Temporal variations in the cooling and denudation history of the Hunza plutonic complex, Karakoram Batholith, revealed by40Ar/39Ar thermochronology

Abstract

The40Ar/39Ar thermochronology of the Late Cretaceous Hunza plutonic complex reveals an episodic cooling and denudation history for this regional‐scale pluton. The40Ar/39Ar analyses of biotites from a vertical relief section of >3200 m reveal a pulse of rapid cooling at ∼20 Ma. In the interval of 110–27 Ma, age‐elevation distributions suggest denudation rates of the order of 0.02 ± 0.003 mm/yr. At ∼20 Ma, denudation rates increased significantly to 2.7 ± 0.7 mm/yr, then returned to much slower rates until 12 Ma. A second pulse of rapid cooling beginning at 12 Ma is revealed by inverse numerical modeling of multidiffusion domain alkali feldspars from a vertical section of 1700 m. Decreasing in elevation, these samples record the onset of rapid cooling at 12, 9, and 7 Ma, respectively. All of the alkali feldspars record a period of nearly isothermal conditions prior to the onset of rapid cooling when rates increased to 30°C/m.y. Assuming a geothermal gradient of 30°C/km, these cooling rates translate into denudation rates of 1.0 mm/yr. Apatite fission track analysis indicates denudation rates of 0.7 ± 0.1 mm/yr over the interval of 6.6 Ma to 2.4 Ma in agreement with the alkali feldspar data. These data suggest denudation of 2.9 ± 0.4 km since the Plio‐Pleistocene. Together, the alkali feldspar and apatite data indicate that a minimum of 10 km of overburden has been removed since the mid‐late Miocene. An electronic supplement of this material may be obtained on a diskette or Anonymous FTP from KOSMOS.AGU.ORG. (LOGIN to AGU's FTP account using ANONYMOUS as the username and GUEST as the password. Go to the right directory by typing CD APEND. Type LS to see what files are available. Type GET and the name of the file to get it. Finally, type EXIT to leave the system).(Paper 95TC02424, Temporal variations in the cooling and denudation history of the Hunza plutonic complex, Karakoram Batholith, revealed by40Ar/39Ar thermochronology, M.A. Krol, P.K. Zeitler, G. Poupeau, and A. Pecher). Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009; $15.00. Payment must accompany order.