P-T history and 40Ar/39Ar dating of the Jinshuikou group, Eastern Kunlun Mountains, Qinghai, China

Abstract

The Kunlun Mountains, located along the northern margin of the Tibetan Plateau in Qinghai, Western China play an important role in the collision history between Eurasia and India. However, not much research has been done on both metamorphic P-T conditions and geochronology. A combined P-T modelling and 40Ar/39Ar dating study on the Jinshuikou group in the Kunlun Mountains revealed a metamorphic event of Silurian – Devonian age. Peak conditions of amphibolite to granulite facies of 750°C, 13 kbar and 800°C, 10 kbar are at higher pressure and lower temperature conditions than previously found by Liu et al. (2005). Dating of hornblende, biotite, muscovite and K-feldspar of metamorphic and intrusive rocks from the Jinshuikou group revealed four age groups: 1) 450.27 ± 22.60 Ma till 403.25 ± 17.89 Ma; 2) 353.40 ± 14.62 Ma till 263.43 ± 9.97 Ma; 3) 252.58 ± 8.87 Ma till 199.86 ± 43.37 Ma and 4) 215.11 ± 5.48 Ma till 184.26 ± 7.52 Ma. These groups all correspond to important accretion phases of microcontinents to the Eurasian continent with corresponding magmatic events. Ages found display cooling rather than forming ages and are (partially) reset by new pulses of magmatic intrusions. By combining P-T modelling with the thermal history based on argon dates from this study and U-Pb zircon and AHe ages from literature, initial cooling rates of 10 – 14°C/Ma between 430 and 400 Ma were found, decreasing exponentially to 2.0°C/Ma between 400 and 300 Ma. Reheating of sample locations 1 and 5 by Permian – Triassic intrusions caused another phase of exponential cooling, followed by a fluctuation around 250°C due to several phases of granitic intrusions. A paleo geothermal gradient of 50°C/km between peak metamorphism and present is assigned to the Jinshuikou group, which implies a denudation rate of 0.1 km/Ma and total unroofing of 40 km.