HR: 1340h
AN: V43D-1645    [Abstracts]
TI: Ages of Igneous Basement From the Komandorsky Islands, Far Western Aleutian Ridge
AU: * Layer, P W
EM: player@gi.alaska.edu
AF: University of Alaska Fairbanks, Geophysical Institute and Dept. of Geology and Geophysics, Fairbanks, AK 99775,
AU: Scholl, D W
EM: dscholl@usgs.gov
AF: Stanford University, Dept. of Geophysics, Palo Alto, CA 94305,
AU: Newberry, R J
EM: ffrn@uaf.edu
AF: University of Alaska Fairbanks, Dept. of Geology and Geophysics, Fairbanks, AK 99775,
AB: Argon-argon ages from four mafic/intermediate rocks from Bering and Medny islands in Komandorsky Islands point to a varied and protracted history for the evolution of the far western Aleutian Ridge, and to the north Pacific as a whole. The oldest sample is low-K tholeiitic basalt with an 40Ar/39Ar isochron age of 46.2 ± 1.5 Ma and some excess argon. It is not significantly altered and compositionally represents immature island arc basalt. This age is identical to the oldest age obtained from dredge samples from the central Aleutians (Jicha et al., 2006, Geology v. 34) and is the oldest reported 40Ar/39Ar age from exposed arc-related igneous rock for the entire Aleutian Ridge. A second sample is similar in composition but contains abundant Na-rich zeolite. It yields a slightly younger age of 42.1 ± 1.8 Ma, perhaps reflecting alteration-related argon loss. Our Eocene basement ages are significantly younger (~10 m.y.) than the paleontologically-based late Paleocene to earliest Eocene age for overlying sedimentary units, which had previously been regarded as representing the oldest ages from the Aleutian Ridge. The most securely dated igneous basement now places the early phase of arc building in the early Eocene at ~47-50 Ma, not Paleocene. The youngest two samples are hornblende- bearing high Mg# andesite ('adakite'), compositionally similar to, but more primitive than, previously-described rocks from the region (Yogodzinski et al.,1995, GSA Bull., v. 107). They have 'classic' adakite trace element characteristics, including high Sr/Y (50-160), low Y (<11 ppm), and high Ni (>80 ppm) and Cr (>100 ppm) and yield ages of 27.4 ± 0.2 and 20.7 ± 0.5 Ma. We interpret these to reflect a transtensional period of deformation in the Komandorsky Islands, which since at least the Oligocene, has resided along the right-lateral transform plate boundary separating the Pacific and North American plates. These Oligocene to Early Miocene ages are significantly older than previously reported Aleutian adakite ages of Middle to Late Miocene and are similar to the initial opening (by spreading) of the Bering Sea's Komandorsky Basin immediately north of the Komandorsky Islands. We consequently suggest that the adakite suite origin is related to formation of a major tear in the subducting Pacific plate beneath the Komandorsky sector.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1065 Major and trace element geochemistry
DE: 1115 Radioisotope geochronology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting