Oligocene-Miocene middle crustal flow in southern Tibet: geochronology of Mabja Dome
J. Lee
W. McClelland
Y. Wang
A. Blythe
M. McWilliams
10.6084/m9.figshare.3454367.v1
https://geolsoc.figshare.com/articles/dataset/Oligocene-Miocene_middle_crustal_flow_in_southern_Tibet_geochronology_of_Mabja_Dome/3454367
<p>New U-Pb zircon, monazite, <sup>40</sup>Ar/<sup>39</sup>Ar, and apatite fission track ages provide constraints on the timing of formation and exhumation of the Mabja Dome, southern
Tibet, shed light on how this gneiss dome formed, and provide important clues on the tectonic evolution of middle crustal
rocks in southern Tibet. Zircons from a deformed leucocratic dyke swarm yield a U-Pb age of 23.1 ± 0.8 Ma, providing the first
age constraint on the timing of middle crustal ductile horizontal extension in the North Himalayan gneiss domes. Zircons and
monazite from a post-tectonic two-mica granite yield ages of 14.2 ± 0.2 Ma and 14.5 ± 0.1, respectively, indicating that vertical
thinning and subhorizontal stretching had ceased by the middle Miocene. Mica <sup>40</sup>Ar/<sup>39</sup>Ar ages from schists and orthogneisses increase structurally down-section from 12.85 ± 0.13 Ma to 17.0 ± 0.19 Ma and then
decrease at the deepest structural levels to 13.29 ± 0.09 Ma. Micas from the leucocratic dyke swarm and post-tectonic two-mica
granites yield similar <sup>40</sup>Ar/<sup>39</sup>Ar cooling ages of 13.48 ± 0.13 to 12.84 ± 0.08 Ma. The low-temperature steps of potassium feldspar <sup>40</sup>Ar/<sup>39</sup>Ar spectra yield ages of <em>c.</em> 11.0–12.5 Ma and apatite fission track analyses indicate the dome uniformly cooled below <em>c.</em> 115°C at 9.5 ± 0.6 Ma. Based on these data, calculated average cooling rates across the dome range from <em>c.</em> 40–60°C/million years in schist and orthogneiss and following emplacement of the leucocratic dyke swarm, to <em>c.</em> 350°C/million years following emplacement of the two-mica granites. The mylonitic foliation, peak metamorphic isograds, and
mica <sup>40</sup>Ar/<sup>39</sup>Ar chrontours are domed, whereas the low-temperature step potassium feldspar <sup>40</sup>Ar/<sup>39</sup>Ar and apatite fission track chrontours are not, suggesting that doming occurred between 13.0 and 12.5 Ma and at temperatures
between 370 and 200°C. Our new ages, along with field, structural and metamorphic data, indicate that the domal geometry observed
at Mabja developed by middle-Miocene southward-directed thrust faulting upward and southward along a north-dipping ramp above
cold Tethyan sediments. The structural, metamorphic and geochronologic histories documented at Mabja Dome are similar to those
of Kangmar Dome, suggesting a common mode of occurrence of these events throughout southern Tibet. Vertical thinning and horizontal
stretching, metamorphism, generation of migmatites, and emplacement of leucogranites in the domes of southern Tibet are synchronous
with similar events in the Greater Himalayan Sequence that underlie the high Himalaya. These relations are consistent with
previously proposed models for a ductile middle-crustal channel bounded above by the South Tibetan detachment system and below
by the Main Central thrust in the High Himalaya that extended northward beneath southern Tibet.
</p>
2016-06-21 12:04:42
Main Central thrust
Mabja Dome
middle crustal ductile
middle crustal rocks
dome
apatite fission track ages
apatite fission track analyses
leucocratic dyke swarm
Tibet
apatite fission track chrontours
Ar
Geology