NEW MONAZITE GEOCHRONOLOGY ACROSS GIANBUL DOME, GREATER HIMALAYAN RANGE, NW INDIA

New monazite geochronology suggests that Gianbul gneiss dome (GD), Greater Himalayan Range, NW India preserves an ~20-25 Ma long record of deformation, metamorphism, and intrusion. The dome is defined by a domal S2 foliation and associated NE-SW-trending L2 stretching lineation (D2 deformation) developed within high Himalayan metapelites and orthogneisses. These rocks preserve a Barrovian metamorphic sequence from garnet- to a sillimanite-grade migmatite core, are intruded by leucogranites, and are bounded by two top-down normal sense shear zones: the NE-dipping Zanskar shear zone (ZSZ), the western continuation of the southern Tibetan detachment system, on the northeast flank of the dome and the SW-dipping Khanjar shear zone (KSZ) on the southwest flank. Metamorphic index minerals grew within the S₂ foliation and quartz and feldspar microstructures suggest deformation temperatures of 400-700°C indicating that D2 structures formed at peak-metamorphic conditions. On the southwest flank of the dome, beneath the KSZ, the D2 deformed Kade orthogneiss yields three sets of monazite ages: ~450 Ma (garnet- zone), 37-33 Ma (garnet-zone), and 22.2 ± 0.4 Ma (kyanite-zone), a deeper syn-D2, kyanite-zone leucocratic dike yields an age of 22.1 ± 0.4 Ma, and a D2 deformed orthogneiss from the deepest structural levels within the sillimanite-zone yields an age of 20.5 ± 0.4 Ma. On the northeast flank of the dome, beneath the ZSZ, D2 deformed orthogneisses within the sillimanite-zone yields an age of 439 ± 9 Ma and in kyanite-zone an age of 37.6 ± 0.8 Ma, a D2 deformed migmatite within the sillimanite-zone an age of 22.8 ± 0.4 Ma, and an undeformed, cross-cutting two-mica granite an age of 21.0 ± 0.4 Ma. These data suggest that metamorphism and ductile deformation may have begun as early as ~37 Ma, continued to ~22-20 Ma, and ceased soon thereafter at ~21 Ma. Dynamic recrystallization of monazite in orthogneiss with melts/fluids associated with muscovite dehydration during exhumation likely resulted in the 22-20 Ma ages. Where melts/fluids were absent or insufficient for recrystallization, ~37 Ma and ~450 Ma ages are preserved. Ar/Ar mica cooling ages young from ~22 Ma in the southwest to ~19 Ma in the northeast across GD and reflect moderate temperature (350-450°C) exhumation along the KSZ prior to exhumation along the ZSZ.