Zircon U-Pb geochronological constraints on the Paleoproterozoic crustal evolution of the Eastern block in the North China Craton

The North China Craton (NCC) is proposed to have formed by collision between the Western and Eastern blocks at ∼1.85 Ga. However, the detailed history of the individual blocks remains unsolved. In the Eastern block, the Paleoproterozoic volcano-sedimentary rocks are mainly exposed along its eastern margin, and are referred to as the Liaohe Group in the Liaodong Peninsula of Liaoning Province and as the Guanghua, Ji'an and Laoling Groups in the Tonghua area of southern Jilin Province. The Guanghua and Ji'an Groups are characterized by the occurrence of volcanic rocks, whereas the Laoling Group is composed of clastic sediments and carbonates. Zircon U-Pb isotopic dating by LA-ICP-MS and SHRIMP methods indicates that the Guanghua and Ji'an Groups were formed between 2.1 and 2.0 Ga, but that the rocks of the Laoling Group may have formed in two episodes at 2.5-2.2 and 2.0-1.9 Ga, respectively. The present data indicate that the Ji'an, Laoling and Liaohe Groups are chaotic, without a common stratigraphy, with intense deformation making it difficult to identify original layering and contact relationships. Combined with age data from rocks intrusive into the sequence, the Paleoproterozoic tectonic evolution of the Liaoning-Jilin area can be related to collision and extension between the Liaonan block to the south and the Longgang block to the north. Extension in the Longgang block occurred at 2.12-1.98 Ga and was represented by eruption of the Guanghua mafic volcanic rocks. However, extension in the Liaonan block is expressed by 2.16-2.09 Ga A-type granites and ∼2.12-1.98 Ga volcano-sedimentary rocks of the Ji'an Group. These two blocks collided at ∼1.93-1.90 Ga, resulting in a clockwise metamorphic P-T path recorded in the N. Liaohe-Laoling Groups, followed by ∼1.85 Ga post-collisional evolution expressed by anticlockwise metamorphic P-T paths and immense granitic intrusions in the S. Liaohe-Ji'an Groups. Therefore, it is suggested that the collision between the Longgang and Liaonan in the Eastern block of the NCC occurred a little earlier than ∼1.85 Ga, when the NCC amalgamated by collision of the Western and Eastern blocks.