epidote blueschist-facies during early exhumation (47.2 ± 3.8 Ma to 41.1 ± 3.1); ii) a late stage of high-pressure exhumation and metasomatism transitioning to blueschist-greenschist-facies (20.8 ± 3.1 Ma to 19.8 ± 5.2 Ma). Anchored white mica Rb/Sr dates thus allow us to discriminate events of fluid-rock interactions and metasomatism associated with shear zone deformation at the subduction interface.