Ca-rich ilvaite-epidote-hydrogarnet endoskarns: A record of late-magmatic fluid influx into the persodic Ilímaussaq complex, South Greenland

Abstract

Endoskarn assemblages involving the Ca-silicates ilvaite, epidote and Ca-rich garnet occur along fracture zones in the persodic Ilímaussaq 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 Ilímaussaq endoskarns contain albite as a main phase and they did not form in metacarbonate rocks, as these are completely lacking in the vicinity of Ilímaussaq. 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 + albite + andradite-rich garnet ± ilvaite ± retrograde prehnite suggest their formation at about 500°C at high oxygen fugacities slightly above the hematite-magnetite oxygen buffer [FMQ (fayalite-magnetite-quartz) + 5 to FMQ + 7] with later small modifications as a result of fluid influx or cooling of the original fluid at about 300-350°C (formation of prehnite) and at about 200-250°C (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 10-3 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 seawater oxygen isotope composition. © The Author 2007. Published by Oxford University Press. All rights reserved.