METAMORPHIC AND STRUCTURAL EVOLUTION OF THE LHAGOI KANGRI GNEISS DOME, SOUTHERN TIBET

The North Himalayan gneiss domes are a series of features exposed in southern Tibet along the transition between the High Himalayas and the Tibetan Plateau that preserve key structural and metamorphic aspects of Himalayan mid-crustal evolution. General characteristics shared by the North Himalayan gneiss domes include: (1) a core of gneiss, granite, and migmatite overlain by metasedimentary rocks that grade into unmetamorphosed Tethyan sediments; (2) Barrovian metamorphism with grade increasing concentrically toward the core; and (3) at least two deformation events, the second of which produces a well-defined, approximately north-south stretching lineation that transitions to mylonite adjacent to the dome contact. The Lhagoi Kangri gneiss dome, located ~100 km northeast of Mt. Everest, typifies many of these geological characteristics.

The Lhagoi Kangri gneiss dome is a 25 x 40 km ellipse with an east-west oriented long axis. The core is composed of orthogneiss and leucogranite and is intruded on its western margin by the Gyaco La granite. The cover rocks grade concentrically from unmetamorphosed at the structurally highest levels to kyanite zone at the lowest levels. The highest structural levels exhibit km-scale, north- and south-vergent folds (D1) that are successively tighter with structural depth. At intermediate structural levels a second event, D2, is recorded by refolded F1 folds and crenulation cleavage (S2). L1x2 intersection lineations are subparallel to stretching lineations (Lm2) that trend NNE and SSW on the north and south sides of the domes, respectively. Near the core-cover contact S1 is transposed, and S2 is oriented sub-parallel to the contact. Rocks throughout the dome contain shear bands or strain shadows that record a mixed top-north and top-south shear sense. Petrographic analysis indicates that porphyroblasts grew prior to or during D2 except for kyanite and fibrolite, which overprint S2 fabric. Compositional x-ray maps and analytical transects of garnet from samples throughout the study area indicate growth during prograde metamorphism, and zoning is commonly truncated on surfaces parallel to S2. Thermobarometric calculations indicate that garnet zone rocks reached conditions of 570° C and 6.6 kbar while kyanite zone rocks reached 625° C and 7.2 kbar.