2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 3
Presentation Time: 8:00 AM-12:00 PM


LEE, Jeffrey1, WANG, Yu2, MCWILLIAMS, Michael3, HOURIGAN, Jeremy3, BLYTHE, Ann4 and MCCLELLAND, William5, (1)Dept. of Geological Sciences, Central Washington Univ, Ellensburg, WA 98926, (2)Dept. of Geology, China Univ. of Geosciences, Beijing, China, (3)Dept. of Geological Sciences, Stanford Univ, Stanford, CA 94305, (4)Dept. of Earth Sciences, Univ. of Southern California, Los Angeles, CA 90089, (5)Dept. of Geological Sciences, Univ of Idaho, Moscow, ID 83844, jeff@geology.cwu.edu

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.