Formation and alteration of a zoned calcsilicate vein from the contact aureole of Kasuga area, central Japan: insights from thorianite and uraninite chemical ages

抄録

To understand fluid activity around a granitic pluton, a petrological study combined with U-Th-total Pb chemical dating of accessory U-Th oxides was conducted on a zoned calcsilicate vein from the contact aureole of the Cretaceous Kaizukiyama granite, Japan. The vein exhibits distinct zoning with the following sequence of major assemblages from the host rock (dolomitic marble) to the vein center: Zone 1 (comprising three mineral assemblages in order away from the host rock: clinohumite + calcite, forsterite + calcite, and diopside + forsterite + calcite), Zone 2 (clintonite + spinel + Al-rich clinopyroxene or pargasite + calcite), and Zone 3 (grossular + anorthite + clinopyroxene + calcite). Additionally, various U-, Th-, and REE-rich accessory minerals occur in Zones 2 and 3. Grossular and anorthite in Zone 3 are extensively replaced by later-stage minerals (clinozoisite or prehnite + muscovite). Zone 1 is likely a metasomatic reaction vein formed by interactions between the host dolomitic marble and aqueous silica in the infiltrating fluid. The mineral assemblage in Zone 2 likely resulted from interactions between Zone 1 and a fluid enriched in alumina. Thorianite grains in Zone 2 yield a chemical age of 97.0 ± 1.1 Ma, contemporaneous with the emplacement age of the Kaizukiyama granite. The primary minerals in Zone 3 probably formed during a crack-sealing process associated with new fluid infiltration, which also slightly interacted with Zone 2. The later-stage minerals in Zone 3 are attributed to low-temperature hydrothermal alteration. Uraninite grains in Zone 3 yield a chemical age of 88.1 ± 0.8 Ma, likely related to the hydrothermal alteration stage. Combining these results with the previously established cooling history of the Kaizukiyama granite suggests that low-temperature hydrothermal activity may have continued for a long period (∼ 6 Myr) in the surrounding area after the rapid initial cooling of the pluton.