Integrated garnet and zircon petrochronology reveals the timing and duration of orogenic events in the North China Craton

Abstract

Radiometric data from accessory zircon in long-lived polymetamorphic granulite facies rocks frequently show a large spread, with no clear relationship to zircon textures. For example, the granulites and associated rocks of the North China Craton which have an apparently continuous spread of metamorphic zircon dates through the Palaeoproterozoic from c. 1.95–1.80 Ga. The data are commonly interpreted to reflect long-lived metamorphism or discrete metamorphic events, with intermediate data reflecting variable Pb loss or a mixture of multiple growth domains. Herein, we present an approach that integrates direct dating of the P–T indexed mineral garnet with accessory zircon petrochronology which allows diagnostic time constraints for polymetamorphic processes in high-temperature rocks. We present high-precision LuHf and SmNd dates for two garnet-bearing mafic granulites and the adjoining granite and quartzite from the Gushan area between the Khondalite Belt and the Trans-North China Orogen, North China Craton. Statistically robust LuHf dates of 1815 ± 4 Ma and 1808 ± 3 Ma and corresponding SmNd isochron dates of 1790 ± 6 Ma and 1800 ± 9 Ma were obtained from the two mafic granulites, overlapping the zircon UPb date range of c. 1959–1785 Ma. Samarium–Nd dates from the accompanying garnet-bearing granite and quartzite are indistinguishable at 1794 ± 6 Ma and 1780 ± 6 Ma. The corresponding bulk garnet LuHf isochrons are poorly constrained—two-point garnet–matrix dates range from 1945 ± 10 Ma to 1849 ± 10 Ma for the granite and from 1936 ± 10 Ma to 1840 ± 9 Ma for the quartzite. Upper intercept UPb dates of 1950 ± 24 Ma (granite) and 1936 ± 17 Ma (quartzite) are obtained from the metamorphic zircons in equilibrium with garnet. The close agreement of SmNd isochron-dates (c. 1.80 Ga), the overlapping clusters of zircon UPb dates (c. 1.96 Ga), the identical LuHf and UPb dates of c. 1.80 Ga within uncertainty in the two granulites, and the older LuHf dates and comparable zircon UPb dates (c. 1.96 Ga) obtained in the garnet-bearing granite and quartzite than in those of the mafic granulites, strongly indicate that these rocks experienced coeval metamorphism from c. 1.95–1.80 Ga. These data require a prolonged episodic tectonometamorphic event in the North China Craton, which is probably related to the amalgamation of the Columbia supercontinent.