3800-MA TO 2500-MA CRUSTAL EVOLUTION IN THE ANSHAN AREA OF LIAONING PROVINCE, NORTHEASTERN CHINA

SHRIMP U-Pb zircon geochronology has revealed several generations of g ranitoids and gneisses from the Anshan area of northeastern China: two samples of the Baijiafen 'granite' (sensu late), 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; Ti ejiashan granite, 2962 +/- 4 and 2964 +/- 6 Ma; and Qidashan granite, similar to 2475 Ma. The similar to 3800 Ma Baijiafen 'granite' consist s 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 an tiquity have been found. The younger suites are predominantly granite (sensu stricto) in composition, and although foliated, they have not b een transformed into banded gneisses. Nd isotopic data suggest that th e < 3800 Ma granites were produced at least in part by remelting of ve ry ancient LREE-enriched sources, perhaps like the exposed remnants of the similar to 3800 Ma rocks. The Chentaigou supracrustals contain la yered mafic and ultramafic rocks, banded iron formation (BIF) and sili ceous layered rocks of probable volcanogenic origin. A sample of the l atter has yielded a concordant U-Pb zircon age of 3362 +/- 5 Ma, inter preted as the age of deposition. A trondhjemitic sheet cutting these r ocks has yielded a U-Pb zircon age of 3342 +/- 6 Ma. The Chentaigou su pracrustals are the oldest recognised in the area. Younger supracrusta l sequences collectively known as the Anshan group contains the main e conomic BIF units in the area. BIF is intruded by the similar to 2475 Ma Qidashan granite, indicating that the Anshan group is Archaean or e arliest Proterozoic in age.