HR: 1340h
AN: T13D-1574    [Abstracts]
TI: New U-Pb SHRIMP-RG Geochronology and Isotope Geochemistry of the Main and Northern Granitoid Belts of NE Russia
AU: * Toro, J
EM: jtoro@wvu.edu
AF: West Virginia University, Department of Geology and Geography, Morgantown, WV 26506, United States
AU: Prokopiev, A V
EM: prokopiev@diamond.ysn.ru
AF: Siberian Branch, Russian Academy of Science, Diamond and Precious Metal Geology Institute, Yakutsk, 677891, Russian Federation
AU: Wiegand, B
EM: bweigand@pangea.stanford.edu
AF: Stanford University, Dept. of Geological and Environmental Sciences, Stanford, CA 94305, United States
AU: Miller, E L
EM: miller@pangea.stanford.edu
AF: Stanford University, Dept. of Geological and Environmental Sciences, Stanford, CA 94305, United States
AU: Wooden, J
EM: jwooden@usgs.gov
AF: U.S. Geological Survey, SHRIMP Lab, Stanford, CA 94305, United States
AB: The Main and Northern granitoid belts of NE Russia extend for 1500 km along the boundary between the Kolyma- Omolon superterrane and the North Asian craton. On the basis of Early Cretaceous 40Ar/39Ar biotite ages (Layer et al., 2001) and of the per-aluminous composition of some of the granitoids, the Main Belt was originally interpreted to have formed by crustal melting during collision of the Kolyma-Omolon superterrane against the North Asia craton (Parfenov, 1991). This interpretation is problematic because there is no evidence that there ever was sufficient crustal thickening in the Verkhoyansk-Kolyma orogen to produce crustal melting. Also, the chemical signature of the granitoids is ambiguous. For example, in a Rb vs. Y+Nb tectonic discrimination diagram the Main Belt compositions overlap those of the Andean granitoids or the Sierra Nevada batholith, and the REE pattern for the Main Belt is similar to that of the Sierra Nevada. New U/Pb SHRIMP-RG zircon ages from 10 granitoids distributed along the length of the Main Belt are Late Jurassic, that is, older the onset of collisional deformation. Specifically, the Dogdo batholith has a 207Pb- corrected 206Pb/238U mean age of 148±2 Ma (n=10/10). The Pravo-Tuostakh batholith is 151±2 Ma (n=9/11). Two samples of the Chibagalakh batholith, which is about 200 km long, have mean ages of 148±2 Ma (n=10/10), and 147±2 Ma (n=9/13) with a second cluster at 154±2 Ma (n=4/13). The Porozhnotsepinsky batholith is155±2 Ma (n=9/11). The Khayargastakh batholith has ages that increase progressively from 142 to 158 Ma with an overall mean of 152±5 Ma (n=8/8). The Chuguluk batholith has a mean age of 151±1 Ma (n=8/9). The isotopic ratios are generally concordant and have very low or no common Pb. Several plutons yielded two populations of Late Jurassic ages with dispersion greater than expected from the analytical uncertainties which we interpret in terms of two zircon components encompasing the time span of magmatism in the area. We only found evidence of significant inheritance of 1.9 to 2.0 Ga crust in zircons of the Trud and Nelkan plutons which are located in the Tas-Kystabyt belt, west of the Main Belt in an area that is underlain by the North Asia craton. Four Northern belt granitoids have 206Pb/238U mean ages ranging from 134 ±0.9 to 126 ±1.2 Ma. Our isotopic studies (in progress) show a pattern of increasing initial 87Sr/86Sr from the Northern belt (0.7052), to the Main Belt (0.7074), to the Tas-Kystabyt Belt (0.714). This reflects progressively greater involvent of crustal material in the source region. The new isotopic data demonstrates that magmatism in the Main Belt started by ~154 Ma and most of the granitoids had been emplaced by ~147 Ma, that is, during the Late Jurassic (Kimmeridgian to Tithonian). Therefore the Main Belt is largely coeval with the Uyandina-Yasachnaya volcanic arc (Oxfordian- Tithonian) located on the western active margin of the Kolyma-Omolon superterrane. The Main belt granitoids were also produced by subduction beneath the Kolyma-Omolon. The Tas-Kystabyt belt requires a second subduction zone along the Verkhoyansk margin of North Asia in order to produce a belt of batholiths on either side of the boundary with the Kolyma-Omolon superterrane.
DE: 1000 GEOCHEMISTRY
DE: 1100 GEOCHRONOLOGY
DE: 8104 Continental margins: convergent
SC: Tectonophysics [T]
MN: 2007 Fall Meeting