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Provenance of Meso- to Neoproterozoic cover sediments at the Ming Tombs, Beijing, North China Craton: An integrated study of U-Pb dating and Hf isotopic measurement of detrital zircons and whole-rock geochemistry

Abstract:

The North China Craton (NCC) was subjected to an extensional regime after the Lüliang movement at ~1.8Ga and then was covered by an extensive Meso- to Neoproterozoic sedimentary succession, namely the Changcheng, Jixian and Qingbaikou Groups in ascending order. We report age spectra for detrital zircons and monazites, Hf isotopic systematics of detrital zircons, and whole-rock chemical and Nd isotopic compositions for sediments from the succession in the Ming Tombs area, Beijing, one of the typical Meso- to Neoproterozoic areas in the NCC. Detrital zircons of six sedimentary samples have two distinct age peaks at ~2.52Ga and ~1.85Ga. There are some detrital zircons at 2.4-2.0Ga but none at 2.3Ga and only a few >2.7Ga. The detrital zircon age spectra change with time. Sediments in the lower succession (Changcheng Group) and in the upper successions (Jixian and Qinbaikou Groups) are dominated by significant detrital zircon populations of late Neoarchean and late Paleoproterozoic ages, respectively. The ~2.5Ga detrital zircons of the Changcheng Group have εHf(2.5Ga) values and tDM(Hf) model ages mainly ranging from -2 to +7 and 2.8 to 2.7Ga, respectively. Detrital monazites of a sample from the Jixian Group exhibit a major age peak between 1.95 and 1.80Ga with some data between 2.0 and 1.95Ga. The sedimentary rocks of the Changcheng Group are characterized by high K2O contents (mostly 7.09-15.20%) and insignificant Eu anomalies (Eu/Eu*=0.71-1.16). They have tDM(Nd) model ages ranging from 2.70 to 2.43Ga, being older than the tDM(Nd) ages (2.11 and 1.99Ga) of sedimentary samples from the Qingbaikou Group. Based on a comparison with ages for the early Precambrian (>1.8Ga) basement of the NCC, it can be concluded that (1) the sediments of the Meso- to Neoproterozoic cover were undoubtedly derived from the NCC itself or once neighboring terranes; (2) variations in the detrital zircon age spectra from the lower to the upper successions reflect provenance evolution in that the lower crustal late Paleoproterozoic rocks were exposed at the surface after the upper crustal late Neoarchean rocks had already been eroded. © 2011 International Association for Gondwana Research.