U-Pb Zircon Dating of Regional Deformation in the Lower Crust of the Kohistan Arc

Felsic veins several cm thick to bodies several hundreds of m wide are sporadically distributed in the Kamila amphibolites of the Kohistan arc sequence. Some small dikes (less than one m thick) were deformed together with ductile deformation of wall-rock amphibolites; other small undeformed dikes crosscut foliation in the amphibolites. Large granitic intrusive bodies include gneissose and massive varieties. Ion-microprobe U-Pb dating of zircon grains reveal ages as follows: three deformed dikes, 107.7 ± 1.8 Ma, 94.0 ± 1.9 Ma, 81.0 ± 1.6 Ma; a single gneissose granitic body, 89.5 ± 4.2 Ma; three undeformed dikes, 82.0 ± 2.0 Ma, 80.6 ± 4.5 Ma, 75.8 ± 1.7 Ma; a massive granitic body, 75.7 ± 2.2 Ma; and a foliated amphibolite xenolith in the massive granitic body, 110.7 ± 4.9 Ma. Ages of the deformed dikes, the gneissose granitic body, and the amphibolite xenolith indicate that ductile deformation in the lower crust of the Kohistan arc took place episodically or successively during the period from 107 to 81 Ma. Ages of the undeformed dikes and the massive granitic body indicate that ductile deformation ceased prior to 80 Ma. Collision between Asia and the Kohistan arc elsewhere was inferred to be before 80 Ma. We conclude that the ductile deformation events did not extend beyond the time of Asia-Kohistan collision.