SHRIMP data on zircons from the Wanzi series: Constraints on the rock formation time and implications of migmatization at 2.1–2.0 Ga in the Fuping Complex, North China Craton

In most of the rocks of the Fuping Complex in the Trans-North China Orogen, a large quantity of leucocratic veins, dykes and granitic intrusions are present as migmatites. The incongruent partial melting of biotite to hornblende suggests the water-fluxed anatexis of the migmatization of which the melt is manifested by the euhedral crystallization of some quartzofeldspathic minerals and presence of albite rim on earlier plagioclase or K-feldspar. Importantly, some melt may be segregated in the sillimanite gneisses or extracted and injected as faint dispersed melt into the competent metasedimentary quartzite of the Wanzi series rocks. The quartzite has plenty of zircon ages cluster at c 2.5 Ga and some at c 2.1–2.0 Ga, the former may be detrital zircon age, while the latter represents the age of migmatization or crystallization of the injected melt, not detrital zircon ages as conventionally believed. The data have constrained that the sedimentation time of the protolith of the quartzite of the Wanzi series was between c 2.5 Ga and c 2.1–2.0 Ga, not younger than c 2.1–2.0 Ga as once proposed. Therefore it must be cautious in applying detrital zircon ages in constraining the time of sedimentation if the rocks have undergone middle-high grade metamorphism, especially when migmatization was involved in the metamorphism process.

► Wanzi series migmatite is formed through water-fluxed anatexis and melt injection.
► The melt in metasediments may grow new zircons and can be misinterpreted as detrital.
► The protolith of the Wanzi series quartzite is older, not younger, than c 2.1 ∼ 2.0 Ga.