Discovery of the Paleoproterozoic complex at Dongwayao in northern Hebei and its constraint on the extensional event of the North China Craton

The formation time of the North China Craton (NCC) is still unclear, and the basement uplift region in northern Hebei is an ideal area for studying the formation and evolution of the NCC. A Paleoproterozoic post−orogenic A−type granite complex newly discovered at Dongwayao has important implications for understanding the formation time of the NCC. This paper present results of petrology, geochemistry, LA−ICP−MS zircon U−Pb geochronology and Hf isotopic composition of the Dongwayao complex to discusse the formation age, petrogenesis and tectonic setting．The LA−ICP−MS zircon U−Pb dating shows that the ages of the syenogranite and quartz syenite are 1920±11 Ma and 1902±12 Ma, respectively, indicating that the complex was emplaced in the Late Paleoproterozoic, not the Early Cretaceous as originally suggested. Petrogeochemical studies show that the syenogranite has high SiO₂(72.07%~75.03%), Na₂O＋K₂O (8.54%~8.99%) and K₂O(5.78%~6.64%) contents, and poor in CaO, Al₂O₃, MgO, P₂O₅, TiO₂, Ba, Sr, Eu, P and Ti contents. The quartz syenite belongs to alkaline series and is relatively rich SiO₂(66.09%~66.65%), Na₂O+K₂O (12.22%~12.35%) and K₂O(10.00%~10.18%) contents, and poor in TiO₂, MgO, CaO, Sr, Eu and Ti contents. The syenogranite and quartz syenite have high TFeO/MgO, K₂O/MgO, (Na₂O＋K₂O)/CaO ratios and high zircon saturation temperatures (average 842°C), indicating the geochemical features of A-type granite. According to the chemical subdivision diagrams of the A−type granitoids, the complex shows the characteristics of A₂−type granitoid, formed in post−orogenic extensional tectonic setting. The ε_Hf (t) values of the syenogranite and quartz syenite range from －2.56 to＋2.42 and－3.27 to＋4.17, respectively, with model ages of 2262~2419 Ma and 2175~2466 Ma. The A−type granites were derived predominantly from partial melting of the ancient crustal materials with involvement of a small amount of mantle materials. The Dongwayao A−type granite complex newly discovered indicates that there is a late Paleoproterozoic post−orogenic extensional tectonic−magmatic event in the northern margin of the NCC, which marks the end of ~1.92 Ga orogenic movement in the NCC and the beginning of the postorogenic evolution.The timing for the collision and assembly of the NCC may be around 1.92 Ga.