Petrology, Mineralogy, and Chemistry of Basaltic Rocks: Leg 81

Abstract

We detail the petrography and mineralogy of 145 basaltic rocks from the top, middle, and base of flow units identified on shipboard along with associated pyroclastic samples. Our account includes representative electron microprobe analyses of primary and secondary minerals; 28 whole-rock major-oxide analyses; 135 whole-rock analyses each for 21 trace elements; 7 whole-rock rare-earth analyses; and 77 whole-rock X-ray-diffraction analyses.These data show generally similar petrography, mineralogy, and chemistry for the basalts from all four sites; they are typically subalkaline and consanguineous with limited evolution along the tholeiite trend. Limited fractionation is indicated by immobile trace elements; some xenocrystic incorporation from more basic material also occurred. Secondary alteration products indicate early subaerial weathering followed by prolonged interaction with seawater, most likely below 150°C at Holes 552, 553A, and 554A. At Hole 555, greenschist alteration affected the deepest rocks (olivine-dolerite) penetrated, at 250-300°C. INTRODUCTIONDeep Sea Drilling Project (DSDP) Leg 81 drilled four sites transecting the rifted "passive" margin of the Rockall microcontinent, with the object of studying the sedimentary and igneous evolution of this type of margin ( Fig. 1). Of particular interest are the age and nature of the westward-dipping reflectors, which dominate shallow seismic records of the Rockall type of margin (Roberts et al., this volume). Leg 81 located Sites 552 and 553 at the midpoint of the transect, and both bottomed in basaltic rocks above or within the upper dipping reflectors after passing through Paleocene and Tertiary successions. In Hole 552, 31.3 m of basalts were encountered and in Hole 553A, 183.0 m of basalts, the lowest 71.0 m corresponding to dipping reflectors.Site 554 was located oceanward of these two sites on the crest of the "outer high," separating the zone of dipping reflectors from the oceanic crust to the west. The outer high lies at the edge of oceanic magnetic Anomaly 24B (52 m.y. in age, corresponding to the initiation of rifting-spreading between Greenland and Rockall). Hole 554A bottomed in 72 m of probable pillow basalts of normal and reverse polarity underlying Pleistocene and Tertiary sediments. The basement of the "outer high" here is formed of overlapping normal-polarity basalts which may represent the first-formed oceanic crust. Site 555 was located midway between Hatton and Edoras banks, some 40 km east of the zone of dipping reflectors drilled at Site 553. The objective here was to compare the subsidence history of these banks with that found at the deeper sites. After penetrating Pleistocene and Miocene sediments, a major hiatus was found between the early Eocene and Miocene. The early Eocene succession contained tuffs and sediments, the former increasing downward and passing into pillow basalts interbedded with autoclastic breccias. These volcanics may represent the dipping reflectors to the west. Late Paleocene marine mudstones underlie the...

40 Ar- 39 Ar ages of lavas from the southeast Greenland Margin, ODP Leg 152, and the Rockall Plateau, DSDP Leg 81

“…These ages are slightly older than the K-Ar ages (52.3 ± 1.7 Ma and 54.5 ± 2.0 Ma) reported by Macintyre and Hamilton (1984). As previously mentioned, the Rockall Plateau lavas have MORB-like compositions (Harrison and Merriman, 1984;Macintyre and Hamilton, 1984), similar to the Sites 915 and 918 lavas, and can be grouped as an oceanic series. Because we have been unable to obtain reliable radiometric ages from the Leg 152 oceanic series, we tentatively assign these lavas to the period 58−57 Ma based on the compositional similarities to the Rockall lavas.…”

“…The lavas at these sites are all consistently MORB-like in composition (Harrison and Merriman, 1984;Macintyre and Hamilton, 1984). The age of the lavas is at least late Paleocene/early Eocene based on the fossil age of the overlying sediments and identification of oceanic magnetic anomalies (Vogt and Avery, 1974).…”

“…To obtain phenocryst and groundmass plagioclase separates from the basalts, samples were crushed and sieved to 0.1−0.5 mm, concentrated using a Franz magnetic separator, and checked for purity under a binocular microscope. The plagioclase separates were briefly washed with 5% HF to remove surficial alter- Larsen, Saunders, Clift, et al (1994), Harrison and Merriman (1984), Joron et al (1984), and Richardson et al (1984). Mineral abbreviations: plag = plagioclase; px = pyroxene; ol = olivine; and cpx = clinopyroxene.…”

The North Atlantic Igneous Province: A review of models for its formation

“…DSDP Leg 81 sampled the Irish SDRS tholeiitic basalt lava flows, extruded under subaerial or very shallow marine conditions. The lavas are all consistently MORB-like in composition (Harrison and Merriman, 1984).…”

Petrology and Geochemistry of Basalts from the Vavilov Basin (Tyrrhenian Sea), Ocean Drilling Program Leg 107, Holes 651A and 655B

“…Basalts from Hole 655B are E-MORB-like, those from Hole 651A progres sively acquire calc-alkaline characteristics as evidenced by the increase of LILE/HFSE ratio and appearance of a negative Nbanomaly. Ayuso et al, 1976, andBryan et al, 1977 (DSDP Leg 11);Flower et al, 1977;Lambert and Hol land, 1977;Blanchard et al, 1976;Gunn and Roobol, 1977 (DSDP Leg 37); Maury et al, 1985 (DSDP Leg 80); Harrison et al, 1984Richardson et al, 1984 (DSDP Leg 81). Samples C, D, and E from Hole 651 A.…”