HR: 0800h
AN: GP41A-0822    [Abstracts]
TI: New Geochemical and Geochronological Constrains on the Tectonic Evolution of the Okhotsk Terrane
AU: Bakharev, A
EM: bakharev@diamond.ysn.ru
AF: Diamond and Precious Metal Geology Institute, 19 Lenin Av., Yakutsk, 678980 Russian Federation
AU: * Prokopiev, A
EM: prokopiev@diamond.ysn.ru
AF: Diamond and Precious Metal Geology Institute, 19 Lenin Av., Yakutsk, 678980 Russian Federation
AU: Hourigan, J
EM: jeremy.hourigan@yale.edu
AF: Dept. of Geology and Geophysics, Yale University, New Haven, CT 06520 United States
AU: Toro, J
EM: jtoro@wvu.edu
AF: Dept. of Geology and Geography, West Virginia University, Morgantown, WV 26506 United States
AB: The Okhotsk Terrane is a continental block with Proterozoic basement which lies east of the South Verkhoyansk fold-and-thrust belt of eastern Siberia. It has been proposed that thrusting in the South Verkhoyansk occurred as a result of Late Jurassic-Early Cretaceous collision of the Okhotsk Terrane against the North Asia craton followed by plate convergence that produced the Uda-Murgal volcanic arc. Ten zircon grains from a biotite plagiogneiss of the Upper Maya uplift, analyzed using the Stanford/USGS SHRIMP-RG, yielded a weighted mean $^{207}$Pb/$^{206}$Pb age of $2624\pm13$ Ma, corrected for $^{204}$Pb. TIMs analysis yielded a statistically undistinguishable upper intercept of $2595\pm26$ Ma. These ages are similar to those reported from the hornblende granulite complex of the Kukhtuy uplift of the central Okhotsk Terrane. 2.6 Ga ages are also common in the North Asia craton. The Neoarchean gneisses of the Upper Maya are intruded by meta-aluminous biotite-hornblende granodiorites and quartz diorites of the Mastakh pluton. Mineralogical composition, REE distribution, and negative Ta, Nb, Zr, and Ti anomalies of the Mastakh pluton are comparable to continental margin, subduction-related plutons in western North America and the Urals. Ten zircons separated from the Mastakh pluton yielded a $^{207}$Pb-corrected weighted mean $^{238}$U/$^{206}$Pb age of $376\pm4$ Ma (Late Devonian). This age suggests that the Mastakh pluton is related to Late Devonian calc-alkaline volcanic rocks of the Matiy formation that are widespread in the Okhotsk Terrane. $^{40}$Ar/$^{39}$Ar analysis of biotites from the Mastakh pluton yielded a weighted mean plateau age of $355\pm1$Ma (Early Carboniferous), therefore that the pluton underwent slow cooling after emplacement. During the Middle Devonian to Early Carboniferous the eastern margin of the North Asia craton underwent a regional episode of rifting. Several major rift grabens and aulacogens formed along the Verkhoyansk margin. The best known of these is the Vilyui basin, but the stratigraphic section of the Sette-Daban zone of the South Verkhoyansk also contains thick rift-related strata. Rift structures are also associated with major fields of basaltic dikes, which in the South Verkhoyansk strike north-south. Stratigraphic links suggest that the Okhotsk Terrane formed part of the North Asia craton until the Middle Paleozoic. We hypothesize that in the Late Devonian to Early Carboniferous the Okhotsk Terrane rifted and moved a relatively small distance away. Because of the existence of coeval subduction-related magmatism on the Okhotsk Terrane, we view this event as back-arc rifting. A poor quality $^{40}$Ar/$^{39}$Ar from phyllites of the Sette-Daban zone suggests that compressional deformation was on-going in the Verkhoyanks fold-and-thrust belt by Latest Jurassic or early Neocomian time. This compressional deformation likely resulted from closure of this back-arc basin and oblique collision of the Okhotsk Terrane against the Verkhoyansk margin along the Bilyakchan fault. Subduction-related magmatism of the Uda-Murgal arc system continued on the Okhotsk Terrane during Triassic to Early Cretaceous time. At about 120 Ma magmatism stepped inboard, perhaps due to flattening of the subduction zone, producing a belt of large calc-alkaline plutons in the core of the Verkhoyansk belt.
DE: 9320 Asia
DE: 8102 Continental contractional orogenic belts
DE: 8110 Continental tectonics--general (0905)
DE: 1035 Geochronology
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting