Orogenic to post-orogenic transition of the Paleoproterozoic Jiao-Liao-Ji Belt: Constraints from geochronology and geochemistry of the granites in the Qingchengzi area

The Jiao-Liao-Ji Belt is the most representative Paleoproterozoic belt in the North China Craton, which hosts huge amounts of Paleoproterozoic volcanic-sedimentary rocks, intermediate-felsic plutonic rocks and some mafic rocks. Multi -stage magmatic metamorphic events occurred to these Paleoproterozoic geologic bodies, which resulted in great confusion about the understanding of the Precambrian geology of this belt. The Qingchengzi area, located in the middle of Jiao-Liao-Ji Belt, is characterized by its huge amounts of Au, Pb, Zn and Ag deposits. Although multi-stage tectonic magmatic events happened in Jiao-Liao-Ji Belt in Paleoproterozoic, Middle Proterozoic to Middle Triassic and Late Triassic in this area, several Paleoproterozoic granites were recognized by petrologic and geochronological work, which offer constraints on the transformation mechanism of the belt in Paleoproterozoic from orogenic period to post-orogeny. In this study, Paleoproterozoic granites with different facies ( Dadingzi monzogranite and Wolongquan porphyritic biotite monzogranite) were chosen to study their petrologic, geochronological and geochemical features. SHRIMP zircon U-Pb geochronological analysis showed that the Dadingzi monzogranite was formed at 1884 +/- 5Ma with abundant inherited zircons aged 2. 1Ga, whereas the Wolongquan monzogranite had an age of 1878 +/- 4Ma, which was synchronous to the Dadingzi monzogranite. Geochemically, the Dadingzi monzogranite is rich in Si0(2) and Al2O3 and poor in K2O and TiO2, with a relatively high MgO content, which is similar to high aluminum adakite; while the Wolongquan porphyritic biotite monzogranite belongs to high Sr + Ba granite, which has high SiO2 and Al2O3 and low TiO2 contents, with a relatively high MgO and obviously high K2O, showing that it is not of adakite characteristics. Combined with the regional tectonic evolution, Hf isotopic and major-trace element compositions, we conclude that the Dadingzi intrusion resulted from partial melting of a thickened lower crust, whereas the Wolongquan intrusion from partial melting of the lower crust mixing with a few appinite magma from enriched mantle. Genetic mechanisms of these two plutons were related to the transitional period from a collisional orogenic event to a strong post-collisional extension in the Jiao-Liao-Ji Belt.