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

• Zircon dating reveals two periods of magmatism at ∼2.5 Ga and 2.2–2.0 Ga and metamorphism at ∼1.95 Ga and ∼1.85 Ga in OB.
• The ∼2.5 Ga granitic magmatism was resulted from crustal reworking, and 2.2–2.0 Ga granitoids was formed in island arc setting.
• Two phases of metamorphism were likely responded to collision between EB and WB.

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 ∼2.5 Ma and 2.2–2.0 Ga, and two phases of metamorphisms at ∼1.95 Ga and ∼1.85 Ga, respectively. Of which most zircons of ∼2.5 Ga granitoids show positive ɛHf(t) values ranging from +0.1 to +4.1 with a peak TDM of ∼2.7 Ga. Minor zircons have negative ɛHf(t  ) values from −3.8 to −0.02 and THfC of 2.8–3.1 Ga. Moreover, the 3.40 Ga inherited zircon and zircons with THfC 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 ∼2.7 Ga and granitic magmatism at ∼2.5 Ga, highly consisting with ∼2.7 Ga crustal growth and ∼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 ɛHf(t) values, much similar to those of 2.2–2.0 Ga granitoids related to continental margin arc setting in the Trans-North China Orogen. They were subsequently subjected to ∼1.95 and ∼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.

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