Fluid evolution during the development of scapolite-bearing assemblages in striped calcsilicate layers between metapelites and metacarbonates

초록

Stratified calcsilicate layers intercalated with metapelites and metacarbonates in the northwestern Gyeonggi Massif, Korea, preserve a two-stage metamorphic and metasomatic history that records the evolution of H2O-CO2-NaCl (+/- HCl) fluids in a lithologically heterogeneous crustal environment. Scapolite-bearing assemblages in garnet-bearing and garnet-free layers display distinct microtextural and mineral-chemical features that reflect contrasting fluid-buffering capacities dur-ing medium pressure regional metamorphism. During peak metamorphism, both lithologies acted as locally internally buffered systems, allowing scapolite-I (XCl < 0.1) to form in response to micro-scale variations in XCO2. Clinozoisite + pla-gioclase (oligoclase to andesine) inclusions within scapolite-I, ferro-pargasite inclusions in clinopyroxene, and pyrite enclosed in scapolite-I indicate alternating CO2-consuming and CO2-producing micro-domains under relatively low fO(2), generating centimeter- to meter-scale heterogeneity in XCO2. During retrograde metamorphism, the system evolved into a locally infiltration-dominated regime. Garnet-free layers selectively incorporated externally derived saline fluids, forming scapolite-II (XCl = 0.1-0.3), albitic plagioclase, and Ca-amphibole, recorded by scapolite-II overgrowths on scapolite-I and albitic rims on plagioclase (oligoclase to andesine). In contrast, garnet-bearing layers experienced anorthitization and destabilization of scapolite-I and clinopyroxene, releasing Na-rich and metal-chloride-bearing fluids. These contrasting metasomatic responses established a lithologic-scale Cl-circulation loop linked to variations in XCO(2 )and in the activities of Na+, Ca2+, and Cl-, explaining the selective stabilization of scapolite-II in garnet-free layers and oscillatory zoning in plagioclase. Mineralogical and geochemical evidence suggests that the saline fluids may have derived from deeply sourced metamorphic brines and/or NaCl (+/- HCl)-bearing hydrothermal systems associated with post-collisional alkali magmatism. Scapolite-plagioclase mineral pairs thus provide robust constraints on fluid composition and metasomatic processes during regional metamorphism and subsequent exhumation.

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