ZIRCON AND APATITE (U-TH)/HE THERMOCHRONOLOGY OF THE DABIE SHAN, CHINA

The Dabie Shan of eastern China is the archetype ultrahigh pressure (UHP) metamorphic terrane, exposing diamond and coesite bearing rocks that were partially exhumed from >120 km depths during Triassic continental collision and Cretaceous magmatism and deformation (e.g., Hacker et al., 1998; 2000; Ratschbacher et al., 2000). Little is known about the mid- to shallow-crustal exhumation of the UHP rocks however, and the post-Mesozoic tectonic and geomorphic history of this ~300 km wide range. We report new zircon and apatite (U-Th)/He ages from 14 samples between 30-530 m elevation that provide several such constraints on the post-collisional history of this complex polygenetic mountain range. Zircon He ages (closure T~180-200°C) show a roughly concentric pattern from about 100-150 Ma on the range flanks to about to 85-100 Ma in the range core. These ages and their concentric pattern are similar to previous K-feldspar low-T domain ages (as well as crustal thickness and barometric constraints) from the range, and reflect a broad mid-crustal thermal dome centered on the presently exposed range core during and following the Cretaceous orogenic episode. Apatite (U-Th)/He ages also show a concentric pattern, from 44-65 Ma on the range flanks, to 32-38 Ma in the range core, constraining the final ~2 km of denudation in the core of the Dabie Shan to no earlier than the Eocene-Oligocene. This distribution of He ages requires a 50-100% difference in time-integrated flank and core exhumation rates. Thermal modeling assuming constant geothermal gradient and cooling rates (but cooling-rate dependent He ages) suggest this could reflect slow, steady post mid-Cretaceous exhumation rates of 0.04-0.05 mm/yr on the range flanks and 0.06-0.08 mm/yr in the core, almost two orders of magnitude slower than published estimates of mid-Cretaceous exhumation rates. However, a relatively short period of rapid exhumation (<2 km) in the core of the range at ~30-40 Ma, to produce the widespread 32-38 Ma apatite He ages in the core, cannot be ruled out. The latter interpretation is consistent with large volumes of Eocene sediments and evidence for Cenozoic faulting near the range (Ratschbacher et al., 2000), and may represent orogenic reactivation of of the range at this time, due to far-field (~2000 km) effects of the Indo-Asian collision.