Siliceous Deposits of the Tethys and Pacific Regions

pp 217-233

Thermal Alteration of Chert in the Ophiolite Basement of Southern Central America

  • Hans-Jürgen Gursky
  • , Monika M. Gursky

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Radiolarian chert and volcaniclastic-rich siliceous rocks of the Nicoya Complex were studied by optical mineralogy, X-ray diffractometry, and SEM. Various phenomena of thermal alteration occur due to post-depositional mafic magmatism. Progressive destruction of sedimentary features is classified into four stages from weak to complete recrystallization. Increases in quartz crystallinity reflecting diagenetic to high-grade thermometamorphic conditions parallel the recrystallization noted in thin sections. Mineral assemblages especially in volcaniclastic-rich rocks help estimate the metamorphic grade. Chert with smectite and illite-smectite mixed-layer minerals or heulandite and clinoptilolite, respectively, indicates low-temperature conditions of less than 200°C. Piemontite, garnet, and diopside reflect low to medium or high (?) grade, very low-pressure contact metamorphism. Mineral assemblages with scapolite were found close to igneous contacts and indicate medium to high grade metamorphism. Secondary minerals in veins with dominantly barite and zeolites commonly formed under low temperature conditions of less than 150°C. These different petrologic changes are discussed as to their usefulness in defining thermal grades.