Abstract

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.

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