40. MAGMATIC DEVELOPMENT OF THE SOUTHEAST GREENLAND MARGIN AND EVOLUTION OF THE ICELAND PLUME: GEOCHEMICAL CONSTRAINTS FROM LEG 1521Andrew D. Saunders,2 Hans Christian Larsen,3 and J. Godfrey Fitton4 |
ABSTRACTLeg 152, located on the southeast Greenland Margin, successfully recovered igneous rocks from three drill sites along a transect at 63°N: 915, 917, and 918. The margin is characterized by a 150-km-wide sequence of seaward-dipping reflectors (SDRS), and thus has an architecture typical of volcanic rifted margins. Site 917 was located close to the inboard, feather-edge of the SDRS, on the continental shelf, and drilling recovered rocks ranging in composition from picrite to dacite. Site 915 was immediately oceanward of Site 917, and Site 918 was located on the continental rise, in the main sequence of the SDRS. Drilling at both Sites 915 and 918 recovered basalt. Ash horizons were cored in the sediment column at Site 918 and Site 919 in the Irminger Basin. The oldest recovered lavas (the Lower and Middle Series from Site 917), erupted approximately 61 m.y. ago, sit on steeply dipping pre-rift metasediments. They are variably contaminated by ancient amphibolite- and granulite-facies crust, consistent with eruption in a continental setting before plate breakup. A few of the Lower Series lavas are high-MgO basalts. The amount of contamination decreased dramatically in the succeeding Upper Series lavas, which also show evidence for a rapid shallowing of the average depth of melting, and contain a high proportion of picrites and high-MgO basalts. These observations are consistent with attenuation of the lithosphere and rapid egress of magmas to the surface during eruption of the Upper Series lavas. Basalt from Site 915 is only slightly contaminated by continental crust, and basalts from Site 918, erupted during C24r time (approximately 56–53 Ma), are uncontaminated. The eruption environment and the composition of the lavas indicate a clear role for the Iceland plume, in that (1) some lava flows from the Lower and Upper Series at Site 917 represent high-MgO magmas and, by inference, had high liquidus and source temperatures; (2) the main sequence of the SDRS were erupted subaerially, thus requiring support from hot, buoyant mantle; and (3) the basalts from Sites 915 and 918 more closely resemble depleted Icelandic tholeiites than normal mid-ocean-ridge basalts. Leg 152 has shown that igneous activity was under way by 61 Ma on the southeast Greenland Margin, at the same time as in West Greenland and the British Isles, over 2000 km apart in their pre-drift locations. |
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