PARTIAL MELTING DURING ASCENT OF GRANITIC XENOLITHS FROM THE PUERCO NECKS, NEW MEXICO: IMPLICATIONS FOR IN SITU MELTING PROCESSES AND FOR CONTAMINATION OF HOST BASALTS

This study investigates in situ partial melting of abundant granitoid xenoliths in Pliocene basaltic necks of the Rio Puerco volcanic field, New Mexico. This setting provides an opportunity to compare partial melting processes in large-volume natural samples with data from experimental melting studies. The samples include granite, granitic gneiss, and charnockite that underwent partial melting following entrainment in the host alkali basalts. Partial melt fractions range from ~3% to ~ 50%. All samples contain clear glass surrounding quartz and feldspars; brown glass is locally present surrounding partially digested Fe-Mg phases. Pyroxene and spinel microlites and plagioclase hopper crystals locally crystallized from the in situ melts. SEM and EMPA analysis show extremely complex melt textures and sharp gradients in melt composition, regardless of the percent melt. SiO2 varies by as much as 8 wt% over short distances, and brown glass is systematically higher in FeO+MgO and lower in alkalis than clear glass in single samples. When plotted on a QAP diagram, most data lie on a linear trend along Or~40%. Overall, the compositions are most consistent with experimental data from muscovite/biotite dehydration melting. Differences in pressure, from an assumed 1 GPa maximum to the surface, control the data trend for low-% melt fractions. Linear trends within single samples likely represent both varying degrees of partial melting and pressure changes during xenolith ascent. Despite the small-scale evidence for disequilibrium, the samples thus mimic general trends observed from equilibrium melting experiments. Cerro Vacio, which contains abundant granitic xenoliths and only rare mantle xenoliths, has modal leucite and the highest normative ne (11-12%) of all 50 necks in the RPV. This neck likely experienced significant contamination via digestion of granitoid xenoliths, and is the only neck at which petrographic evidence of magma mixing between basalt and xenoliths is common. At necks containing abundant mantle xenoliths, even high % melts remained in situ within granitoid xenoliths during ascent. Ascent rate thus likely played a primary role in melt communication between xenoliths and basalt. Melt % was more likely controlled by xenolith composition, size, and depth of entrainment.