Thermochronologic constraints on deformation and cooling history of high‐ and ultrahigh‐pressure rocks in the Qinling‐Dabie orogen, eastern China

The Hong'an block is the best place to study the exhumation of high‐pressure (HP) and ultrahigh‐pressure (UHP) metamorphic rocks in the Qinling‐Dabie orogen of eastern China because it lacks the extensive Cretaceous tectonic and thermal overprint observed in the Dabie Shan. We measured timing of deformation and rate of cooling of the HP‐UHP rocks by 40Ar/39Ar analyses of synkinematic minerals from tectonites of key structural zones in the Hong'an and Tongbai Shan. Normal‐sense shear along the north dipping Huwan detachment at the northern edge of the Hong'an block occurred between 237 and 231 Ma; this detachment facilitated the bulk of the exhumation of the HP‐UHP rocks. Our new 40Ar/39Ar ages, combined with U/Pb zircon and Sm/Nd ages of 245–240 Ma, suggest that exhumation of UHP rocks from mantle depths occurred at rates of 5–25 mm/yr from ∼245 to 230 Ma. The mountain range is a warped extensional footwall, within which white mica cooled from 225 to 205 Ma. Locally, younger extension is recorded by white mica recrystallization at 198–194 Ma, after which the entire block had cooled to below 300°C. Early Cretaceous 40Ar/39Ar ages from the Tongbai shear zone indicate that dextral shear along the southwest boundary of the orogen was contemporaneous with normal to sinistral‐oblique slip along the Xiaotian‐Mozitan fault along the northern boundary. Coeval dextral and sinistral shear zones along the northern and southwestern margins of the Hong'an and Dabie Shan would have caused eastward lateral extrusion of these two blocks, perhaps driven by collision of the Lhasa block with Eurasia.