HR: 1340h
AN: V43A-1556    [Abstracts]
TI: Geochronology of prograde and eclogite-facies zircon growth, North Qaidam UHP terrane, NW China
AU: * Mattinson, C G
EM: cgm@pangea.stanford.edu
AF: Dept. of Geological & Environmental Sciences, Stanford University, Stanford, CA 94305-2115 United States
AU: Liou, J G
V43A-1556 AF: Dept. of Geological & Environmental Sciences, Stanford University, Stanford, CA 94305-2115 United States
AU: Bird, D K
V43A-1556 AF: Dept. of Geological & Environmental Sciences, Stanford University, Stanford, CA 94305-2115 United States
AU: Wooden, J L
V43A-1556 AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: Wu, C
V43A-1556 AF: Chinese Academy of Geological Sciences, Institute of Geology, Beijing, 100037 China
AU: Yang, J
V43A-1556 AF: Chinese Academy of Geological Sciences, Institute of Geology, Beijing, 100037 China
AB: Amphibolite-facies felsic gneisses near Dulan, at the southeast end of the North Qaidam terrane, enclose minor eclogite and peridotite which record ultra-high pressure (UHP) metamorphism. Field relations, and coesite inclusions in zircons from paragneiss suggest that felsic, mafic, and ultramafic rocks all experienced UHP metamorphism and a common amphibolite-facies retrogression. Cathodoluminescence (CL) imaging of zircons separated from three eclogite samples reveal dark CL cores and medium CL rims + whole grains. SHRIMP-RG U/Pb and REE analyses of the zircon cores (partial overlap on rims) yield high Th/U and U, and ages up to 475 Ma, which places a minimum age on eclogite protolith crystallization. Albite and Th + REE-rich epidote inclusions suggest that the cores record prograde metamorphism, rather than magmatic growth, and REE patterns are steep and contain negative Eu anomalies, suggesting the presence of plagioclase and the absence of garnet during zircon core growth. Blocky polycrystalline clinozoisite inclusions (X_Fe = 0.03--0.14) in garnet cores probably represent lawsonite pseudomorphs, indicating a cold, prograde subduction path. The medium CL zircon rims yield low to moderate Th/U, U, and weighted mean ages of 449 ± 3 Ma, 440 ± 4 Ma, and 422 ± 4 Ma. Garnet, omphacite (X_Jd = 0.3--0.5), and rutile inclusions, and REE patterns (low HREE, no Eu anomalies) demonstrate that these ages reflect eclogite-facies metamorphism. Garnet rims in the youngest sample contain polycrystalline quartz inclusions which probably represent coesite pseudomorphs. The spread in eclogite-facies ages (449--422 Ma) may represent either protracted zircon growth at eclogite-facies conditions, or discrete events prior to tectonic juxtaposition. The ages of eclogite-facies metamorphism obtained in this study are similar to, but slightly younger than ca. 486 Ma HP/UHP metamorphism 400 km NE, near Da Qaidam, and ca. 495 Ma HP metamorphism on the NE side of the Altyn Tagh fault, in the south Altyn Mountains near Bashiwake, supporting the proposed correlation of these localities in a HP/UHP belt.
DE: 1115 Radioisotope geochronology
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3625 Petrography, microstructures, and textures
DE: 3654 Ultra-high pressure metamorphism
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005