HR: 0830h
AN: T31F-0891    [PDF]
TI: Lower Paleozoic Through Archean Detrital Zircon Ages From Metasedimentary Rocks of the Nome Group, Seward Peninsula, Alaska
AU: * Amato, J M
EM: amato@nmsu.edu
AF: New Mexico State University, Department of Geological Sciences MSC 3AB/P.O. Box 30001, Las Cruces, NM 88003 United States
AU: Miller, E L
EM: miller@pangea.stanford.edu
AF: Stanford University, Department of Geological and Environmental Sciences, Stanford, CA 94305 United States
AU: Gehrels, G
EM: ggehrels@geo.arizona.edu
AF: University of Arizona, Department of Geosciences, Tucson, AZ 85721 United States
AB: Metamorphic rocks of Seward Peninsula have been divided into two groups based on their metamorphic grade and history: The Nome Group and the Kigluaik Group. Although it is sometime been assumed that the higher structural position of the Nome Group versus the Kigluaik Group indicates the Kigluaik Group is older, this relationship and the age of the protoliths of these rocks has never been well-established. The Nome Group includes (delete the) lower grade blueschist and greenschist facies rocks which are widespread across the Seward Peninsula (delete) Rock types include pelitic schist, more mafic chlorite-white mica-albite schist, marble, quartzite, and metabasite. An early metamorphic event (pre-120 Ma) occurred at high pressure and relatively low temperature, and is everywhere overprinted by younger deformation and greenschist facies Rare eclogite facies assemblages are preserved in metabasites, and garnet-glaucophane in some of the pelitic schists. The Kigluaik Group includes upper greenschist to granulite facies rocks that are exposed in the core of a gneiss dome. They record a younger event (~91 Ma) that occurred at higher temperatures and resulted in partial thermal overprinting of the Nome Group and upper greenschist to granulite facies assemblages forming in the Kigluaik Group. The Kigluaik Group and equivalent rocks in the Bendeleben and Darby Mountains represent at least in part similar protoliths to many of the units in the Nome Group (Till and Dumoulin, 1994). The boundary between the rocks of the Nome Group and those clearly affected by the second metamorphic event is placed arbitrarily at the "Biotite-in" isograd along the flanks of the gneiss dome. In order to assess the protolith ages and source rock ages for these units, detrital zircon ages were obtained from three samples from the Nome Group, with Kigluaik Group ages forthcoming. LA-MC-ICPMS U/Pb isotope analysis was used for dating. Two samples were collected from the western Kigluaik Mountains near Eldorado Creek and one further south along the Feather River. Each sample yielded 90-105 analyses and all uncertainties are 1 sigma. Chlorite schist MC-74 has a range of ages from the two youngest grains at 484 $\pm$ 18 Ma and 510 $\pm$ 7 Ma to 2984 $\pm$ 2 Ma. Chlorite schist LMC-30 has a youngest grain at 521 $\pm$ 2 Ma and an oldest grain of 2027 $\pm$ 12. Quartz-mica schist LMC-58 also has a youngest grain at 521 $\pm$ 2 Ma and an oldest grain of 2655 $\pm$ 7 Ma. All three therefore have lower Paleozoic zircons, suggesting Lower Cambrian or younger depositional ages. Combining the data from all three rocks results in peaks on a cumulative probability plot at (in descending order of importance): 600 Ma, 683 Ma, 1593 Ma, 522 Ma, and 2985 Ma, with several smaller peaks between 774-1540 Ma and 1685-1960 Ma. Published ages from Nome Group orthogneisses are 680 Ma, suggesting the samples so far analyzed are likely in part sourced from local basement rocks that were eroded to provide ~680 Ma detrital zircons to sedimentary protoliths of part of the Nome Group.
DE: 1035 Geochronology
DE: 8105 Continental margins and sedimentary basins
DE: 9614 Paleozoic
DE: 9619 Precambrian
SC: Tectonophysics [T]
MN: 2003 Fall Meeting