Formation age and genesis of the banded iron formations from the Guyang Greenstone Belt, Western North China Craton

摘要

Previous studies focused on banded iron formations (BIFs) from the eastern North China Craton (NCC), few were conducted on those of the western NCC. This paper concentrates on two BIFs, Sanheming and Gongyiming, from the Guyang Greenstone Belt (GB), western NCC. SIMS U-Pb analyses of zircons from the amphibolite interlayers of Sanheming and Gongyiming BIFs gave upper intercept ages of 2562 ± 14 Ma and 2569 ± 78 Ma, respectively, coupled with 2555 ± 56 Ma for the tonalite with BIF xenoliths, the formation age of the BIFs is inferred to be 2.56-2.57 Ga. Protolith reconstruction based on geochemistry and field occurrence of the amphibolites shows that they are originally basaltic. Trace elements reveal that the basalts of Sanheming and Gongyiming are T-MORB and N-MORB, respectively. Metabasites from other locations of the Guyang GB in previous studies display island arc geochemical characteristics. Plus the lithologic assemblage of komatiite, basaltic komatiite, boninite, tholeiite, high-Mg andesite and