IMPRINT OF METEORIC WATER ON THE STABLE-ISOTOPE COMPOSITIONS OF IGNEOUS AND SECONDARY MINERALS, KAP-EDVARD-HOLM-COMPLEX, EAST GREENLAND

Hydrogen and oxygen isotope analyses have been made of hydrous mineral s in gabbros and basaltic xenoliths from the Eocene Kap Edvard Helm in trusive complex of East Greenland, The analyzed samples are of three t ypes: (1) primary igneous hornblendes and phlogopites that crystallize d from partial melts of hydrothermally altered basaltic xenoliths, (2) primary igneous hornblendes that formed during late-magmatic recrysta llization of layered gabbroic cumulates, and (3) secondary actinolite, epidote and chlorite that formed during subsolidus alteration of both xenoliths and gabbros. Secondary actinolite has a delta(18)O value of - 5.8 parts per thousand and a delta D value of - 158 parts per thous and. These low values reflect subsolidus alteration by low-delta(18)O, low-delta D hydrothermal fluids of meteoric origin. The delta D value is lower than the - 146 to - 112 parts per thousand values previously reported for amphiboles from other early Tertiary meteoric-hydrotherm al systems in East Greenland and Scotland, indicating that the meteori c waters at Kap Edvard Helm were isotopically lighter than typical ear ly Tertiary meteoric waters in the North Atlantic region. This probabl y reflects local climatic variations caused by formation of a major to pographic dome at about the time of plutonism and hydrothermal activit y. The calculated isotopic composition of the meteoric water is SD = - 110 +/- 10 parts per thousand, delta(18)O approximate to - 15 parts p er thousand. Igneous hornblendes and phlogopites from pegmatitic pods in hornfelsed basaltic xenoliths have delta(18)O values between - 6.0 and -3.8 parts per thousand and delta D values between - 155 and - 140 parts per thousand. These are both much lower than typical values of fresh basalts. The oxygen isotope fractionations between pegmatitic ho rnblendes and surrounding hornfelsic minerals are close to equilibrium fractionations for magmatic temperatures, indicating that the pegmati tes crystallized from low-delta(18)O partial melts of xenoliths that h ad been hydrothermally altered and depleted in O-18 prior to stoping. The pegmatitic minerals may have crystallized with low primary delta D values inherited from the altered country rocks, but these values wer e probably overprinted extensively by subsolidus isotopic exchange wit h low-delta D meteoric-hydrothermal fluids. This exchange was facilita ted by rapid self-diffusion of hydrogen through the crystal structures . Primary igneous hornblendes from the plutonic rocks have delta(18)O values between + 2.0 and + 3.2 parts per thousand and delta D values b etween - 166 and - 146 parts per thousand. The O-18 fractionations bet ween hornblendes and coexisting augites are close tn equilibrium fract ionations for magmatic temperatures, indicating that the hornblendes c rystallized directly from the magma and subsequently underwent little or no oxygen exchange. The hornblendes may have crystallized with low primary delta D values, due to contamination of the magma with altered xenolithic material, but the final delta D values were probably contr olled largely by subsolidus isotopic exchange, This inference is based partly on the observation that coexisting plagioclase has been extens ively depleted in O-18 via a mineral-fluid exchange reaction that is m uch slower than the hydrogen exchange reaction in hornblende. It is co ncluded that all hydrous minerals in the study area, whether igneous o r secondary, have delta D value that reflect extensive subsolidus isot opic equilibration with meteolic-hydrothermal fluids.