Geochemistry and paragenesis of heulandite cements in a Miocene marine fan-delta system of the Pohang Basin, Republic of Korea

초록

Heulandite cements in the Pohang area of Korea formed by alternation of volcaniclastic sediment during shallow burial and uplift in marine pore fluid diluted by meteoric water. SR isotope data for heulandiate II and heulandite II indicate that the pure fluid was progressively mixed with meteoric water during burial and uplift but the Ca in the pore-filling heulandites has been derived mainly from dissolution of carbonate cements. Heulanite III, heulanite-clinoptilolite, was formed with an unusual coarsening and chemical zoning at somewhat diluted and disequilibrium conditions caused by the migration of oxygen-rich meteoric water during or after uplifting.; In the Pohang area of Korea, heulandite occurs as cements in conglomerate and sandstone of a Miocene marine fan-delta system resting on Eocene dacitic volcanics. three types of heulandite cements are distinguishable in the fan-delta sediments on the basis of texture, chemical composition and authigenic mineral association. The earliest type I heulandite (Si/(Al + Fe): 3.5-3.8) occurs as microcrystalline (10-30 μm) in situ crystallites that replce volcanic matrix and are intermixed with early-formed smectite. Type II heulandite (Si/(Al + Fe): 3.2-3.6) occurs as medium-grained (30-60 μm) crystal aggregates rimming intergranular cavities. Type III heulandite (Si/(Al + Fe): 3.6-4.1) is the last to form and is a composite phase of heulandite-clinoptiloite, which occurs as unusually coarse (50-200 μm) single-crystal cement associating with late-formed smectite and hematitel These characteristic heulandite cements were formed by alteration of volcaniclastic sediment during shallow burial (burial temperature: 40-60 °C) and uplift in marien pore fluid diluted by meteoric water. Sr isotope data for heulandite II (87Sr/86Sr: 0.706565-0.706598) and heulandite III (87Sr/86Sr: 0.707347-0.707432) indicate that the pore fluid was progressively mixed with meteoric water during burial and uplift but the ca in the pore-filling heulandites has been derived mainly from dissolution of carbonate cements. Heulandite III, heulandite-clinoptilolite, was formed with an unusual coarsening and chemical zoning at somewhat diluted and disequilibrium conditions caused by the migration of oxygen-rich meteoric water during or after uplifting.