SYNOPSIS The Larne No. 2 Borehole is situated in an area where Mesozoic and Tertiary rocks cover the assumed westerly extension of the Midland Valley of Scotland. It penetrated 2880-36 m of New Red Sandstone sediments and volcanic rocks and is one of the deepest boreholes drilled in Ireland. Beneath thin Quaternary clays, a Triassic succession consisting of saliferous Mercia Mudstone Group (958-3 m), overlying Sherwood Sandstone Group (648-3 m) was encountered. Permian Upper Marls (185-4 m), including a thick (113-1 m) basal halite, and Magnesian Limestone (21-6 m) successively underlie the Triassic rocks and with the latter may be correlated with similar sequences in Northern Ireland, England and the Irish Sea. The Permian succession beneath the Magnesian Limestone resembles that of south-west Scotland and comprises 440-7 m of sandstone and pebble conglomerate overlying a hitherto unsuspected 616-7 m proven thickness of intermediate to basic volcanic rocks. These altered lavas have yielded K/Ar ages of c. 245 ± 1 3 Ma and the balance of evidence suggests that they are of early Permian age.

Quaternary, middle Eocene, lower Eocene and Paleocene volcanogenic sediments were cored at DSDP Leg 48 Sites 403 and 404, on the rifted southwest margin of the Rockall Plateau. The Paleocene-lower Eocene tuffaceous sequence is thickest (140 m) at Site 403, but tuff bands are attenuated and diluted by orthoclastic sediment at Site 404. Site 403 tuffs are mainly of granule, sand-grade, and silt-grade with sparse lapilli. Glass shards are almost entirely argillized, but sparse unaltered shards with pumice and microlitic lava particles indicate mainly basaltic parent magmas with minor rhyolite. The Paleocene-lower Eocene cores indicate a succession of mainly effusive eruptive events, probably originating in the Reykjanes Ridge, Iceland province. Heavy mineral suites group as amphibole-epidote, diopside-hypersthene, and augite-olivine; they are largely unrelated to the pyroclasts and were derived mainly from metamorphic basement including granulites and amphibolites. Vitroclastic foraminiferal ooze of Quaternary age at Site 404 contains abundant fresh glass shards of basaltic and minor rhyolitic types. The record of Paleocene to lower Eocene vulcanism greatly extends the lower Tertiary volcanic province of northwest Europe, the North Sea Basins, Iceland, and Greenland into the northeast Atlantic; and correlates widespread magmatism with rifting and subsidence in the North Sea and the major opening of the North Atlantic.

No 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...

Sills of olivine-microgabbro and olivine-dolerite 0.5–180 m thick intrude Lower Liassic (Hettangian–Sinemurian) sediments in the Fastnet Basin, a SW extension of the N Celtic Sea Basin, about Lat. 50°N, Long. 10°W. The evidence of 3 wells and 2300 km of seismic reflection profiles indicates 6 major sills and sill-complexes, and 3 possible igneous centres or plugs. A high-level sill may intrude Lower Cretaceous sediments, and one plug apparently deforms Cretaceous reflectors. K-Ar ages on separated biotites from a thick sill intruding Liassic sediments gave 170 ± 4 Ma (Bajocian). This sill contains 30% olivine (FO 70-73 ), 20% clinopyroxene (magnesian salite), 45% plagioclase (An 60-78 ), &, with accessory magnetite, ilmenite, edenitic hornblende, Ti-phlogopite, apatite, zircon, spinel (in olivine) and sulphides. Alteration to serpentine minerals, chlorite, sericitic mica and leucoxene is sporadic. The magmatism in the Fastnet province thus appears to have been active from the mid-Jurassic to the early Tertiary, related initially to the time of initial rifting between the European and American plates, and subsequently to their final separation between Greenland and Rockall and associated Thulean magmatism.