40Ar/39Ar thermochronological constraints on the retrogression and exhumation of ultra-high pressure (UHP) metamorphic rocks from Xitieshan terrane, North Qaidam, China

Abstract

Paleozoic ultra-high pressure (UHP) eclogite-bearing mafic boudins are exposed in gneiss in the Xitieshan terrane, North Qaidam. Petrological and thermobarometric studies show that garnet-amphibolite and amphibolite rocks record upper amphibolite-facies and lower amphibolite-facies retrograde metamorphic conditions of 8.1–9.1kbar, 630–670°C and 5.0–7.2kbar, 530–630°C, respectively. To better constrain the timing of retrogression and exhumation, 40Ar/39Ar stepwise heating analyses have been performed on minerals that were extracted from these UHP mafic rocks and their country rocks. Amphibole yields disturbed release spectra marked by anomalously old apparent ages in low temperature steps, suggesting the presence of extraneous 40Ar. Isochron ages of ~425Ma and ~419Ma are obtained for amphibole from the garnet-amphibolite and amphibolite, respectively, which are interpreted as the best estimate for the age of amphibolite-facies retrogression during exhumation. Muscovite from the gneiss yields consistent plateau and isochron ages of ~409Ma. K-feldspar from the same gneiss yields rising staircase-shaped age spectra, which are characterized by younger apparent ages in the first steps (263–281Ma) and higher plateau ages in the last segment (294–321Ma), indicating partial diffusive loss of argon from the K-feldspar samples. Biotite from schist yields flat age spectra with a plateau age of ~397Ma that is interpreted as a cooling age. Our new 40Ar/39Ar data, together with published LA-ICPMS ages of zircon from a coesite-bearing amphibolite indicate that the Xitieshan UHP metamorphic rocks may have experienced two stages of exhumation. During the first stage, from ~95–31km depth, the UHP metamorphic rocks were exhumed with a relatively rapid velocity of ~4.0km/Myr, resulting from buoyancy forces. Subsequently, the rate of exhumation decreased gradually from ~1.1 to 0.1km/Myr between ~31–7km depth. This later gentle exhumation was probably governed by extension and/or erosion.