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

• Published in: Journal of Petrology
• Main Authors: Graser, Gesa, Markl, Gregor
• Format: Text
• Language: English
• Published: Oxford University Press 2008
• Subjects: Original Papers; Greenland; Ilimaussaq
• Online Access: http://petrology.oxfordjournals.org/cgi/content/short/49/2/239; https://doi.org/10.1093/petrology/egm079

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.