Petrogenesis And Metamorphic Age Of Palaeoproterozoic Granitic Gneisses In Luliang Area: Constraints From Zircon And Monazite U-Pb Ages And Hf Isotopes.

The Luliang area, occupying an important position in the Precambrian research on the North China Craton (NCC), developed extensive Palaeoproterozoic metamorphic supracrustal rocks and granitic rocks. It is of great significance to study the paleoproterozoic geological evolution of the NCC. This paper presents zircon and monazite U-Pb ages and zircon Hf isotopes of Palaeoproterozoic Baijiatan granitic gneisses in the Luliang area. Two granitic gneiss samples obtain LA-ICP-MS zircon U-Pb ages of 2182 +/- 16Ma and 2185 +/- 24Ma, respectively, representing their emplacement ages. New LA-ICP-MS dating of monazites from two samples yielded U-Pb ages of 1898 +/- 7Ma and 1899 +/- 14Ma, respectively, which are much younger than the concordant Pb-207/Pb-206 ages (2180 similar to 2032Ma) of zircon overgrowth rims. It indicates that monazites thoroughly suffered from the late metamorphism and their U-Pb ages showed the granitic gneiss underwent similar to 1900Ma metamorphism, which is synchronous with the metamorphic age of the Central Zone of the NCC. Zircon depleted mantle model ages are 2473 similar to 2598Ma and two-stage model ages are 2646 similar to 2839Ma, which are distinctively older than the zircon U-Pb ages. Zircon epsilon(Hf)(t) values are -1. 2 similar to + 1. 8 and -1. 3 similar to + 1. 4, respectively, suggesting the granitic gneiss were originated from partial melting of the late Archean crustal materials, without any mantle-derived materials input. Combined with the available zircon Hf isotopic data from middle Palaeoproterozoic (2. 2 similar to 2. 1Ga) magmatic rocks, the Late Archean crust of the NCC suffered from extensive remelting effect during 2. 2 similar to 2. 1Ga. The 2. 2 similar to 2. 1Ga magmatic activity widely distributed in the Luliang, Zhongtiao, Wutai and Jiao-Liao-Ji areas and elsewhere in the NCC, which may develop in an intracontinental rift setting.