Anatomy, age and evolution of a collisional mountain belt : the Baltoro granite batholith and Karakoram Metamorphic Complex, Pakistani Karakoram

Geological mapping and U–(Th)–Pb geochronology from the Karakoram Metamorphic Complex and Baltoro granite batholith has resulted in more detailed timing constraints on tectonic evolution of Asian crust. During and following collision and accretion of the Kohistan Arc and the Indian plate to the southern margin of Asia, crustal thickening along the Karakoram resulted in polyphase deformation, metamorphism and melting. U–Pb zircon and monazite ages show that kyanite- and sillimanite-grade metamorphism in the Baltoro region peaked during Oligocene–Early Miocene (c. 28–22 Ma) times. At structurally deeper levels sillimanite-grade migmatites of the Dassu dome have Pliocene (c. 5.4–3.5 Ma) ages. New U–Pb ages from crustal melt biotite + muscovite + garnet leucogranites from the Trango Towers, Cathedral peak and Paiyu peak across the Baltoro batholith range from 19.8 ± 0.5 Ma to perhaps as young as 14 Ma. Using data from both the Hunza and Baltoro regions we suggest that regional high-grade metamorphism was diachronous in space and time and could have lasted continuously for at least 37 Ma (c. 50–13 Ma) or as much as 50 Ma (c. 63–13 Ma) along the southern margin of the Asian plate in the Karakoram. Crustal melting along the Baltoro granite batholith spanned at least 13 Ma (26.4–13 Ma).