HR: 16:35h
AN: T24A-03    [Abstracts]
TI: Geology of the North Qaidam HP-UHP Terrane, Western China
AU: * Menold, C A
EM: cmenold@ess.ucla.edu
AF: University of California, Los Angeles, 595 Charles Young Dr., Los Angeles, CA 90095-1567 United States
AU: Manning, C E
EM: manning@ess.ucla.edu
AF: University of California, Los Angeles, 595 Charles Young Dr., Los Angeles, CA 90095-1567 United States
AU: Yin, A
EM: yin@ess.ucla.edu
AF: University of California, Los Angeles, 595 Charles Young Dr., Los Angeles, CA 90095-1567 United States
AU: Chen, X
EM: xhchen@cags.net.cn
AF: Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing, 100081 China
AB: The Early Paleozoic North Qaidam ultrahigh-pressure (UHP) metamorphic belt is a recently discovered UHP terrane, located along the southwestern boundary of the Qilian Shan orogenic belt, Western China. It preserves Precambrian to Mesozoic rocks with continuous exposure over tens of kilometers. Pre-Mesozoic rocks can be divided into two units: epidote-amphibolite grade late Proterozoic rocks (Dakendaben gneiss) which include heterogeneous biotite gneiss, amphibolite, and quartzo-feldspathic gneiss containing eclogite and minor garnet peridotite. The second major unit is a suite of early Cambrian (545.5$\pm$6.0 Ma) epidote-amphibolite facies ophiolitic rocks: vari-textured gabbro, plagiogranite, mafic dikes, vesicular basalt, and metasediments (marble, chert and flysch). The contact between the ophiolite and UHP gneiss is folded and strongly transposed by later ductile folding, making it approximately parallel to the pervasive regional foliation (N10-45$\deg$W), with top to N and top-NE sense kinematic indicators which become subhorizonital when the effects of Cenozoic thrusting are removed. Serpentinized ultramafic bodies commonly occur close to this contact. Middle Paleozoic granites (510-400 Ma) intrude the belt placing a lower bound on the cessation of the ductile phase of deformation. Regional muscovite cooling ages of 460-360 Ma indicate that the metamorphic complex reached the middle crust in the Ordovician and unmetamorphosed near-source Devonian and Carboniferous sediments lie unconformably on the ophiolite indicating that the metamorphic complex was at the surface by 360 Ma. Early Cenozoic imbricate thrust faults placed UHP gneiss over unlithified Tertiary sediments of the Qaidam Basin and generated broad regional folding and repetition of UHP units. Preliminary data indicate regional variations in age and peak metamorphic pressure within the eclogite-bearing gneisses. U/Pb dating of metamorphic zircon from individual eclogite blocks yields statistically different ages of UHP metamorphism, ranging from 425$\pm$7 (MSWD = 0.26) to 523$\pm$8 Ma (MSWD = 3.9). Preliminary regional thermobarometry also indicates spatial variation in peak pressure ranging from 25 to 20 kbar. No regional variation in peak T has been identified. A random spatial distribution of peak pressure and metamorphic zircon age implies mixing of blocks within the eclogite-bearing gneiss during the early stages of exhumation. Whereas, a systematic spatial variation in pressure and time imply a coherent unit which preservation of geometric relationships developed during UHP metamorphism. Preliminary data in the North Qaidam support the former and predicts regionally variable pressure-temperature-time (P-T-t) paths.
DE: 8102 Continental contractional orogenic belts
DE: 3660 Metamorphic petrology
DE: 1035 Geochronology
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting