Earliest Paleoproterozoic supracrustal rocks in the North China Craton recognized from the Daqingshan area of the Khondalite Belt: Constraints on craton evolution

Abstract

The Upper Wulashan “Subgroup” at Daqingshan in the Khondalite Belt of the Western Block of the North China Craton contains an Early Paleoproterozoic (2.5–2.45 Ga) supracrustal sequence, incorporating banded iron for- mations,whichwerefertoastheDaqingshanSupracrustalRocks.Theycontainroundedtoellipticalzircongrains that commonly show core–rim or core–mantle–rim structures and precise SHRIMP U–Pb dating of these do- mains allows the discrimination of a series of tectono-thermal events that straddle the Archean/Proterozoic boundary. Detrital zircon cores with oscillatory zoning have formation ages ranging from 2.55 to 2.50 Ga, indi- cating derivation from Late Neoarchean magmatic rocks. As a result of exhumation and erosion, these became incorporated in earliest Paleoproterozoic sediments that were deposited between 2.50 and 2.45 Ga and then underwent high-grade metamorphism at 2.45–2.40 Ga. This event variously recrystallized the cores into two main domains: a dark inner domain and a gray outer domain, as imaged in cathodoluminescence. The gray do- mains commonly show sector zoning and have Th/U ratios of 0.1–0.5, a feature commonly noted in zircons recrystallized under high-grade metamorphic conditions. Metamorphic mantles are generally more homoge- neous in structure and have lower Th/U ratios (commonly b0.1) than the recrystallized domains, although they show similar age distributions. However, they are difficult to distinguish when only two domains are pres- entinthezirconandtheyrepresentacontinuumofrecrystallization.Therocksthenunderwentasecondepisode of high-grade metamorphism in the Late Paleoproterozoic, as revealed by thin overgrowth rims that are homo- geneous or show only weak zoning, have low Th/U ratios (commonly b0.1), and record ages of 1.95–1.90 Ga. Thisisthe first time that two tectono-thermaleventsof Early and Late Paleoproterozoicage havebeenidentified in single rock samples from the Western Block of the North China Craton. The conclusion that unequivocal ear- liest Paleoproterozoic sediments are present in the North China Craton is also supported by evidence that the Daqingshan Supracrustal Rocks are cut by anatectic garnet granite, previously dated at 2.39 Ga. Combined withevidenceobtainedhereandelsewhere inthe North ChinaCraton,weconcludethat the2.45–2.40 Gameta- morphism was not an extension of the well-established, craton-wide Late Neoarchean tectono-thermal event, but a previously unrecognized episode that followed a short quiescent period that allowed sedimentation of the Daqingshan Supracrustal Rocks.