Timescales of Barrovian metamorphism, ductile deformation and near-isothermal decompression in the Lhagoi Kangri gneiss dome, south–central Tibet

The Lhagoi Kangri dome is one of a series of gneiss-cored domes that crop out along the southern margin of the Tibetan Plateau. The dome is composed of orthogneiss separated from overlying amphibolite-facies to unmetamorphosed sedimentary rocks by a nonconformity and intruded by leucogranite. At the lowest exposed structural levels a distributed, ≥2.5 km thick shear zone is characterized by a penetrative foliation and lineation in the orthogneiss and amphibolite-facies metasedimentary rocks. The penetrative L–S fabric overprints an earlier ductile fabric associated with burial and Barrovian metamorphism, and the shear zone contains both top-to-north and top-to-south shear sense indicators throughout, although top-to-north shear dominates at the lowest structural levels of the metasedimentary rocks. Thermobarometry coupled with monazite, titanite and rutile petrochronology constrains the timing and conditions of metamorphism, melt crystallization and ductile deformation in the dome. Garnet-zone rocks reached peak metamorphic conditions of 550–570°C and 6–7 kbar during Barrovian-style metamorphism, whereas staurolite–kyanite-zone rocks reached peak metamorphic conditions of 640–660°C and 8–11 kbar. Garnet growth in the staurolite–kyanite-zone rocks began at c. 45 Ma, and its subsequent breakdown to form Y + HREE-enriched monazite and titanite at 40–34 Ma brackets timing of peak metamorphism. Development of the L–S fabric began at 36–24 Ma contemporaneously with peak metamorphism as indicated by microstructural observations and growth of fabric-parallel monazite. Ductile deformation was also accompanied by near-isothermal decompression to form cordierite + biotite ± sillimanite assemblages, quartz–cordierite symplectite on garnet, and leucogranite dykes that crystallized at 24.8–22.7 Ma. The Lhagoi Kangri dome records early crustal thickening ( c. 45 Ma) that began soon after collision of India with Asia and a protracted, nearly continuous Eocene ( c. 40 Ma) to late Oligocene ( c. 24 Ma) episode of high-temperature metamorphism, ductile deformation and melt generation followed by cooling and exhumation at 17–13 Ma. The Eocene thermal development of the Lhagoi Kangri dome provides a window into the early crustal thickening history of the Himalayan orogen that is elsewhere, particularly in the Himalayan metamorphic core, heavily masked by subsequent deformation, metamorphism and large-scale crustal anatexis. These new data provide further support for the notion that the middle crust of the Himalaya and southern Tibet resided at sufficiently high temperatures to induce large-scale weakening and lateral flow.