TIMING OF BARROVIAN METAMORPHISM AND DUCTILE THINNING IN LHAGOI KANGRI DOME, SOUTHERN TIBET

The North Himalayan antiform marks the location of a series of gneiss-cored domes along the southern Tibetan Plateau that record a history of Eocene convergence followed by Oligocene to Miocene exhumation as the locus of India-Asia convergence was transferred southward. Key features common to these North Himalayan gneiss domes (NHGD) include: (1) a core of Precambrian to Cambrian granitic gneiss (+/- Miocene granite/granitic gneiss), (2) a carapace of Paleozoic to Mesozoic metasedimentary rocks that record Barrovian metamorphism, (3) two deformation events that reflect crustal thickening (D1) and ductile vertical thinning (D2), and (4) an Oligocene to Miocene period of crustal melting and leucogranite emplacement. The relative timing of deformation, metamorphism, partial melting, and doming varies between several previously studied NHGD, which has resulted in variable interpretations regarding the processes behind dome formation. Our structural, petrologic, and geochronologic data from Lhagoi Kangri dome (LKD) provide new constraints on the NHGD evolution and the early development of Himalayan middle crust.

LKD is a 20 x 40 km elliptical dome located ~100 km NNW of Mt. Everest. Lhagoi Kangri is cored by orthogneiss that is overlain by Paleozoic to Triassic mixed siliciclastic and carbonate metasedimentary rocks. At the highest structural levels deformation is characterized by km-scale N- and S-vergent folds and slaty cleavage (collectively, D1). A later generation of ductile deformation (D2) resulted in tightening of F1 folds and transposition of S1 cleavage at intermediate structural levels. At deeper structural positions, D2 resulted in the formation of mylonitic foliation characterized by C and C’ shear bands frequently consisting of biotite, muscovite, and/or silliamanite laths. Pelitic intervals within the metasedimentary strata record concentric chlorite to sillimanite Barrovian metamorphic isograds (M1), and garnet- to sillimanite-zone rocks record nearly ubiquitous retrograde metamorphism (M2). Preliminary electron probe microanalysis and in-situ U(-Th)-Pb geochronology of monazite grains from staurolite- to sillimanite-grade samples constrain timing of M1, D2, and M2 between early Eocene and late Oligocene.