The East Greenland Early Tertiary rifted margin is composed of extensive seaward-dipping reflector sequences (SDRS) and continental flood basalts. The magmatic development during the continental rifting and initiation of oceanic spreading were investigated by researchers on both Leg 152 (Holes 915-919) and Leg 163 (Holes 988-990) of the Ocean Drilling Program. Hole 917A penetrated the early part of a subaerially erupted continental succession divided into a Lower Series of basalt and subordinate picrite, dominantly pahoehoe flows (444 m), and a Middle Series of basaltic and dacitic aa flows and minor tuffs (193 m) (Larsen, Saunders, Clift, et al., 1994). Werner et al. (1998) showed that the volcaniclastic deposits of the Middle Series represented primary or reworked tephra that originated from rare explosive hydroclastic and pyroclastic eruptions during the terminal phases of the effusive continental flood basalt succession. The continental succession was shown to have been derived from a relatively depleted mantle and was modified by crustal contamination (Fitton et al., 1998a, 1998b; Saunders et al., 1998). Thy et al. (1998) suggested that the assimilation of a hydrous silicic component suppressed plagioclase crystallization and permitted extended olivine fractionation and formation of accumulative picrites. Fitton et al. (1998a, 1998b) postulated that granulitic gneiss was the principal contaminant for the lower part of the Lower Series, while above this level, including the Middle Series, contamination by amphibolitic gneiss became increasingly important. The most heavily contaminated lavas were recovered from the Middle Series (Larsen, Saunders, Clift, et al., 1994). Fitton et al. (1998b) conjectured that the compositional variations up through the Lower and Middle Series at Site 917 reflected a temporal decline in the supply of primitive melts to the crust and a progressive shoaling of the site by magma storage and fractionation.

The transition into the oceanic succession is represented by the Upper Series at Site 917 and is composed of basaltic and picritic flows compositionally and morphologically resembling the Lower Series flows (Larsen, Saunders, Clift, et al., 1994; Larsen and Saunders, 1998). The Upper Series is less contaminated than the Lower and Middle Series (Fitton et al., 1998a, 1998b; Saunders et al., 1998). The slight evidence of contamination and the mid-ocean-ridge basalt (MORB)-like source characteristics of most lavas of the Upper Series lead Fitton et al. (1998b) to include the Upper Series in the synrift succession. Fram et al. (1998) showed that trace-element compositions of the succession of Upper Series lavas recorded a rapid removal of the thick continental lithosphere that accompanied plate separation and initiation of subaerial seafloor spreading in the region. This was reflected in an increase in the extent of partial melting and a decrease in pressure of melt separation from the mantle. The oceanic succession was drilled at Site 918 (and 915) in the main portion of seaward-dipping reflectors. In contrast to the interpretation of Fitton et al. (1998b) and Fram et al. (1998), the boundary between the continental and oceanic successions was defined by Larsen and Saunders (1998) by the unconformity and thin sediment horizon between the Middle and Upper Series of Hole 917A.

The principal scientific drilling objectives of Leg 163 were to sample the basal portion of the continental succession (Site 989), to drill into the breakup unconformity, and to document more completely the continent/ocean transition (Site 990). Thereby, Leg 163 would bridge the gaps in the record believed to exist between the Upper Series (Site 917) and lavas recovered at Sites 915 and 918 (Duncan, Larsen, Allan, et al., 1996). Only two units of aphyric basalt were recovered at Site 989. Based on flow composition and morphology, the Shipboard Scientific Party (Duncan, Larsen, Allan, et al., 1996) concluded that the lavas belonged to the oceanic succession and not to the Lower Series. Lesher et al. (Chap. 12, this volume) concluded that this unusual compound flow most likely represents a lava fan delta formed by multiple breakouts from a lava tube at a considerable distance from the eruption site at the axial rift. At Site 990, 13 units of aa and pahoehoe flows were recovered that also resembled the aphyric to phyric basalts of the oceanic succession recovered from Sites 915 and 918 (Duncan, Larsen, Allan, et al., 1996). At Site 988 we drilled north of the main transect into the featheredge of the seaward-dipping reflectors and sampled two flows.

Most of the lava succession of the Southeast Greenland margin displays systematic compositional variations that have previously been related qualitatively to crystal fractionation and assimilation of primitive, mantle-derived melts (e.g., Fitton et al., 1998b; Larsen et al., 1998). Quantitative information on the low-pressure phase relations and liquid lines of descent of these primitive melts was provided in the experimental study of Thy et al. (1998). Here we present new low-pressure experimental data for a number of more evolved compositions belonging to the oceanic succession (Sites 918 and 989) and from the silicic flows of the Middle Series of the continental succession (Site 917). These results constrain the locations of low-pressure cotectics that permit assessments of fractionation assemblages involved in crustal differentiation and of the generation of the silicic lavas of the Middle Series.