The isotope geochemistry of host rocks of the late Archean Guandi and Banshigou banded iron formations, southern Jilin Province: temporal and tectonic significance

Abstract

Abstract The Guandi and Banshigou iron deposits of southern Jilin Province, China, are Algoma-type banded iron formations (BIFs) located in the Eastern Block of the North China Craton (NCC). LA–ICP–MS U–Pb dating shows the magmatic zircons in plagioclase amphibolite from the Guandi BIF (PA–GD) and plagioclase hornblende gneiss from the Banshigou BIF (PHG–BSG) were crystallised at 2556 ± 17 Ma and 2545 ± 27 Ma, respectively, representing the formation age of the BIFs. Metamorphic overgrowth zircons from both samples gave two U–Pb ages of 2452 ± 23 Ma and 2446 ± 70 Ma, indicating the age of metamorphism. The zircon εHf(t) values of PA–GD (–1.3 to +2.8) and PHG–BSG (–1.5 to +2.7) indicate that the magma source was derived from depleted mantle with contamination of crustal material. According to our study, the PA–GD and PHG–BSG were formed in an island arc setting and support previously proposed subduction zone tectonic models for the northeastern NCC during the Neoarchean. KEY POINTS The magmatic zircons from the PA–GD and PHG–BSG were crystallised at 2556 ± 17 Ma and 2545 ± 27 Ma, which represents the formation age of the BIFs. The metamorphic overgrowth zircons from the PA–GD and PHG–BSG gave two ages of 2452 ± 23 Ma and 2446 ± 70 Ma, which represent the age of metamorphism. Major-element and REE data indicate that BIFs precipitated in a mixture of seawater and hydrothermal fluids in an anoxic environment. Geochemical and Hf isotope characteristics of BIFs’ wall rock indicate that the source magma was derived from a depleted mantle wedge metasomatised by subduction-slab melts and contaminated by ancient crust, formed in an island arc setting.