Evaluation of the volatilization and infiltration effects on the stable isotopic and mineralogical variations in the carbonate rocks adjacent to the Cretaceous Muamsa Granite, South Korea

摘要

The Muamsa Granite in the Hwanggangri Mineralized Zone, South Korea, intruded the Cambro-Ordovician Samtaesan Formation, which dominantly consists of calcitic marble and dolomitic marble with minor intercalations of pelitic rock and amphibolite. Intrusion of the granite caused an isotopic depletion of ~(13)C and ~(18)O in the carbonate rocks. Calcitic carbonates show systematic depletion from unrecrystallized limestone through calcitic marble to skarn. It was initiated with volatilization accompanying calcsilicate-forming reactions, and then, with increased permeability, H_2O-rich siliceous fluids of igneous origin actively infiltrated the system promoting fluid-rock interaction. For the skarn fluid infiltration yielded fluid-rock ratios as high as 4.2 in open system, and caused increased ~(18)O depletion rates compared to the volatilization stage. More than 70% of the isotopic depletion in the calcitic marble is attributed to the infiltration process. With such strong infiltration, mineral assemblages such as phlogopite in the absence of K-feldspar, phlogopite + diopside, and wollastonite are commonly observed in the marble. The dolomitic marble was comparatively less depleted in heavy isotopes compared to the calcitic marble and formed under a wide range of X_(CO2) and low fluid-rock ratios (< 1.0). Active decarbonation in dolomitic marble was restricted by silica-poor dolomite protolith, which caused limited fluid infiltration and resulted in high X_(CO2) mineral assemblages represented by dolomite + calcite + diopside + forsterite and more ~(13)C-depleted nature compared to dolomite + calcite + tremolite + forsterite assemblage.