Petrography and zircon U–Pb isotopic study of the Bayanwulashan Complex: Constrains on the Paleoproterozoic evolution of the Alxa Block, westernmost North China Craton

• The Bayanwulashan Metamorphic Complex was formed in the Early Paleoproterozoic, not in the Archean.
• It was subjected to two metamorphic events at ca. 1.94–1.91 Ga and ca. 1.88–1.86 Ga.
• The Alxa Block joined the NCC with the final amalgamation at ca. 1.85 Ga.

The Bayanwulashan Metamorphic Complex (BMC) exposes along the eastern margin of the Alxa Block, the westernmost part of the North China Craton (NCC). BMC is principally composed of metamorphic rocks with amphibole plagiogneiss, biotite plagioclase gneiss and granitic gneiss. Our research has been focused on the petrography and zircon U–Pb geochronology of the BMC to better understand the evolution of the Alxa Block and its relationship with the NCC. Evidences from field geology, petrography, and mineral chemistry indicate that two distinct metamorphic assemblages, the amphibolite and greenschist facies, had overprinted the preexisting granitic gneiss and suggest that the BMC experienced retrograde metamorphic episodes. The LA-ICP-MS zircon U–Pb ages reveal that the primary magmatic activities of BMC were at ca. 2.30–2.24 Ga and the two metamorphic events were at ca. 1.95–1.91 Ga and ca. 1.88–1.85 Ga respectively. These ages indicate that BMC initially intruded during Paleoproterozoic, not as previously suggested at Archean period. The Early Paleoproterozoic metamorphic records and the magmatic thermochronological data in BMC exhibit different evolution paths between the Alxa Block and the NCC. The Alxa Block was most likely an independent Early Paleoproterozoic terrain. Following different amalgamation processes, The Alxa Block combined with Western Block at ca. 1.95 Ga and then united with NCC at ca. 1.85 Ga.