Ca-rich Ilvaite-Epidote-Hydrogarnet Endoskarns: a Record of Late-Magmatic Fluid Influx into the Persodic Ilimaussaq Complex, South Greenland
Journal of Petrology
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 1Á16 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
... king in the vicinity of Il|¤ maussaq. Instead, they record late-to 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 epidote þ albite þ andradite-rich garnet AE ilvaite AE 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 FMQ þ 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-bearing minerals, such as primary eudialyte, clinopyroxene or ternary feldspar, and redistribution of the Ca by a metasomatizing 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 À3 mol/l Ca along fractures into the metasomatized rocks. Fluid^rock interaction in the Eriksfjord basalts is documented by abundant chlorite^epidote^q uartz assemblages; high fluid/rock ratios allowed the fluid to retain its seawater oxygen isotope composition.