TIMING OF METAMORPHISM, DUCTILE DEFORMATION, AND EXHUMATION IN THE LHAGOI KANGRI DOME, SOUTH-CENTRAL TIBET

The Lhagoi Kangri dome (LKD) is an orthogneiss- and granite-cored structural dome located in south-central Tibet. The dome is mantled by a ~6 km-thick succession of siliciclastic and minor carbonate rocks that range from weakly metamorphosed at the highest structural levels to upper amphibolite facies at the base of the section. Monazite petrochronology, thermobarometry, and microstructural analysis of metapelitic rocks from the LKD constrain its pre-doming geologic history and provide insight into the relationship between crustal thickening, metamorphism, and deformation in the Himalayan middle crust.

Mesoscale fabrics in the LKD transition down-section from slaty cleavage (S1), to crenulation cleavage (S1/S2), and finally to a pervasive mylonitic foliation (S2) at the lowest structural levels. This fabric transition marks the approximate upper boundary of a ≥2.5 km-thick, distributed shear zone (D2) that encompasses the underlying orthogneiss as well as the metasedimentary rocks. Metapelites within the shear zone contain pre- to syn-kinematic garnet, staurolite, and kyanite porphyroblasts. In the highest grade metapelites, sillimanite occurs in shear bands, and cordierite overprints garnet and kyanite porphyroblasts. These observations indicate that the rocks in the LKD shear zone underwent Barrovian metamorphism (M1) followed by D2 deformation and high temperature decompression (M2), consistent with pressure-temperature estimates and monazite rare-earth element (REE) geochemistry. Ages of monazite included in garnet suggest crustal thickening began by at least middle Eocene time. Matrix monazite ages and monazite REE trends indicate that peak M1 metamorphism (growth of staurolite or kyanite) occurred in the early Oligocene. Some matrix monazite grains preserve syn-kinematic overgrowths that constrain onset of D2 fabric development to the late Eocene to early Oligocene. Finally, ages of monazite included in cordierite constrain decompression of the LKD to no earlier than the late Oligocene. Results of this study suggest that the LKD shear zone formed shortly after crustal thickening, remained active at high temperatures, and accommodated a component of vertical thinning that was partially responsible for exhuming rocks in the LKD.