Studies on the 2.2 Ga monzogranite and its wall rocks in western part of North Qinling Orogen: Constraints on early Precambrian tectonic attributes and evolution of the southwestern margin of Ordos Block

Abstract

Precambrian continent remnants in Phanerozoic orogenic belts contain vital information about not only the Precambrian evolution history of continent remnants but also the relationship between orogenic belts and adjacent blocks. This paper presents an integrated study of petrology, geochemistry and accessory mineral chronology on a monzogranite, its wall rocks biotite-quartz schist and an amphibolite interlayer in the schist in the Kuanping Group, from the western part of the North Qinling Orogen (NQO), adjacent to the southwestern margin of the Ordos Block (OB). The results reveal that the crystallization age of the monzogranite is 2200 ± 12 Ma and the protolith of the biotite-quartz schist deposited during 526–447 Ma, and they experienced similar multistage metamorphism during 450–400 Ma. The 2.2 Ga formation age of the monzogranite is significantly older than the protolith deposition age of the biotite-quartz schist, and there is almost no ca. 2.2 Ga detrital zircon age in the schist, indicating that the monzogranite is neither an intrusive body in the Kuanping Group, nor its provenance. Considering the ca. 2.2 Ga magmatism is widely distributed in North China Craton but never found in the NQO, and the monzogranite and host schist recorded similar metamorphic ages, we proposed that the monzogranite initially formed at the southwestern margin of the North China Craton and involved into the NQO during the closure of the Kuanping sedimentary basin at early Paleozoic. Combined with existing studies, three Paleoproterozoic magmatic events of ca. 2.5 Ga, 2.2–2.0 Ga and 1.8–1.75 Ga, and one ca. 1.9–1.85 Ga metamorphic event occurred at the southwestern margin of OB. These tectono-thermal events as well as their geological setting are very similar to the basement rocks of the central-southern part of the OB. Therefore, the early Precambrian rocks at the southwestern margin of the OB should belong to the basement of the central-southern part of OB, and the late tectonic events placed them at their present position.