LA-ICP-MS U-Pb zircon geochronology of the Yushulazi Group in the Eastern Block, North China Craton

Abstract

The Paleoproterozoic Jiao-Liao-Ji belt in the North China craton is composed mainly of the Liaohe Group (and its equivalents) and the Liaoji Granites. They are uncomformably overlain by the sedimentary Yushulazi Group, which itself is overlain by the Neoproterozoic (0.9-0.8 Ga) Yongning Group. The Yushulazi Group has long been considered as a Proterozoic assemblage, but its depositional age has never been precisely constrained in terms of modern geochronology. Cathodoluminescence images reveal that most detrital zircon grains from this formation are characterized by concentric-oscillatory zoning, with comparatively low luminescence and high Th/U ratios, suggesting an igneous origin, whereas minor grains have structureless overgrowth or recrystallization rims that are characterized by low Th/U ratios, indicating a metamorphic origin. The results of LA-ICP-MS U-Pb zircon analyses show that the detrital zircon grains have concordant U-Pb ages of two populations - i.e., 1.05-1.37 Ga and 1.87-2.51 Ga - with an inherited zircon core giving a concordant age of 3.35 Ga. The U-Pb age range of 1.87-2.51 Ga is in good accordance with that of the Paleoproterozoic Liaohe Group and other basement rocks in the Eastern Block of the North China craton. Zircon ages of 1.05-1.31 Ga are, for the first time, reported for the Precambrian basement. The recognition of these Mesoproterozoic (1.05-1.31 Ga) igneous zircons from the Yushulazi Group indicate the existence of Grenvillian-aged magmatism in the Eastern Block of the North China craton. The Yushulazi Group must have deposited at some time between 1.05 Ga and 0.9 Ga, prior to sedimentation of the 0.9-0.8 Ga Yongning Group and it is not the upper part of the Paleoproterozoic Liaohe Group, as previously suggested. Copyright © 2006 by V. H. Winston & Son, Inc. All rights reserved.