(Table 2) Fission track ages of apatite and sphene from Paleogene sediments at DSDP Hole 81-555
2019-11-23T05:18:16Z (GMT) by
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 (Roberts et al., 1973) 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. Sediment depth is given in mbsf. All track densities expressed as for internal surfaces. Neutron dosimetry for reference glass SRM612: B = 6.005 * 109 neutrons/track, Zeta = 360 ± 10. Equivalent to NBS Cu dosimetry via reference glass SRM962.