Low-pressure melting studies of basalt and basaltic andesite from the Southeast Greenland continental margin and the origin of dacites at Site 917

Abstract

A series of 1-atm. melting experiments on basaltic flows collected from Holes 918D and 989B in the oceanic succession of the East Greenland continental margin can be used to define possible phase equilibria and liquid lines of descent. A sample from Site 918 (Section 152-918D-108R-2) shows the melting order low-Ca pyroxene (1153°C), augite (1182°C), olivine (1192°C), and plagioclase (1192°C). A sample from Site 989 (Section 163-989B-10R-7) melts in the order of low-Ca pyroxene (1113°C), augite (1167°C), plagioclase (1177°C), and olivine (1184°C). In particular, the relatively early appearance of low-Ca pyroxene distinguishes the melting relations for the oceanic succession from those observed for the basaltic continental succession at Site 917. A basaltic andesite flow from the Middle Series at Site 917 (Section 152-917A-27R-4) shows the melting order of low-Ca pyroxene (1142°C), plagioclase (1173°C), and olivine (1173°C). This melting order is difficult to reconcile with the observed large compositional variations in SiO 2 and FeO for the Middle Series, which imply early magnetite fractionation. Major element considerations and rare-earth element modeling of the dacites of the Middle Series suggest that they formed by low extent of melting (<20%) of continental hydrated gabbroic or mafic amphibolite at pressures <8 kbar. These crustally derived melts represent possible contaminants of basaltic magmas of the Lower and Middle Series at Site 917.