8. CHEMICAL COMPOSITION, AGE, AND SOURCES OF VOLCANICLASTIC SEDIMENTS FROM SITES 917 AND 9181Reinhard Werner,2 Paul van
den Bogaard,2 Christian Lacasse,3
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ABSTRACTVolcaniclastic deposits drilled at Leg 152 Sites 917 (East Greenland Shelf) and 918 (Irminger Basin) comprise 21 Neogene ash-bearing intervals and numerous Paleogene ash layers, tuffs, lapillistones, breccias, and conglomerates. The volcanogenic components of the Neogene ash deposits consist mainly of brown to colorless glass particles with an average median grain size of 30 µm (± 20 µm). Colorless glass particles are bubble wall shards and pumice fragments. Brown glass shards are slightly vesicular or dense blocky shards, rarely highly vesicular or pumiceous. The glass shards have subalkalic and low-K subalkalic basaltic, as well as low-K rhyolitic compositions. Rhyolitic ash layers are attributed to highly explosive plinian eruptions. Textural criteria and degree of sulfur degassing of most mafic shards suggest an origin from hydroclastic explosive fragmentation processes in a subglacial/sublacustrine eruption environment. Major element compositions indicate that most ash deposits are derived from Iceland. Only a few mafic deposits may have originated at the Reykjanes Ridge. Abundant, highly altered Paleogene ash layers in the middle Eocene sequence of Hole 918D confirm a peak in the volcanic activity in the North Atlantic area at this period of time, as postulated in the literature. Polymict volcanic breccias and conglomerates immediately overlying flood basalts of the East Greenland Shelf are interpreted as lahars and fluvial deposits that mark the end of the intense volcanic activity in East Greenland. Altered tuffs and lapillistones within the flood basalt sequence of Site 917 are primary or reworked hydroclastic and pyroclastic deposits, reflecting rare explosive phases during the effusive eruption of the flood basalts. Laser probe 40Ar/39Ar dating of anorthoclase and plagioclase crystals from two successions of Paleogene tuffs and lapilli-stones intercalated with the lavas of the Middle Series at Site 917 give mean apparent ages from 60.5 ± 0.2 Ma to 60.3 ± 0.2 Ma. |
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