28. VOLCANIC ROCKS FROM THE SOUTHEAST GREENLAND MARGIN AT 63°N: COMPOSITION, PETROGENESIS, AND MANTLE SOURCES1J.G. Fitton,2 A.D. Saunders,3 L.M. Larsen,4 B.S. Hardarson,2 and M.J. Norry3 |
ABSTRACTEarly Tertiary volcanic rocks recovered from the southeast Greenland margin represent the transition from continental tholeiitic flood basalt to voluminous oceanic magmatism. This magmatism accompanied continental breakup and resulted in the formation of seaward-dipping reflector sequences (SDRS) characteristic of volcanic rifted margins. The earliest, and most landward, lava flows (Hole 917A, Lower and Middle Series) comprise a pre-breakup continental sequence ranging in composition from olivine basalt to dacite. Evolution in crustal magma reservoirs, with a dwindling supply of primitive magma and an increasing role of crustal contamination, can account for the variations in magma composition. Changes in the inferred nature of the contaminant suggest that the site of magma storage may have moved to shallower levels in the crust during the pre-breakup period. The subsequent eruption of picrite and olivine basalt magmas (Hole 917A, Upper Series) marked a dramatic change in the style of magmatism to one of unrestrained passage of primitive magma from the mantle to the surface during the final stages of breakup. The younger parts of the SDRS (Sites 915 and 918) are composed of compositionally uniform basalt (7.6 ± 0.8% MgO), suggesting that an effective magmatic filtering system was established, soon after breakup, in magma chambers associated with a spreading axis. An increase in degree of mantle melting (<5% to 10%–20%), accompanied by a decrease in depth of melt segregation, marked the transition from continental to oceanic volcanism. The continental volcanic rocks had a garnet lherzolite source, whereas the post-breakup magmas had a shallower, spinel lherzolite source. Most of the older basalts (Site 917, Lower to Upper Series) had a mantle source similar to that of normal mid-ocean-ridge basalt (N-MORB), although a group of flows with compositions similar to typical Icelandic basalt occurs high in the Lower Series. All the post-breakup basalt lava flows (Sites 915 and 918) had a depleted Icelandic mantle source. The head of the Iceland plume may have been zoned at the time of continental breakup with a core of Icelandic mantle surrounded by a carapace of anomalously hot N-MORB-source mantle. |
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