Archean-Paleoproterozoic crustal evolution of the Ordos Block in the North China Craton: Constraints from zircon U-Pb geochronology and Hf isotopes for gneissic granitoids of the basement

The Ordos Block has been considered as an Archean micro-block in North China Craton. However, its formation and evolution are poorly understood for a long time due to lack of available basement rocks. Our LA-ICPMS and SIMS zircon U-Pb dating of gneissic granitoids from the Ordos Block basement identified two distinct periods of granitic magmatism at similar to 2.5 Ma and 2.2-2.0 Ga, and two phases of metamorphisms at similar to 1.95 Ga and similar to 1.85 Ga, respectively. Of which most zircons of similar to 2.5 Ga granitoids show positive epsilon(Hf)(t) values ranging from +0.1 to +4.1 with a peak TEN of similar to 2.7 Ga. Minor zircons have negative epsilon(Hf)(t) values from -3.8 to -0.02 and T-Hf(C) of 2.8-3.1 Ga. Moreover, the 3.40 Ga inherited zircon and zircons with T-Hf(C) of 3.1-4.2 Ga were found from 2.2 Ga to 2.0 Ga granitoids. These suggest that there existed a Neoarchean crust in the Ordos Block, which underwent a major crustal growth at similar to 2.7 Ga and granitic magmatism at similar to 2.5 Ga, highly consisting with similar to 2.7 Ga crustal growth and similar to 2.5 Ga magmatism resulted from juvenile crustal reworking in North China Craton. The zircons from 2.2 Ga to 2.0 Ga granitoids in Ordos Block basement display variable Hf isotopic composition of positive to negative epsilon(Hf)(t) values, much similar to those of 22-2.0 Ga granitoids related to continental margin arc setting in the Trans-North China Orogen. They were subsequently subjected to similar to 1.95 and similar to 1.85 Ga metamorphisms, which were widespread throughout the Trans-North China Orogen. Based on the highly coherent zircon geochronology and Hf isotopic composition between Ordos Block and other micro-blocks of North China Craton, we propose that there is a Neoarchean crust in Ordos Block, which underwent a tectonic evolution similar to those in East Block in later Neoarchean, and then experienced 2.2-2.0 Ga granitic magmatism related to continental margin arc and 1.95-1.85 Ga metamorphism resulted from collision between the East and West Blocks along TNCO. (C) 2015 Elsevier B.V. All rights reserved.