HR: 0800h
AN: T41A-1284    [Abstracts]
TI: Onset of Middle Crustal Flow, Southern Tibet: Evidence From U/Pb Zircon Ages
AU: * Lee, J
EM: jeff@geology.cwu.edu
AF: Dept. of Geological Sciences, Central Washington Univ., Ellensburg, WA 98926 United States
AU: Whitehouse, M
EM: Martin.Whitehouse@nrm.se
AF: Laboratory for Isotope Geology, Swedish Museum of Natural History, Stockholm, SE-104 05 Sweden
AB: New U/Pb zircon ion-microprobe ages from core rocks of Mabja Dome constrain the timing and duration of migmatite formation, peak metamorphism, and onset of middle crustal flow in southern Tibet. Mabja Dome is one of several North Himalayan gneiss domes which are characterized by doubly plunging antiforms cored by middle crustal rocks including high grade metapelites, orthogneisses, and migmatites. These rocks record two primary deformational events: D1 characterized by EW-trending folds of S0 with an associated N-dipping axial planar foliation S1, and D2 characterized by a domed mylonitic foliation S2, and associated NS-trending stretching lineation. Peak metamorphism, ranging from 500°C/300 MPa in chtd-zone rocks to 705°C/870 MPa in sill-zone rocks, was syntectonic with D2 and defines a set of isograds which are subparallel to structure. D2 horizontal extension and vertical thinning was simultaneous with development of migmatites and emplacement of a pegmatite and aplite dike swarm; two-mica granites were emplaced after D2 ductile deformation had ceased. Migmatites from the deepest structural levels at Mabja yielded zircons with oscillatory zoned cores and recrystallized or newly grown high U rims and concordant ion-microprobe ages of 532 ± 16 Ma and 35.0 ± 0.8 Ma, respectively. Zircons from a syn-tectonic pegmatite dike yielded an U/Pb age of 23.1 ± 0.8 Ma (Lee et al., in press). The deepest post-tectonic granite yielded zircons with oscillatory zoned cores and overgrowths of high U oscillatory zoned rims; these zircons yielded ion-microprobe ages of ~450 Ma and 16.1 ± 0.4 Ma, respectively. These data suggest that D2 vertical thinning and horizontal stretching, simultaneous with peak metamorphism and migmatization, in the middle crust of southern Tibet began at 35 Ma, was ongoing at 23 Ma and had ceased by 16 Ma. The structural, metamorphic, intrusive, and geochronologic histories in the North Himalayan gneiss domes are similar to those recorded in the Greater Himalayan sequence, suggesting that during Eocene to Miocene time high grade middle crustal rocks were once continuous from beneath the high Himalaya northward beneath southern Tibet. Exposures of the Greater Himalayan sequence have been interpreted as the leading edge of an eroding and southward extruding tabular or wedge-shaped body of ductile middle-crustal rocks bounded above by the STDS and below by the MCT. We suggest that the once hot and weak middle crustal rocks now exposed in the core of the North Himalayan gneiss domes represent the interior of such a middle-crustal channel and that onset of flow within this channel began in the late Eocene. To maintain strain compatibility, slip along the STDS and MCT must have begun in the late Eocene to accommodate the onset of middle crustal flow.
DE: 8102 Continental contractional orogenic belts and inversion tectonics
DE: 8108 Continental tectonics: compressional
DE: 8109 Continental tectonics: extensional (0905)
DE: 8110 Continental tectonics: general (0905)
SC: Tectonophysics [T]
MN: Fall Meeting 2005