Timing of deposition and tectonothermal events of banded iron formations in the Anshan–Benxi area, Liaoning Province, China: Evidence from SHRIMP U-Pb zircon geochronology of the wall rocks

Abstract

Most of the Algoma-type BIFs and associated volcanic suites in the North China Craton (NCC) have undergone high-grade metamorphism during the ∼2.5Ga tectonothermal event. Isotopic systems in high-grade metamorphic rocks are often reset, thereby losing the earlier records, whereas zircons in low-grade metamorphic rocks are potential candidates to evaluate earlier thermal events. Detailed textural relationship and internal structures of zircons and their age data for eight wall-rock samples with metamorphic grades from greenschist to upper amphibolite facies of the Algoma-type BIFs in the Anshan–Benxi area are used to constrain the age of protolith. Zircons in the greenschist to lower amphibolite facies are generally metamorphism-resistant, whereas those in the upper amphibolite facies rocks show variable recrystallization. SHRIMP zircon U–Pb dating of magmatic zircons in six samples constrains the peak of BIF-deposition at 2.56–2.53Ga, which is consistent with ∼2.5Ga crustal growth in the NCC. In addition, the greenschist-facies metamorphic rocks in the Anshan area also record an event of volcanic activity at 2.72–2.65Ga. We consider that low-grade metamorphic wall rocks were not significantly affected by the ∼2.5Ga tectonothermal event, and the 2.72–2.65Ga age group records another important magmatic event corresponding to ∼2.7Ga crustal growth. The metamorphic zircons from these rocks record a thermal event during 1.88–1.84Ga.