Composition of volcanic rocks from the Southeast Greenland margin, Leg 163; major and trace element geochemistry

During Leg 163, in the Southeast Greenland margin, drilling penetrated a thick succession of volcanic rocks that erupted during breakup of the North Atlantic in the early Tertiary. Samples recovered during Leg 163 supplement the samples recovered during Leg 152 from the transect across the volcanic margin at 63 °N. During Leg 163, drilling at Site 989 was intended to recover the oldest part of the continental prebreakup series, and drilling at Site 990 was intended to penetrate the transition zone from synbreakup, compositionally variable volcanic products, to postbreakup volcanics with a limited compositional range and a depleted chemical character similar to mid-ocean-ridge basalts (oceanic character). All the lava flows recovered from Sites 989 and 990, however, have an oceanic chemical character with low contents of incompatible elements and high contents of Sc. A dikelet from Site 990 and a previously drilled dike from Site 917 are likewise oceanic. We consider that the two drilled lava flows from Site 989 were emplaced after breakup despite their setting on the innermost part of the continental margin. The succession at Site 990 consists of 13 units of lava flow that show a slight compositional development upsection with Mg# decreasing from ~62 to ~49. This variation is within the limits known from the slightly younger oceanic succession drilled earlier at Site 918, in which the variation is thought to reflect fractionation in magma chambers in the oceanic crust. The ree stablishment of magma chambers after the breakup must have been achieved during the interval that the short, undrilled lava succession between uppermost Site 917 and lowermost Site 990 was deposited. All the lava flows from Sites 989 and 990, except perhaps one (Unit 989-1), are crustally contaminated, as judged from their high Ba/Zr (>0.42) ratios. The two dikes appear to be uncontaminated. The contamination most likely took place in magma chambers in the young oceanic rift, which, at this early stage of spreading, could still have contained fragments of continenta l crust. Most lavas were probably erupted within the rift and flowed subaerially away from it toward the edge of the continent. The dikes show that lateral injection of magma into the continental crust also took place without resulting in crustal contamin ation. The oceanic lava flows at Site 989 on the innermost margin have probably been erupted through such laterally injected dikes. The primary magma for the postbreakup rocks is estimated to have contained ~18% MgO. After fractionation of 30 mol% olivine (Fo 91–82 ), it shifted to gabbro fractionation, and the magmas started to erupt. The modal composition of the gabbroic cumulate is 8% olivine (Fo 82–74 ), 50% plagioclase (An 76–66 ), and 42% clinopyroxene (Fs 9–15 ). The erupted oceanic magmas are produced by <24 mol% gabbro fractionation, with an average of 14%. Thus, geochemical modeling indicates that the oceanic crust is composed of olivine cumulates (30%), gabbro cumulates (14%), and melts (lava flows and dikes, 56%), which is in accordance with models based on geophysical data.