Ca-rich Ilvaite-Epidote-Hydrogarnet Endoskarns: a Record of Late-Magmatic Fluid Influx into the Persodic Ilimaussaq Complex, South Greenland

Abstract

Endoskarn assemblages involving the Ca-silicates ilvaite, epidote and Ca-rich garnet occur along fracture zones in the persodic Il|¤ maussaq intrusion, South Greenland. The 1 16 Ga intrusion solidified at a depth of about 3^4 km, below a cover of sandstones and pillow-basalts of the Eriksfjord Formation. In contrast to typical skarn assemblages, the Il|¤ maussaq endoskarns contain albite as a main phase and they did not form in metacarbonate rocks, as these are completely lacking in the vicinity of Il|¤ maussaq. Instead, they record lateto post-magmatic interaction of possibly external Ca-rich fluids with the alkaline to agpaitic magmatic rocks. Accordingly, endoskarn textures clearly reflect the magmatic textures of the precursor rocks. Phase relations in two endoskarn varieties with epidoteþ albiteþ andradite-rich garnet ilvaite retrograde prehnite suggest their formation at about 5008C at high oxygen fugacities slightly above the hematite^magnetite oxygen buffer [FMQ (fayalite^magnetite^quartz)þ 5 to FMQþ 7] with later small modifications as a result of fluid influx or cooling of the original fluid at about 300^3508C (formation of prehnite) and at about 200^2508C (oxygen isotopic re-equilibration of the albite). One model for the formation of the observed assemblages is the decomposition of Ca-bearingminerals, such as primary eudialyte, clinopyroxene or ternary feldspar, and redistribution of the Ca by ametasomatizing late-magmatic fluid. Stable isotope (O, H) investigations, however, favour a model in which seawater was the metasomatizing fluid, which entered the Eriksfjord basalts above the intrusion, reacted with them (spilitization) and brought about 10 3 mol/l Ca along fractures into the metasomatized rocks. Fluid^rock interaction in the Eriksfjord basalts is documented by abundant chlorite^epidote^ quartz assemblages; high fluid/rock ratios allowed the fluid to retain its seawateroxygen isotope composition.