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Ocean Drilling Program Leg 152 drilled Hole 917A first through Quaternary and Eocene sediments, then through about 779 m of Paleocene basalt and interlayered dacite that comprise the dipping seismic reflectors along the southeast Greenland margin, and thence into about 53.7 m of steeply dipping and metamorphosed sandstone and siltstone. Ten centimeters of quartz sandstone, probably of fluvial origin, was recovered between the basalt and metamorphosed sedimentary rocks. The sedimentary rocks, believed to be in part volcaniclastic, are mostly well-bedded turbidites and laminites and, in places, are intensely burrowed. Minerals are characteristic of the greenschist facies and consist of albite, chlorite, quartz, graphite, white mica, and epidote with lesser amounts of biotite, sphene/leucoxene, and pyrite. Normalized SiO2 contents range from about 50 to 60.6 wt% and average about 54.4 wt%. Na2O is highly variable with albite-rich specimens having as much as 7.6 wt% and albitepoor specimens as little as 1.5 wt%. K2O contents range from a low of 0.04 wt% to a high of 2.32 wt%; variation is related to white mica content. Organic carbon contents range from about 0.3 wt% to 3.28 wt%, with an average of about 1.0 wt%. Mineralogy and chemistry suggest a volcanic (basaltic) source. Temperature of metamorphism was greater than 350°C, and possibly as high as 450°C, as shown by the crystallinity of graphite, chlorite geothermometry, and the presence of metamorphic biotite. The characteristics of similar onshore rocks from East Greenland, within the Late Cretaceous (Maastrichtian)–early Paleocene Ryberg Formation, may suggest a correlation with the metamorphosed sedimentary rocks recovered near the base of Hole 917A. We presume that the sediments were deposited in a basin during early rifting of the continents that led to the formation o the North Atlantic Ocean basin, possibly during the Late Cretaceous–earliest Paleocene interval. The original sediments were tilted, raised above sea level and eroded, and then buried beneath a thick section of basalt and dacite. Metamorphism at high temperatures and low pressure (2–4 kg?) probably took place prior to burial by the volcanic cover. The entire sequence was faulted and tilted during subsequent opening and spreading apart of the North Atlantic Ocean basin. ing 52o a rs, edINTRODUCTION Metamorphosed sedimentary rocks were unexpectedly recovered beneath a thick pile of Paleocene basalt and dacite in Hole 917A (Figs. 1, 2). Outcrops of crystalline Precambrian rocks along the east coast of Greenland, about 50 km west of Site 917, led us to believe that similar crystalline rocks would be encountered beneath the Tertiary volcanic rocks. However, pre-basaltic sediments, in part as thin layers, are fairly widespread farther north along the margin (see Larsen, 1980, for review). Thus, the occurrence of the metamorphosed sedimentary rocks has important implications for the early opening of the North Atlantic Ocean basin, and for the tectonic and hydrothermal regimes that led to metamorphism. This paper (1) describes the structure, stratigraphy, sedimentology, mineralogy, and chemistry of the rocks; (2) interprets the source rocks and environment of deposition of the sediments; (3) speculates about temperatures, pressures, and ion mobilities during metamorphism; and (4) briefly discusses the implications of these rocks for North Atlantic Ocean basin history. We studied thin sections of 25 samples; obtained X-ray diffraction (XRD) patterns of 13 specimens; conducted carbon and carbonate analyses of 13 samples; submitted 16 samples (two on board) for 1Saunders, A.D., Larsen, H.C., and Wise, S.W., Jr. (Eds.), 1998. Proc. ODP, Sci. Results,152: College Station, TX (Ocean Drilling Program). 2U.S. Geological Survey, MS-919, 345 Middlefield Road, Menlo Park, CA 94025, U.S.A. tvallier@usgs.gov 3U.S. Geological Survey, MS-910, 345 Middlefield Road, Menlo Park, CA 94025, U.S.A. 4Institute for Geology and Mineralogy, University of Erlangen, Schlossgarten 5, 91054 Erlangen, Federal Republic of Germany. 5Laboratoire de Pétrologie Magmatique, Université d’Aix-Marseille III, 1339 Marseille Cedex 20, France. major, minor, and trace element chemistry; and completed microprobe analyses on chlorite and albite in selected polished thin sections. We attempted to separate muscovite grains for K-Ar radiometric dating, but the very fine grain size precluded a clean separation by the techniques we used. Details of methods used in the analyses are described in the sections that present the data. STRUCTURE, STRATIGRAPHY, AND SEDIMENTOLOGY A total of 874.9 m was cored in Hole 917A (Shipboard Scientific Party, 1994) which included, from top to bottom: (1) Quaternary glaciomarine sediments; (2) middle Eocene siltstone; (3) a small interval (about 4 m) of weathered basalt interlayered with conglomerate, breccia, and paleosols; (4) a thick (about 775 m) sequence of Paleocene basalt and dacite in three units or series (for age of these, see Sinton and Duncan, this volume); (5) a very thin (about 10 cm) layer of quartz sandstone; and (6) a sequence of metamorphosed sedimentary (volcaniclastic?) rocks that were encountered at a depth of about 821 meters below seafloor (mbsf). Approximately 15.9 m of these sedimentary rocks was recovered within the 53.7 m cored (Cores 152-917A-103R through 110R) for a recovery rate of about 30%. These metamorphosed volcaniclastic(?) sedimentary rocks in the bottom unit of Hole 917A are the subject of this paper. The metamorphosed sedimentary rocks, originally sand, silt, clay, and possibly tuffs, were lithified and then deformed. Bedding dips within the sequence range from about 30° −35° to overturned, and all beds dip more steeply (Fig. 3) than the overlying lava flows. Bedd dips increase downcore from about 30° near the top of Core 1 917A-103R to vertical (and overturned) in Core 104R, and then t steep dip of about 70° −80° in Cores 105R–110R (Larsen, Saunde Clift, et al., 1994). The presence of microfaults indicates that the s 7

Metamorphosed sedimentary (volcaniclastic?) rocks beneath Paleocene basalt in Hole 917A, East Greenland Margin