Age of crystallization and cooling of the K2 gneiss in the Baltoro Karakoram

The mountains of K2 (8611 m) and Broad Peak (8047 m) in the Baltoro Karakoram (northern Pakistan) are composed of plagioclase–homblende and biotite–hornblende-K-feldspar orthogneisses and amphibolite–facies paragneisses, intruded by garnet–biotite–muscovite–tourmaline leucogranitic veins. A U-Pb zircon age of 115–120 Ma was obtained on an orthogneiss from the south face of K2. ⁴⁰Ar-³⁹Ar analysis on hornblende yields a plateau age of 90.6 ± 1.8 Ma, consistent with a mid-Cretaceous phase of magmatism, concomitant with early subduction-related components of the Karakoram batholith (Muztagh Tower unit, Hunza plutonic unit). We interpret the K2 gneiss as representing a culmination of mid-crustal rocks along a discontinuous but wide zone north of the Karakoram batholith.