HR: 10:35h
AN: T22C-02    [Abstracts]
TI: Large-Scale, Long-Lived Subduction of Ultrahigh-Pressure Terranes: Western Gneiss Region, Norway
AU: * Kylander-Clark, A R
EM: akylander@umail.ucsb.edu
AF: University of California, Santa Barbara, Department of Earth Science University of California Webb Hall, BLDG 526, Santa Barbara, CA 93106-9630, United States
AU: Hacker, B R
EM: hacker@geol.ucsb.edu
AF: University of California, Santa Barbara, Department of Earth Science University of California Webb Hall, BLDG 526, Santa Barbara, CA 93106-9630, United States
AU: Johnson, C M
EM: clarkj@geology.wisc.edu
AF: University of Wisconsin, Madison, 1215 W Dayton St., Madison, WI 53706, United States
AU: Beard, B L
EM: beardb@geology.wisc.edu
AF: University of Wisconsin, Madison, 1215 W Dayton St., Madison, WI 53706, United States
AU: Corfu, F
EM: fernando.corfu@geo.uio.no
AF: University of Oslo, P.O.Box 1047 Blindern, Oslo, CA NO-0316, Norway
AU: Mahlen, N J
EM: mahlen@geology.wisc.edu
AF: University of Wisconsin, Madison, 1215 W Dayton St., Madison, WI 53706, United States
AB: Recent Lu-Hf and Sm-Nd ages of garnets and a U-Pb age of zircon of eclogites from the Western Gneiss Region (WGR) ultrahigh-pressure (UHP) terrane, Norway, demonstrate that eclogite-facies metamorphism occurred over a large area (60,000 km²) for an unexpectedly long time. This observation stands in stark contrast to the general belief that continental subduction, and attendant (U)HP metamorphism, occurs over short timescales. Four HP eclogites (~700-800°C, ~2.0-2.5 GPa) from the central WGR yielded equivalent Lu-Hf ages of ~416 Ma; three of these samples gave Sm-Nd ages of ~400 Ma. Given the distribution coefficients for Lu and Sm, the older Lu-Hf ages reflect prograde growth, but are younger than the initiation of garnet crystallization. The younger Sm-Nd ages represent either eclogite-facies cooling through the blocking temperature of the Sm-Nd system or an ‘average' age of garnet growth. Both cases imply >16 m.y. of eclogite- facies conditions. Two UHP eclogites (~750-850°C, ~3 GPa) from the same region yielded significantly younger, but equivalent Lu-Hf and Sm-Nd ages of ~380 Ma, which likely indicate passage through the blocking temperature of both systems up to 20 m.y. after the HP eclogites had passed through the blocking temperature. Because these eclogites are unretrogressed, their ages are the youngest known for eclogite stability in the WGR. An eclogite from the northern WGR yielded a Sm-Nd age of 413.9 ± 3.7 Ma. This could represent a different HP history than that of the central WGR: U/Pb ages in the north are also ~15 m.y. older. Two HP (~650°C, ~2 GPa) eclogites from the southern WGR yielded Lu-Hf ages of 410.2 ± 3.1 and 427.5 ± 7.7 Ma, indicating a similar garnet growth history to the central WGR eclogites. A retrogressed eclogite from the undated eastern portion of the WGR gave a ²⁰⁶Pb/²³⁸U age of 408.0 ± 1.7 Ma. Thermal models mirror results from similar studies (Roselle et al., 2002), and confirm that slow subduction likely produced the P-T-t record observed in the WGR. For subduction of a continent into a formerly Andean-style subduction zone, Calculated temperatures of ~700-800°C are reached at 100 km depth only at burial rates slower than 4 mm/yr. These same temperatures are achieved with slow burial (4-7 mm/yr) during intracontinental subduction.
DE: 1115 Radioisotope geochronology
DE: 3611 Thermodynamics (0766, 1011, 8411)
DE: 3654 Ultra-high pressure metamorphism
DE: 8104 Continental margins: convergent
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting