Earliest paleoproterozoic supracrustal rocks in the north china craton recognized from the daqingshan area of the khondalite belt: Constraints on craton evolution
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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 formations, which we refer to as the Daqingshan Supracrustal Rocks. They contain rounded to elliptical zircon grains that commonly show core–rim or core–mantle–rim structures and precise SHRIMP U–Pb dating of these domains 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, indicating 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 domains 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 homogeneous in structure and have lower Th/U ratios (commonly < 0.1) than the recrystallized domains, although they show similar age distributions. However, they are difficult to distinguish when only two domains are present in the zircon and they represent a continuum of recrystallization.The rocks then underwent a second episode of high-grade metamorphism in the Late Paleoproterozoic, as revealed by thin overgrowth rims that are homogeneous or show only weak zoning, have low Th/U ratios (commonly < 0.1), and record ages of 1.95–1.90 Ga. This is the first time that two tectono-thermal events of Early and Late Paleoproterozoic age have been identified in single rock samples from the Western Block of the North China Craton. The conclusion that unequivocal earliest 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 with evidence obtained here and elsewhere in the North China Craton, we conclude that the 2.45–2.40 Ga metamorphism 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.
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