Volcanic sandstones were recovered from sites along seamount chains in the North Pacific. Three different modes of origin are proposed for them. The well-rounded, commonly weathered basalt sand and fine gravel from Köko Guyot in the southern part of the Emperor Seamount chain is accompanied by whole and abraded shallow-water fossils, and is epiclastic, reworked at shallow depths. The fine, angular altered hyaloclastite sand from a submarine eruption of the Hawaiian Ridge west of Midway Islands is of turbidity current origin and helped to build an archipelagic apron. The third site, among seamounts and guyots at the east end of the Mid-Pacific Mountains, has volcanic-bearing sediments of mixed texture, composition, and origin: partially epiclastic, partly hyaloclastic, and partly pyroclastic, and largely emplaced by turbidity currents that ponded between the volcanoes. The three types of sediment have had diagenetic histories that reflect, among other factors, their textures, glass, and carbonate content.
Fission track ages of individual crystals of detrital apatite and sphene separated from early Eocene and Paleocene sandstones at Site 555 indicate the contribution of detritus from source rocks of at least three distinct age groups. A major group of Tertiary (56 ± 3 m.y.) apatite ages is compatible with either contemporaneous or near contemporaneous volcanism or rapid unroofing of high level intrusives like those identified previously on Rockall Island (Jones et al., 1972) and Greenland (Gleadow and Brooks, 1979). Cretaceous (100 ± 20 m.y.) apatite ages are consistent with known high level microgabbroic intrusives on Helen's Reef (Roberts et al., 1974). Such apatite ages could also be derived from basement rocks of very ancient terrains, especially around rifted continental margins where low level heating or uplift has affected the fission track ages of apatites (Gleadow, 1978).Minor contributions were also identified from a terrain with apatite ages around 150 to 200 m.y. Similar ages are known from granitic and metamorphic rocks in Greenland (Gleadow, 1978;Gleadow and Brooks, 1979). From experience elsewhere in the North Atlantic Craton (Gleadow, 1978), such apatite ages are also likely in the basement rocks of Rockall, but direct fission track results are lacking.With the identification of at least two nonmetamorphic sources, these data enable a refinement of the source of apatite suggested by heavy mineral studies (Morton, this volume).Two distinct groups of sphene ages of around 60 and 1,380 m.y. are also consistent with sources either on the Rockall Bank or in Greenland (Gleadow, 1978).Apatites from around 900 m depth have a combined age very close to the age of deposition, and they have track length distributions indicating that present temperatures in the hole are at, or close to, the maximum experienced since deposition.
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