CONTRACTION, EXTENSION, AND DIAPIRISM IN MABJA DOME: IMPLICATIONS FOR TECTONICS OF SOUTHERN TIBET

The Mabja Dome (MD), southern Tibet, is one of a series of gneiss domes located south of the Indus-Tsangpo suture zone and north of the high Himalaya. The MD consists of a migmatitic orthogneiss core mantled by high grade metapelites and orthogneisses. These rocks record two primary deformational events: an older deformational event, D1, characterized by ~EW-trending folds of S0 with an associated moderately N-dipping axial planar foliation, S1, and a younger event, D2, characterized by a domed mylonitic foliation, S2, and associated NS-trending mineral stretching lineation. Peak metamorphism is pre- to early syntectonic with D2 structures. Qualitative thermobarometry suggests peak metamorphic conditions of ~500°C and 2.5 kbar in chloritoid-zone rocks increasing to ~650°C and >10 kbar in kyanite-zone rocks. At the deepest structural levels, sillimanite + staurolite assemblages suggest peak conditions of ~625-675°C and 4.5-7.5 kb. Two-mica granites were emplaced during the latest stages of D2 deformation and yield an U/Pb monazite age of 14.5±0.1 Ma. Muscovite from orthogneisses and metasediments yield Ar/Ar cooling ages of ~12.8 Ma on the northern flank of the dome, increasing to ~17.0 Ma at deeper structural levels, and decreasing to ~13.3 Ma at the deepest structural levels. Apatite fission track (AFT) analyses across the dome yield a mean age of 9.5 Ma indicating symmetric cooling. Two-mica granites yield concordant muscovite and biotite Ar/Ar ages of 13.1-13.5 Ma, Ar/Ar ages of ~11 Ma at the low temperature part of kspar spectra and ages of ~13 Ma at the high temperature part, and AFT ages of 9.2-9.9 Ma. These data indicate cooling rates that vary from ~55-75°C/Ma in the migmatitic core and two-mica granites to ~25-35°C/Ma in the overlying metapelite and orthogneiss. We interpret these structural, metamorphic, and cooling relations as suggesting: (1) contraction (D1) leading to thickening followed by thermal re-equilibration (peak metamorphism), (2) mid-crustal extension (D2), (3) doming, exhumation, and erosion as a consequence of thrusting upward and southward over a north-dipping ramp above cold Tethyan sediments during early to middle Miocene, and (4) reheating of the deepest structural levels coincident with the emplacement of two-mica granites and continued doming and exhumation as a consequence of diapirism and erosion during middle to late Miocene.