Journal of Petrology Advance Access published online on December 26, 2007
Journal of Petrology, doi:10.1093/petrology/egm079
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Ca-rich Ilvaite–Epidote–Hydrogarnet Endoskarns: a Record of Late-Magmatic Fluid Influx into the Persodic Ilímaussaq Complex, South Greenland
INSTITUT FÜR GEOWISSENSCHAFTEN, AB MINERALOGIE UND GEODYNAMIK, EBERHARD-KARLS-UNIVERSITÄT, WILHELMSTR. 56, 72074 TÜBINGEN, GERMANY
Received June 26, 2006; Revised typescript accepted November 15, 2007
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Abstract |
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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.
KEY WORDS: agpaite; endoskarn; Ilímaussaq; ilvaite; metasomatism; seawater
*Corresponding author. E-mail: markl{at}uni-tuebingen.de
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