Heavy minerals from Paleogene sediments at DSDP Leg 81 Holes

Five heavy mineral associations occur in the Paleocene and Eocene sediments recovered during Leg 81 of the Deep Sea Drilling Project (DSDP) in the SW Rockall area. Association 1, consisting of augite, iddingsite, and olivine, was derived from the basaltic rocks of the northern part of the Rockall Plateau. Association 2 consists of epidote group minerals, including piedmontite, and amphiboles of actinolite, actinolitic hornblende, and magnesio-hornblende compositions, and was derived from the metamorphic basement of south Greenland. Association 3 comprises garnet, augite, apatite, and edenitic and pargasitic amphiboles and has a provenance in the southern Rockall Plateau. Associations 4 (garnet, apatite, edenitic/pargasitic amphiboles) and 5 (garnet, apatite) are intrastratal solution derivatives of Association 3, with successive removal of first pyroxene and then amphibole with increasing depth of burial. Throughout the SW Rockall Plateau area there is a significant change in the spectrum of the above assemblages in the lower part of the Eocene. This change has been noted at Sites 403, 404, 553, and 555 and is defined by the last appearance of Association 2. This level therefore marks the cessation of sediment supply from southern Greenland and is the result of the final separation of Rockall and Greenland immediately prior to magnetic Anomaly 24.

The petrology of the lower series volcanics of ODP Hole 104-642E

Between 1086.6 and 1229.4 m below seafloor at Site 642 on the Outer Vøring Plateau, a series of intermediate volcanic extrusive flow units and volcaniclastic sediments was sampled. A mixed sequence of dacitic subaerial flows, andesitic basalts, intermediate volcaniclastics, subordinate mid-ocean ridge basalt, (MORB) lithologies, and intrusives was recovered, in sharp contrast to the more uniform tholeiitic T-type MORB units of the overlying upper series. This lower series of volcanics is composed of three chemically distinct groups, (B, A2, A1), rather than the two previously identified. Flows of the dacitic group (B) have trace-element and initial Sr isotope signatures which indicate that their source magma derived from the partial melting of a component of continental material in a magma chamber at a relatively high level in the crust. The relative proportions of crustal components in this complex melt are not known precisely. The most basic group (A2) probably represents a mixture of this material with MORB-type tholeiitic melt. A third group (A1), of which there was only one representative flow recovered, is chemically intermediate between the two groups above, and may suggest a repetition of, or a transition phase in, the mixing processes.

Heavy minerals, grain size distribution, and ice-rafted debris in the Greenland Sea during 44 to 30 Ma

The widely accepted age estimate for the onset of glaciation in the Northern Hemisphere ranges between 2 and 15 million years ago (Ma). However, recent studies indicate the date for glacial onset may be significantly older. We report the presence of ice-rafted debris (IRD) in ~44 to 30 Ma sediments from the Greenland Sea, evidence for glaciation in the North Atlantic during the Middle Eocene to Early Oligocene. Detailed sedimentological evidence indicates that glaciers extended to sea level in the region, allowing icebergs to be produced. IRD may have been sourced from tidewater glaciers, small ice caps, and/or a continental ice sheet.

Chemical analysis of ODP Hole 104-642E

During Ocean Drilling Program Leg 104 a 900-m-thick sequence of volcanic rocks was drilled at Hole 642E on the Vøring Plateau, Norwegian Sea. This sequence erupted in two series (upper and lower series) upon continental basement. The upper series corresponds to the seaward-dipping seismic reflectors and comprises a succession of about 122 flows of transitional oceanic tholeiite composition. They have been subdivided into several formations consisting of flows related to each other by crystal fractionation processes, magma mixing, or both. Major- and trace-element chemistry indicates affinities to Tertiary plateau lavas of northeast Greenland and to Holocene lavas from shallow transitional segments of the Mid-Atlantic Ridge, such as Reykjanes Ridge. The tholeiitic magmas have been derived from a slightly LREE-depleted mantle source. Two tholeiitic dikes that intruded the lower series derive from an extremely depleted mantle source. Interlayered volcaniclastic sediments are dominantly ferrobasaltic and more differentiated. They appear to come from a LREE-enriched mantle source, and may have been erupted in close vicinity of the Vøring Plateau during hydroclastic eruptions. The two tholeiitic dikes that intruded the lower series as well as some flows at the base of the upper series show evidence of assimilation of continental upper crustal material.