Post‑magmatic fracturing, fluid flow, and vein mineralization in supra‑subduction zones: a comparative study on vein calcites from the Troodos ophiolite and the Izu-Bonin forearc and rear arc

摘要

Based on the published data of pillow lava-hosted mineralized veins, this study compares post-magmatic fracturing, fluid flow, and secondary mineralization processes in the Troodos and Izu-Bonin supra-subduction zone (SSZ) and discusses the crucial factors for the development of distinct vein types. Thin section and cathodoluminescence petrography, Raman spectroscopy, fluid inclusion microthermometry, and trace element and isotope (Sr-87/Sr-86, delta O-18, delta C-13, Delta(47)) geochemistry indicate that most veins consist of calcite that precipitated from pristine to slightly modified seawater at temperatures 50 degrees C. In response to the mode of fracturing, fluid supply, and mineral growth dynamics, calcites developed distinct blocky (precipitation into fluid-filled fractures), syntaxial (crack and sealing), and antitaxial (diffusion-fed displacive growth) vein microtextures with vein type-specific geochemical signatures. Blocky veins predominate in all study areas, whereas syntaxial veins represent subordinate structures. Antitaxial veins occur in all study areas but are particularly abundant in the Izu-Bonin rear arc where the local geological setting was conducive of antitaxial veining. The temporal framework of major calcite veining coincides with the onset of extensional faulting in the respective areas and points to a tectonic control on veining. Thus, major calcite veining in the Troodos SSZ began contemporaneously with volcanic activity and extensional faulting and completed within similar to 10-20 Myr. This enabled deep seawater downflow and hydrothermal fluid upflow. In the Izu-Bonin forearc, reliable ages of vein calcites point to vein formation 15 Myr after subduction initiation. Therefore, high-T mineralization (calcite, quartz, analcime) up to 230 degrees C is restricted to the Troodos SSZ.