Chronology of deformation, metamorphism, and magmatism in the southern Karakoram Mountains

U-Pb dating of metamorphic and igneous rocks from the Hunza Valley and Balt oro regions of the Karakoram Mountains in northern Pakistan addresses the t hermal and magmatic evolution of the thickened Asian plate crust before, du ring, and after the collision of the Kohistan arc and the Indian plate. Cru stal thickening and high-temperature, sillimanite-grade metamorphism in the southern Karakoram Mountains followed the collision and accretion of the K ohistan arc during the Late Cretaceous. U-Pb ages of metamorphic monazites from sillimanite gneisses in the Hunza Valley are 63.3 +/- 0.4 Ma, ca. 50-5 2 Ma, and 44.0 +/- 2.0 Ma, and monazites from a kyanite-grade schist from t he Baltoro region are 28.0 +/- 0.5 Ma. Metamorphic monazites from a highly graphitic garnet + staurolite schist from the Hunza Valley yield a crystall ization age of 16.0 +/- 1.0 Ma. Sillimanite gneisses from the Dassu dome ha ve magmatic zircons of 1855 +/- 11 Ma, reflecting a Proterozoic continental crustal source, and metamorphic monazites of 5.4 +/- 0.2 Ma. Magmatism was also sporadic; early granodiorite, monzogranite, and leucogranite dikes yi eld zircon, monazite, and uraninite ages of 50-52 Ma and 35.0 1.0 Ma. Wides pread lower crustal melting during the latest Oligocene-early Miocene culmi nated with emplacement of the Baltoro Plutonic Unit in the Karakoram bathol ith that cuts deformation fabrics in the high-grade gneisses to the south. The youngest magmatic phase dated is the 9.3 +/- 0.2 Ma Sumayar leucogranit e pluton. On the basis of detailed structural field studies combined with U -Pb geochronology, sillimanite-grade metamorphism was either a protracted e vent lasting as long as 20 m.y. (64-44 Ma) or peaked at different times wit hin the lower crust following collision of first, the Kohistan arc, and lat er, the Indian plate. We also present evidence for southward propagation of peak metamorphism and postmetamorphic thrusting and folding of isograds wi thin the past 5 m.y. Detailed geochronology shows that deformation, metamor phism, and magmatism in the middle and lower crust of the south Asian margi n has been occurring within the Karakoram metamorphic complex for more than 60 m.y. Similar processes may also have affected the unplumbed depths of t he south Tibetan crust.