PbSL dating of garnet and staurolite: Constraints on the Paleoproterozoic crustal evolution of the Eastern Block, North China Craton

Abstract

It is well accepted that the North China Craton formed by assembly of the Eastern and Western blocksalong the Trans-North China Orogen, with cratonization in the Paleoproterozoic (1.85 Ma). However,there is controversy about the formation of the Eastern and Western blocks themselves, especially thetiming of cratonization of the Eastern Block. The Jiao-Liao-Ji orogenic belt is the most important Paleoproterozoic Orogenic belt in the Eastern Block and consists mainly of the North and South Liaohe Groups and the Liaoji Granitoids. The timing of metamorphism of the Liaohe Group is the key to understanding the evolution of the Eastern Block of the North China Craton. Because the isotope systematics ofmetamorphic minerals can be used to date parts of the prograde thermal histories of metamorphic belts, dating of minerals, such as garnet and staurolite has the distinct advantage that time information can be related directly to the pressure–temperature-deformation history recorded by the same mineral. In this paper we present results of precise 207Pb/206Pb stepwise-leaching (PbSL) dating of garnet and staurolite from the metasedimentary rocks of the south Liaohe Group. The primitive Pb and radiogenic Pb are selectively extracted from garnet and straurolite by leach steps and determined by multiple collector (MC-) ICP-MS. The garnet and staurolite record two groups of 207Pb/206Pb ages, at 1.93–1.91 Ga and 1.86 Ga, respectively. These constrain the time of peak metamorphism of the Liaohe Group during collision between the Longgang Block in the north and the Liaonan (Nangrim) Block in the south and a subsequent post-collisional event, respectively. Our data for the South Liaohe Group and previously published data for the Liaoji Granite, combined with key lithological, structural and metamorphic information, indicate that the cratonization of the Eastern Block of the North China Craton occurred at 1.93–1.91 Ga.