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- Zeolites and associated minerals as indicators of post-magmatic hydrothermal alteration in Mesozoic tholeiitic basalts in Northeastern BrazilPublication . Araujo, Lidyane; Castro Jobim Vilalva, Frederico; Franco de Souza, Raquel; Bustamante, Andres; Souza, LaécioABSTRACT: In northeastern Brazil, tholeiitic basalts and microgabbros from the Rio Ceará-Mirim dike swarm and Serra do Cuó basaltic flow experienced post-magmatic hydrothermal alteration, forming secondary minerals that replaced magmatic phases and filled amygdules. Petrographic, X-ray diffraction, thermogravimetric, and chemical analyses identified laumontite, quartz, and calcite as the dominant amygdule minerals in the Rio Ceará-Mirim dikes, formed at temperatures < 150 °C (Stage II) following an earlier low to moderate-temperature (< 150 – 200 ºC) alteration phase (Stage I). In the Serra do Cuó basalts, polymineralic amygdule formation began with mafic phyllosilicates (from < 150 to ~200 °C, Stage I) and proceeded to Ca-Na zeolites (~250 °C, Stage II). These assemblages reflect fluid composition changes due to primary mineral destabilization by heated meteoric fluids. Results indicate fluid composition, rather than temperature, as the primary control on mineral variability, highlighting the role of host-rock chemistry in hydrothermal alteration.