Syn-exhumation metasomatic glaucophane-phengite-quartz veins formed at moderate pressures

Author(s): T. A. Ducharme, D. A. Schneider, B. Grasemann, M. Bukała, A. Camacho, K. P. Larson, K. Soukis
Abstract: Veins composed of glaucophane + phengite + quartz cross-cut the high pressure-low temperature (HP-LT) Cycladic Blueschist Unit (CBU) of southern Evia, Greece. The veins exhibit a rheology-dependent distribution within layered metamorphic rock comprising cm-scale intercalations of albite-clinopyroxene metabasalt and schistose quartzite. Strain was accommodated by ductile processes in the quartzite, whereas brittle deformation produced four sets of crack-seal syntaxial veins in the coarser-grained metabasalt. All vein sets are subvertical to steeply-dipping and are oriented at high angles to one another. The geometry of the planar vein walls suggests the veins are mode-I (opening mode) fractures, whose sub-vertical orientations indicate formation during extension. Oxygen isotope thermometry using phengite-quartz pairs provides crystallization temperatures of 315–335 °C. Combined ⁴⁰Ar/³⁹Ar and in-situ⁸⁷Rb/⁸⁷Sr geochronology of vein-hosted phengite and glaucophane indicate crystallization and vein sealing at c. 22–23 Ma when the CBU is predicted to be undergoing greenschist facies metamorphism coincident with regional extension. The structural and stable isotope data are likewise consistent with a syn-exhumation extensional setting, and easily reconciled with existing petrological data indicating the CBU sustained prolonged residence near the greenschist-blueschist facies boundary. We propose a model whereby phengite and glaucophane were stabilized at greenschist facies conditions by the elevated α_SiO2 and fO₂ in the fluid parental to the veins. Our data provide strong new evidence for the sensitivity of nominally blueschist facies minerals to bulk system chemistry, supported by thermodynamic modelling evidence from other orogens that such HP-LT minerals may exhibit stability that spans multiple stages of orogenesis.
Organisation(s): Department of Geology
External organisation(s): University of Ottawa, Instituto Andaluz de Ciencias de La Tierra (IACT), University of Manitoba, University of British Columbia (UBC), National & Kapodistrian University of Athens
Journal: Contributions to Mineralogy and Petrology
Volume: 179
ISSN: 0010-7999
DOI: https://doi.org/10.1007/s00410-024-02101-8
Publication date: 03-2024
Peer reviewed: Yes
Austrian Fields of Science 2012: 105124 Tectonics
Keywords
ASJC Scopus subject areas: Geophysics, Geochemistry and Petrology
Portal url: https://ucrisportal.univie.ac.at/en/publications/ca6ff5de-4ce9-4aed-aefc-365daba89e2e

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