It’s on the way up! Syn-exhumation paragenesis of glaucophane-phengite-quartz veins
- 1Department of Earth and Environmental Sciences, University of Ottawa, Canada (david.schneider@uottawa.ca)
- 2Department of Geology, University of Vienna, Austria
- 3Instituto Andaluz de Ciencias de la Tierra, CSIC-UGR, Spain
- 4Department of Earth Sciences, University of Manitoba, Canada
- 5Department of Earth and Environmental Sciences, University of British Columbia Okanagan, Canada
- 6Department of Geology and Geoenvironment, National and Kapodistrian University of Athens, Greece
Subvertical glaucophane + quartz ± phengite veins exposed in the footwall of a major extensional detachment on southern Evia (NW Cyclades, Greece) record mode-I brittle fracturing with variable principal stress orientations. The dense network of veins displays systematic cross-cutting relationships with a prominent youngest vein set spaced 5-10 cm apart transecting all other veins. Interlayered jadeite-rich metabasalt and quartzite host the veins and promote a pronounced rheological control on fracturing, with veins preferentially hosted in metabasalt layers and terminating abruptly as pressure solution seams or shear bands at contacts with the ductily deformed quartzite. Vein-hosted ferromagnesian minerals do not exhibit recrystallization or dissolution-reprecipitation microstructures. Jadeite in the metabasalt forms both i) 'fuzzy' euhedral crystals overgrown by hematite and ii) symplectitic intergrowths with quartz and albite. Vein-hosted glaucophane and phengite grew at fixed angles (normal and oblique) to vein walls. Phengite is compositionally homogeneous with elevated Si content (3.41–3.52 apfu). Glaucophane from all veins shows a homologous concentric compositional zoning with core chemistry intermediate between glaucophane and magnesioriebeckite, glaucophane-rich mantles, and rims of magnesioriebeckite or winchite. Phengite yields consistent single-grain total-fusion 40Ar/39Ar dates with a weighted mean of 22 ± 1 Ma (n: 22), whereas the low-K glaucophane produced equivocal and dispersed dates. Phengite (n: 44, 20) and glaucophane (n: 8, 42) in-situ 87Rb/87Sr isochrons from two samples yield mutually indistinguishable dates of 21 ± 5 Ma and 25 ± 4 Ma, within uncertainty of the 40Ar/39Ar dates. The uniform mineral chemistry, compositional zoning, and geochronology indicate that the veins formed over a short time without major shifts in ambient pressure-temperature conditions. Contrary to the apparent mineralogically-defined high pressure-low temperature paragenesis of the veins, dates obtained from pristine high-Si phengite support crystallization in the latest Oligocene to earliest Miocene, coincident with regional extension and widespread greenschist-facies retrogradation in the Aegean. Abundant hematite and the predominance of Fe3+-rich sodic amphibole and clinopyroxene species implicates a highly oxidizing fluid in the stabilization of these nominally ‘high-pressure’ minerals under conditions widely responsible for producing retrogradational greenschist-facies assemblages throughout the Cyclades.
How to cite: Schneider, D., Ducharme, T., Grasemann, B., Bukala, M., Camacho, A., Larson, K., and Soukis, K.: It’s on the way up! Syn-exhumation paragenesis of glaucophane-phengite-quartz veins, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11595, https://doi.org/10.5194/egusphere-egu24-11595, 2024.
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