Zircon growth and ages of migmatites in the Algoma-type BIF-hosted iron deposits in Qianxi Group from eastern Hebei Province, China: Timing of BIF deposition and anatexis
Algoma-type BIFs and associated volcanic suites of the Qianxi Group in eastern Hebei Province have undergone high-grade metamorphism and anatexis. The anatectic event is genetically related to high-grade magnetite ores, but the age of the anatectic melting has not been well constrained. We present detailed textural relationship and internal structures of zircon grains and their age data from eight samples of migmatitic rocks representing the different Algoma-type BIF-hosted iron deposits to constrain the formation age of BIF deposition and subsequent anatexis. Six continuous zircon growth stages are distinguished by a series of low-CL and high-CL zones outside from center to edge: inherited magmatic zircon, bright-CL resorption domain, dark-CL recrystallization front, dark-gray-CL diffusion domain, light-gray-CL overgrowth and bright-CL resorption edge. The overgrowths are interpreted as a solid-state diffusion of Zr of primary zircon during interaction with anatectic melt, which resulted in different stages of chemical re-equilibration of primary domains and local re-deposition of newly grown domains on the suitable isostructural substrate of residual magmatic zircon. SHRIMP zircon U-Pb dating of inherited magmatic cores and discrete magmatic grains constrains the peak BIF-deposition age at 2520 Ma, which is different from the peak at 2.75-2.70 Ga for Algoma-type BIFs elsewhere in the world. Zircon U-Pb dating of light-gray-CL rims and newly grown homogeneous grains indicates that the anatectic event lasted from 2511 to 2485 Ma at least, immediately following the BIF deposition. The BIF depositional event is consistent with widespread late Neoarchean magmatism, and the anatectic event is consistent with regional metamorphic events in the eastern part of the North China Craton.