Age of crustal melting, emplacement and exhumation history of the Shivlingleucogranite, Garhwal Himalaya

We report a U-Pb monazite age of 23.0 +/- 0.2 Ma for the Shivling leucogran ite, a tourmaline + muscovite +/- biotite leucogranite at the top of the Hi gh Himalayan slab in the Garhwal Himalaya, north India. The Shivling-Bhagir athi leucogranite is a viscous near-minimum melt, emplaced as a foliation p arallel laccolith via a dyke network not far from its source region. Progra de heating occurred soon after the India-Asia collision at c. 50 Ma up to m elting at 23 Ma and high temperatures (>550 degrees C) were maintained for at least 15 Ma aftergarnet growth. The leucogranite was emplaced at mid-cru stal depths along the footwall of the Jhala fault, a large-scale low-angle normal fault, part of theSouth Tibetan Detachment system, above kyanite and sillimanite grade gneisses. The geometry of the leucogranite laccolith sho ws biaxial extension and boudinage both perpendicular (north-northeast-sout h-southwest) and parallel to the strike (west-northwest-east-southeast) of the mountain range. Unroofing occurred by underthrusting beneath the High H imalayan slab along the Main Central Thrust zone, progressively 'jacking up ' the leucogranites, removal of material above by low-angle normal faulting , and erosion. Very rapid cooling at rates of200-350 degrees C/Ma between 2 3-21 Ma immediately followed crystallization, as tectonic unroofing and ero sion removed 24-28 km of overburden during this time. K-Armuscovite ages ar e 22 +/- 1.0 Ma and fission track ages of zircons from > 5000 m on the Nort h Ridge of Shivling are 14.2 +/- 2.1 and 8.8 +/- 1.2 Ma and apatites are 3. 5 +/- 0.79 and 2.61 +/- 0.23 Ma. Slow steady state cooling at rates of 20-3 0 degrees C/Ma from 20-1 Ma shows that maximum erosion rates and unroofing of the leucogranite occurred during the early Miocene. This timing coincide s with initiation of low-angle, north-dipping normal faulting along the Sou th Tibetan Detachment system.