Dehydration, partial melting, and assimilation of metabasaltic xenoliths in gabbros of the Kap Edvard Holm Complex, East Greenland

in Eocene gabbros of the Kap Edvard Helm Complex in East Greenland, The xenoliths range from centimeters to >100 m in length and consist mainly of hornfelsic augite, orthopyroxene, plagioclase, olivine, magnetite, and ilmenite, with two-pyroxene equilibration temperatures of similar to 1050 degrees C, The hornfelses have been strongly depleted in incompatible elements due to extraction of an anatectic liquid, Melt fractions of roughly 30 to 50 percent are estimated from trace element modeling, Devolatilization or dehydration-melting of xenoliths released hydrous liquids into the gabbroic magma, locally causing replacement of gabbroic cumulates by bodies of peridotite and coarse-grained to pegmatitic gabbro. The xenoliths contain hornfelsic minerals with delta(18)O values as low as -5.5 permil, The low-delta(18)O values were inherited from the country rock protoliths, which had previously been altered be low-delta(18)O hydrothermal fluids of meteoric origin, Oxygen isotope fractionations between plagioclase and pyroxene are close to equilibrium values for magmatic temperatures, Indicating that the xenoliths recrystallized in isotopic equilibrium during heating and subsequently underwent little or no oxygen isotope exchange during subsolidus cooling and alteration of the pluton. Many xenoliths contain igneous bodies that crystallized from trapped partial melts, These include pegmatitic pods consisting of hornblende, andesine, apatite and phlogopite, and plagioclase-rich leucosomes in which pockets of hornblende-rich pegmatite are locally developed, The abundance of hydrous minerals In these melt bodies suggests that melting was facilitated by the presence of H2O in the hydrothermally altered metabasaltic protoliths. Alteration and hydration of the country rock metabasalts thus enhanced the potential for contamination of magmas with hydrous fluids and partial melts.