Metamorphism of the Chengde complex in the Trans-North China Orogen and implications for Paleoproterozoic assembly of the North China Craton

Abstract

The North China Craton (NCC) is one of the oldest cratons in China, which can be further divided into the Eastern Block, the Western Block, and the Trans-North China Orogen (TNCO) between them. The TNCO comprises several terranes which have experienced amphibolite facies to granulite facies metamorphism. The Chengde area is located in the northern part of the TNCO, which exposes Precambrian basement of TTG gneisses, (medium-pressure and high-pressure) mafic granulite, garnet-bearing amphibolite, garnet-free amphibolite, metapelite, and metagraywacke. Mafic granulite and amphibolite generally occur as different-sized enclaves within felsic gneiss or paragneiss, displaying the “blocks-in-matrix” feature. Some of these rocks preserve retrograde-stage reaction textures, while other rocks display equilibrium textures. Metamorphic rocks in the Chengde Complex generally preserve one- to four-stage mineral assemblages. Phase equilibrium modeling and geothermobarometers reveal that some of these rocks record clockwise P-T paths with prograde heating and decompression process, post-peak near isothermal decompression (ITD) process, and later near isobaric cooling (IBC) process, indicative of subduction, fast exhumation and final cooling processes. These metamorphic rocks record diverse peak metamorphic conditions (pressures from 4.0 kbar to14.9 kbar and temperatures from 500 ℃ to 920 ℃), indicating different subduction depths. Zircon U-Pb dating suggests that metamorphic ages of mafic granulite, amphibolite, and paragneiss are variable (1910-1810 Ma), implying that these rocks were metamorphosed diachronously. Thus, it can be concluded that the Chengde Complex recorded the Paleoproterozoic orogenic event rather than a late Archean orogenic event. Metamorphic rocks in the Chengde Complex display “blocks-in-matrix” features in the field, and have variable protoliths, diverse metamorphic peak P-T conditions, and different metamorphic ages, which are typical features of Phanerozoic orogen. Therefore, it is concluded that such an unordered juxtaposition of diverse metamorphic rocks could also be a typical feature of Precambrian orogens.