U-Pb SHRIMP geochronology of leucogranites from the Greater Himalayan Sequence in Zanskar and from the Karakoram fault zone, NW India

New U-Pb SHRIMP ages have been obtained from the westernmost limb of the South Tibetan Detachment in northwest India, locally known as the Zanskar Shear Zone (ZSZ), and from the dextral strike-slip Karakoram fault zone. This research investigates the extent of mid-crustal anatexis and ductile exhumation of the Greater Himalaya Sequence (GHS) in the western Himalaya, and explores a possible relationship between Zanskar and Karakoram fault zone leucogranites. Zircon Hf isotope signatures, measured with laser ablation ICP-MS, identify potential sources of the anatectic leucogranites. Along the ZSZ, samples were collected from the Nun-Kun and Suru valleys in the west and from Haptal valley ~100 km to the southeast. Leucogranite from the lowest structurally-exposed GHS in Suru valley yields a monazite age of 19.2±0.4 Ma, which is significantly younger than the 25.1±0.6 Ma monazite age of a psammitic schist near the ZSZ. Monazites give an age of 20.7±0.4 Ma for a Haptal valley migmatite and an adjacent late-stage pegmatite dike in a small leucogranite pluton has monazite ages ranging from 25.9±1.3 to 19.0±0.9 Ma. While these ages suggest coeval leucogranite emplacement along the entire ZSZ, only leucogranites in the east yielded inherited monazite ages of ~470 Ma and ~450 Ma that are indicative of a Cambro-Ordovician pan-African source. The western segment of the ZSZ exhibits less extensional offset and leucocratic melts appear to have migrated further from migmatite source regions than in the east. These along-strike variations suggest that the structural transition from a compact shear zone in the east to dispersed extensional shear zones near Pensi La may represent the westernmost extent of ductile melt-facilitated GHS exhumation. Miocene leucogranite emplacement also occurred along the Karakoram fault, which raises the question of whether the fault served as a conduit for mid-crustal GHS melts. In Nubra Valley, a leucogranite from the Karakoram fault zone yielded an average U-Pb zircon age of 15.0±0.2 Ma. In the Pangong mountains, leucogranite intruded into a psammite host from the northern end of Tangste gorge gave a zircon age range of 19.8±0.1 to 12.7±0.5 Ma. A two-mica leucogranite from the southern end of Tangste gorge yielded zircon core ages ranging from 69.0±0.3 to 35.7±0.2 Ma and zircon rim ages that cluster at ~20 Ma. While Karakoram fault and Zanskar leucogranite crystallization ages overlap, the inherited Paleocene-Eocene zircon cores from the Pangong leucogranites suggest that Karakoram leucogranites were derived from Neo-Tethyan oceanic subduction-related granites of the Ladakh batholith, rather than Indian metasediments that record >450 Ma pan-African magmatism. Positive zircon ɛHf(t) signatures of Karakoram leucogranites (averaging 2.4-4.9) also suggest that the Ladakh batholith was the primary anatectic source of these melts. However, the GHS and Karakoram batholiths—with average ɛHf signatures of -2.2 and -3.5 respectively—cannot be ruled out as possible partial sources for the Pangong leucogranites. Anomalously low zircon ɛHf(t) values reported by Ravikant and others (2009) suggest that Indian crust may have been a significant melt source for some Karakoram fault leucogranites.