A polyphase metamorphic evolution for the Xitieshan paragneiss of the north Qaidam UHP metamorphic belt, western China: In-situ EMP monazite- and U–Pb zircon SHRIMP dating

Abstract

In-situ electron microprobe (EMP) U–Th–Pb monazite-, sensitive high-resolution ion microprobe (SHRIMP) zircon analyses, metamorphic phase equilibrium (Domino/Theriak)- and geothermobarometric calculations are performed on kyanite/sillimanite-bearing garnet biotite gneisses forming part of the dominant rock association in the Xitieshan ultra-high pressure metamorphic belt, north Qaidam, western China. Results are consistent with the following complex polyphase tectono-metamorphic evolution.The kyanite/sillimanite bearing garnet biotite gneisses contain monazite ages of 938±23Ma and zircon SHRIMP ages of 945±7Ma, referring to a Neoproterozoic metamorphism, i.e. similar to the age of the Jinning orogeny in the Yangtze block of southern China. This correlation suggests that the paragneiss has affinities with the Yangtze block (South China block). The Neoproterozoic monazites were found inside coarse grained porphyroblastic garnets containing amphibolite facies mineral inclusion assemblages.The kyanite/sillimanite-bearing garnet biotite gneisses also contain early Paleozoic monazite ages of 422–425Ma and 455–460Ma, detected in amphibolite facies mineral assemblages associated with matrix minerals. Using phase equilibrium- and geothermobarometric calculations, PT conditions of 560–610°C/5.8–7.0kbar and 610–675°C/4.6–6.5kbar were calculated respectively for both amphibolite facies assemblages. The early Paleozoic ages of 422–425Ma and 455–460Ma were detected in 8 monazite grains from the investigated paragneiss samples. Based on the Y and Eu contents variation of the early Paleozoic monazite domains (measured by EMP), the 422–425Ma monazite ages are interpreted to have formed during an amphibolite facies tectono-metamorphic overprint that post-dates (U)HP metamorphism and can thus be related to exhumation of previously deeply subducted rocks. Alternatively, the 455–460Ma monazite ages are interpreted to represent the age of the prograde subduction zone metamorphism.We conclude therefore that the mineral assemblage of the kyanite/sillimanite-bearing garnet–biotite gneiss (and associated retro-eclogite) was formed during an early Paleozoic subduction/collision event, which involved late Proterozoic (938–945Ma) crystalline basement inliers of minimal mid amphibolite facies grade. Early Paleozoic deep subduction towards (U)HP depths occurred around 455–460Ma followed by retrograde amphibolite facies metamorphism at 422–425Ma during exhumation.