Metamorphism, melting, and extension: Age constraints from the High Himalayan Slab of southeast Zanskar and northwest Lahaul

We present evidence for two distinct stages of Tertiary metamorphism (M-1 and M-2) in the High Himalayan Slab of southeast Zanskar and northwest Lahaul, as well as evidence for an older, pre-Himalayan metamorphism (pre-M-1). The M-1 was a regional Barrovian-type event related to crustal shortening and thickening of the Indian plate margin, while M-2 was associated with crustal melting and the emplacement of the Gumburanjon leucogranite into the Zanskar Shear Zone at the top of the slab. U-Pb dating of metamorphic and magmatic accessory phases constrains the timing of M-1 between 30 and 37 Ma and the crystallization and emplacement age of the Gumburanjon leucogranite at 21-22 Ma. Inherited accessory phases in metamorphic and magmatic samples suggest that the protoliths of the slab are at least Lower Paleozoic in age and that they experienced a major pre-M-1 thermal perturbation at ca. 450-500 Ma. Whether this was associated with a regional Barrovian-type metamorphism or whether it was a thermal event related to the intrusion of large Cambro-Ordovician granites remains uncertain. The 40Ar/39Ar cooling ages of muscovites from metamorphic schists range from 22.0 +/- 0.6 to 20.4 +/- 0.6 Ma, suggesting that cooling of the High Himalayan Slab below the nominal closure temperature for Ar loss in muscovite (similar to 400 degrees-350 degrees C) occurred during the Early Miocene. Muscovite and biotite 40Ar/39Ar cooling ages from the Gumburanjon leucogranite are virtually identical at ca. 20-21 Ma and are only slightly younger than the U-Pb crystallization ages, suggesting rapid, postemplacement cooling at the top of the slab. Rapid cooling, together with the emplacement of the Gumburanjon leucogranite into the immediate footwall of the Zanskar Shear Zone, suggests a direct link between normal faulting and crustal anatexis.