Zircon U-Pb age and Hf isotopic compositions of the granitic gneisses from the Sanggan complex in the Zhangjiakouk area: Constraints on the early evolution of North China Craton

The Sanggan metamorphic complex in the northwestern Hebei Province consists mainly of mediate-to high-grade metamorphic rocks, including granodioritic, monzonitic and banded gneisses. It is located to the south of the Chicheng-Chongli fault and to the north of the Huai an metamorphic complex. SHRIMP and LA-ICPMS zircon U-Pb dating results reveal that the granodioritic gneisses were formed at 2.54 similar to 2.50Ga, while the K-rich monzonitic gneisses were emplaced at about 2.44Ga. An important metamorphic event of 2.37 similar to 2.30Ga was recorded in the zircons from the banded gneisses. The granodioritic geneisses exhibits rather radiogenic Nd (epsilon(Nd)(t) = -1.06) and Hf (average epsilon(Hf) (t) of zircon = -1. 6), suggesting the involvement of recycled old crustal material in origin. The K-rich monzonlitic geneisses are characterized by high SiO,, low Mg' and more radiogenic Nd (epsilon(Nd)(t) = -0.39 similar to +0.92) and Hf (epsilon(Hf)(t) = +0. 11 similar to +1.85) compositions. They were considered to be formed through melting of earlier tonalite and underplated mafic magmas under amphibolite-facies conditions, with an important contribution of juvenile crustal component. Combined with the previous geochronological and geochemical studies on these metamorphic complexes, we proposed that the study area was in a post-collional tectonic setting during 2.50 similar to 2.40Ga, and that it was metamorphically overprinted at 2.37 similar to 2.30Ga in response to lithospheric extension.