10. METAMORPHOSED SEDIMENTARY (VOLCANICLASTIC?) ROCKS BENEATH PALEOCENE BASALT IN HOLE 917A, EAST GREENLAND MARGIN1Tracy Vallier,2 Lewis Calk,3 Rainer Stax,4 and Alain Demant5 |
ABSTRACTOcean 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 albite-poor 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 of 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. |
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