LA-ICP-MS U-Pb zircon ages of the Liaohe Group in the Eastern Block of the North China Craton: Constraints on the evolution of the Jiao-Liao-Ji Belt

Abstract

As a major lithostratigraphic unit of the Palaeoproterozoic Jiao-Liao-Ji Belt in the Eastern Block of the North China Craton, the Liaohe Group consists of an assemblage that is transitional from a lower arkose- and volcanic-rich sequence (Langzishan, Li'eryu and Gaojiayu Formations), through a middle carbonate-rich sequence (Dashiqiao Formation) to an upper argillaceous sequence (Gaixian Formation). Tectonically associated with the Liaohe Group are large volumes of granitoid rocks, named the Liaoji Granite, whose emplacement age has been dated at about 2.1-2.2 Ga. Regionally, the Liaohe Group and the associated Liaoji Granite extend along a NEE-trending belt that is flanked by 2.45-2.55 Ga basement gneisses. U-Pb zircon data and cathodoluminescence (CL) imaging are used to constrain the maximum depositional and metamorphic ages of the Liaohe Group, which are critical in understanding the tectonic setting and history of the Jiao-Liao-Ji Belt. Cathodoluminescence images reveal that most detrital zircon grains from the Liaohe Group are characterized by concentric oscillatory zoning, with comparatively low luminescence and high Th/U ratios, suggesting magmatic origin, but minor zircon grains have structureless overgrowth or recrystallization rims that are characterized by low Th/U ratio, indicating metamorphic origin. Results of this study, show that the detrital zircon grains of the Langzishan Formation, the lowest sequence of the Liaohe Group, have concordant U-Pb ages ranging from 2.05 to 2.24 Ga, suggesting that the Liaohe Group was deposited at some time after 2.05 Ga. Although most overgrowth or recrystallization rims are too narrow to be analyzed, some rims yield concordant or upper intercept ages around 1.93 Ga, interpreted as the peak metamorphic age of the Liaohe Group. Detrital zircon grains from the Dashiqiao Formation in the middle sequence of the Liaohe Group have concordant U-Pb ages ranging from 2.25 to 2.53 Ga, some of which are similar to those of the basement gneisses in the area. This study indicates that the Liaohe Group was deposited after the emplacement of the Liaoji Granite, and the latter may be a major component of the provenance for the Langzishan Formation. In contrast, the protoliths of the middle sequence of the Liaohe Group were most probably derived from the basement gneisses. These new zircon ages for the Liaohe Group and recent data for the Liaoji Granite, combined with lithological, structural and metamorphic considerations, support a rifting model for the evolution of the Jiao-Liao-Ji Belt. © 2004 Elsevier B.V. All rights reserved.