Two occurrences of paired metamorphic belts in central Inner Mongolia, China: Implications for two-stage subduction of the Paleo-Asian Ocean

Abstract

The paired metamorphic belts in orogenic areas record the dual thermal regimes associated with subduction. In this study, based on detailed petrographic observations, phase equilibrium modelling, and zircon and rutile U–Pb dating of samples from different metamorphic units, we report two occurrences of paired metamorphic belts in central Inner Mongolia. The first included a HP–LT and a HT–LP metamorphic belt in which garnet mica schist sample XL04 and garnet-sillimanite biotite gneiss sample XL16 yielded peak P–T conditions, respectively of ∼19 kbar/∼510℃ (∼9℃/km) and ∼ 6 kbar/∼676℃ (∼32℃/km). The second includes a MP–LT and a HT–LP metamorphic belt in which blueschist sample WD13 and amphibolite sample XL10 yielded peak P–T condition of 8.6–8.8 kbar/520–530℃ (∼19℃/km) and ∼ 4.5 kbar/870–910℃ (∼60℃/km), respectively. Zircon and rutile U–Pb results indicate that the first pair of metamorphic belts (∼9℃/km and ∼ 32℃/km) were formed at 455–442 Ma, while the second (∼19℃/km and ∼ 60℃/km) were formed at 383–306 Ma based on previous studies. These ages are consistent with those of two arc magmatic pulses in central Inner Mongolia. Consequently, we suppose that the two occurrences of paired metamorphic belts are attributed to a trench–continental margin in the Early Paleozoic and a trench–basin–arc system in the Late Paleozoic. It is likely that the two occurrences of paired metamorphic belts record the tectonic transition from a completely active continental margin setting in the Early Paleozoic to a fore-arc extension system in the Late Paleozoic, driven by slab rollback mechanisms with trench retreat of the Paleo-Asian Ocean.