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, Th e xenoliths range from centimeters to >100 m in length and consist mai nly of hornfelsic augite, orthopyroxene, plagioclase, olivine, magneti te, and ilmenite, with two-pyroxene equilibration temperatures of simi lar to 1050 degrees C, The hornfelses have been strongly depleted in i ncompatible elements due to extraction of an anatectic liquid, Melt fr actions of roughly 30 to 50 percent are estimated from trace element m odeling, 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 pe gmatitic gabbro. The xenoliths contain hornfelsic minerals with delta( 18)O values as low as -5.5 permil, The low-delta(18)O values were inhe rited from the country rock protoliths, which had previously been alte red be low-delta(18)O hydrothermal fluids of meteoric origin, Oxygen i sotope fractionations between plagioclase and pyroxene are close to eq uilibrium values for magmatic temperatures, Indicating that the xenoli ths recrystallized in isotopic equilibrium during heating and subseque ntly underwent little or no oxygen isotope exchange during subsolidus cooling and alteration of the pluton. Many xenoliths contain igneous b odies that crystallized from trapped partial melts, These include pegm atitic 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 presenc e of H2O in the hydrothermally altered metabasaltic protoliths. Altera tion and hydration of the country rock metabasalts thus enhanced the p otential for contamination of magmas with hydrous fluids and partial m elts.