3800 to 2500 Ma crustal evolution in the Anshan area of Liaoning Province, northeastern China

Abstract

SHRIMP UPb zircon geochronology has revealed several generations of granitoids and gneisses from the Anshan area of northeastern China: two samples of the Baijiafen ‘granite’ (sensu lato), 3804 ± 5 and 3812 ± 4 Ma; Chentaigou granite, 3306 ± 13 Ma; Lishan granite, 3142 ± 7 Ma; East and West Anshan granites, 3001 ± 4 and 2994 ± 8 Ma; Tiejiashan granite, 2962 ± 4 and 2964 ± 6 Ma; and Qidashan granite, ∼ 2475 Ma. The ∼ 3800 Ma Baijiafen ‘granite’ consists of banded, migmatitic trondhjemitic gneisses and forms only a small part of the area. Nonetheless, they are the oldest-known rocks in Asia and are one of very few locations in the world where rocks of this antiquity have been found. The younger suites are predominantly granite (sensu stricto) in composition, and although foliated, they have not been transformed into banded gneisses. Nd isotopic data suggest that the < 3800 Ma granites were produced at least in part by remelting of very ancient LREE-enriched sources, perhaps like the exposed remnants of the ∼ 3800 Ma rocks. The Chentaigou supracrustals contain layered mafic and ultramafic rocks, banded iron formation (BIF) and siliceous layered rocks of probable volcanogenic origin. A sample of the latter has yielded a concordant UPb zircon age of 3362 ± 5 Ma, interpreted as the age of deposition. A trondhjemitic sheet cutting these rocks has yielded a UPb zircon age of 3342 ± 6 Ma. The Chentaigou supracrustals are the oldest recognised in the area. Younger supracrustal sequences collectively known as the Anshan group contains the main economic BIF units in the area. BIF is intruded by the ∼ 2475 Ma Qidashan granite, indicating that the Anshan group is Archaean or earliest Proterozoic in age.