Geochronology and zircon geochemistry of Greater Himalaya leucogranites in Zanskar, NW India

Samples were collected along the Karakoram Shear Zone (KSZ) in the Pangong Mountains and from the Greater Himalaya Sequence (GHS) in Zanskar to constrain the timing of magmatism in the northwest Himalaya. Geochronology and zircon geochemistry analyses were conducted using a sensitive high-resolution ion microprobe and laser ablation inductively-coupled plasma mass spectrometry. KSZ leucogranites have crystallization ages ranging from 22 to 9 Ma, contain Late Cretaceous through Paleocene inherited cores, and have sHf (T) values from +1 to +9. These positive sHf (T) values and inherited ages compare closely to the adjacent Ladakh batholith, but anomalously low sHf (T) values can be attributed to additional input of older crustal material from the Karakoram terrane or subducted Indian crust. Zircon and monazite ages suggest that Permian and Cambro-Ordovician granites are more widespread in Zanskar than previously noted. Zanskar granites with Cenozoic zircon rim ages and monazite ages ranging from 27-17 Ma are interpreted either as crystallization ages from Cenozoic anatectic melts or as pre-Himalayan granites with Himalayan metamorphic zircon growth. An abundance of pre-Himalayan granites suggests that insufficient anatectic partial melting occurred in Zanskar for mid-crustal channel flow. Low melt percentages in Zanskar can be explained by the presence of a mica-poor igneous protolith or the interruption of a partially-molten mid-crustal channel by the KSZ.