⁴⁰Ar/³⁹Ar dating of plutonic/metamorphic rocks from Chinmen Island off southeast China and its tectonic implications

To reconstruct the thermal history of the Yanshanian Orogeny in SE China, a combination of ⁴⁰Ar/³⁹Ar thermochronological and thermobarometrical techniques have been performed on hornblende, biotite and K-feldspar separates from a granite and gneiss sample from Chinmen Island. Based on closure temperature calculations and the isotopic dates of the minerals, the thermal history of the area is derived as follows: the region experienced plutonism/metamorphism prior to 101 Ma, cooled rapidly in the initial stages at a rate of ~80°C/Ma between 101 and 97 Ma, and then cooled slowly to 180°C at a rate of ~8°C/Ma until ~82 Ma. The fast cooling may reflect rapid tectonic uplifting. However, the plutonic/metamorphic complex was not exposed at the surface until the Miocene. A paleo-geothermal gradient of about 30°C/km has also been inferred. -from Authors