Tectonic evolution of the Dadongcha Formation, Ji'an group, Qinghe area, Tonghua, NE China: Implications from geochronological and geochemical evidence of metapelites

摘要

The Jiao-Liao-Ji Belt (JLJB) is an important Paleoproterozoic orogenic belt that has been a focus of research to understand the Precambrian evolution of the North China Craton (NCC). The widespread Paleoproterozoic Ji'an group, which was exposed in the northern margin of JLJB, is the key to solving controversies in the tectonic setting of this special belt. An intensive study on metamorphic reactions, zircon U-Pb ages, mineral thermo-barometry, geochemistry, and quantitative phase equilibria modeling (pressure-temperature pseudosections) of typical metapelites from the Dadongcha Formation of the Ji'an group provides important insight to understand the tectonic setting and processes of the Paleoproterozoic JLJB. The analyzed metapelites are exposed in Toudao and Qinghe towns in NE China, include garnet-biotite schist, garnet-biotite-plagioclase gneiss, garnet-sillimanite-cordierite-biotite gneiss, and hypersthene-spinel-cordierite-biotite gneiss. Garnets contain mineral inclusions such as sillimanite, biotite, feldspar, and quartz and retain the typical cordierite reactive edge structures. The protoliths of metapelites are sets of claystone or miscellaneous sandstone with an average chemical composition that is close to that of the upper continental crust (UCC). Chemical Index of Alteration (CIA) and Index of Compositional Variability (ICV) values suggest a high degree of weathering of their sources and low compositional maturity of clastic sediments. The chondrite-normalized rare earth element (REE) pattern of analyzed samples displays characteristics of medium fractionation in light and heavy REEs, and strong enrichment in light REEs. The primitive mantle-normalized trace element pattern shows enrichment of large ion lithophile elements (LILEs) such as Ba, and the content of high field strength elements (HFSEs) such as Zr and Hf is relatively low; furthermore, Nb, Ta, P, and Ti are distinctly depleted. The emphasis is on the metamorphism of metapelites, which shows a clockwise P-T path. In combination with zircon U-Pb dating, a general evolution model for the metapelites was drawn. The metapelites were formed in an active continental margin environment where the evolved island arcs developed around 1.95-2.02 Ga, then were involved in the collision between the Longgang and Langrim blocks in the eastern part of the NCC, and returned to the surface after undergoing regional metamorphism with widespread anatexis around 1.8-1.95 Ga.