Constraining the mid-crustal channel flow beneath the Tibetan Plateau: data from the Nielaxiongbo gneiss dome, SE Tibet
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Gneiss domes involving the South Tibetan Detachment System provide evidence for crustal extension simultaneous with shortening. The Nielaxiongbo gneiss dome is composed of a metamorphic complex of granitic gneiss, amphibolite, and migmatite; a ductilely deformed middle crustal layer of staurolite- or garnet-bearing schist; and a cover sequence of weakly metamorphosed Triassic and Lower Cretaceous strata. The middle crust ductilely deformed layer is separated from both the basement complex and the cover sequence by lower and upper detachments, respectively, with a smaller detachment fault occurring within the ductilely deformed layer. Leucogranites crosscut the basement complex, the lower detachment, and the middle crustal layer, but do not intrude the upper detachment or the cover sequence. Three deformational fabrics are recognized: a N–S compressional fabric (D1) in the cover sequence, a north- and south-directed extensional fabric (D2) in the upper detachment and lower tectonic units, and a deformation (D3) related to the leucogranite intrusion. SHRIMP zircon U–Pb dating yielded a metamorphic age of 514 million years for the amphibolite and a crystallization age of 20 million years for the leucogranite. Hornblende from the amphibolite has an 40Ar/39Ar age of 18 ± 0.3 million years, whereas muscovites from the schist and leucogranite yielded 40Ar/39Ar ages between 13.5 ± 0.2 and 13.0 ± 0.2 million years.These results suggest that the basement was consolidated at 510 Ma and then exhumed during extension and silicic plutonism at 20 Ma. Continuing exhumation led to cooling through the 500°C Ar closure temperature in hornblende at 18 Ma to the 350°C Ar closure temperature in muscovite at 13 Ma. The middle crustal ductilely deformed layer within gneiss domes of southern Tibet defines a southward-extruding ductile channel, marked by leucogranites emplaced into migmatites and amphibolites. We propose a model involving thinned upper crust for the initial extension of the Tibetan Plateau in the early Miocene.
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