Tectonic setting of the South Liaohe Group in the Jiao‐Liao‐Ji Belt, North China Craton: Geochemical and geochronological constraints from metasedimentary rocks. Issue 4 (9th December 2021) - Wang, Pengsen
Abstract : The metasedimentary rocks of the South Liaohe Group are widely distributed along the Jiao‐Liao‐Ji Belt (JLJB) in eastern Liaoning Province. It is important to understanding tectonic setting and evolution of the JLJB by determining its sediment sources and properties of the provenances, sedimentary facies, metamorphic ages, and facies. After the detailed field geological surveys and comprehensive laboratory observation, many high‐grade metapelitic rocks are identified in the South Liaohe Group. The main rock type is sillimanite‐garnet‐cordierite‐biotite plagiogneiss. The zircon U–Pb geochronology of three samples shows that these high‐grade meta‐pelitic rocks are mainly derived from the Liaoji Granites and the Archean basement. The protolith formation ages are between 2.00 and 1.93 Ga. Subsequently, these protolithes underwent high‐pressure metamorphism resulted from collisional orogeny, with peak metamorphism at ca. 1.88 Ga. Then, at 1.88–1.84 Ga, these rocks experienced the retrograde metamorphism and exhumed to the surface during crustal uplift. Geochemical data show that the protoliths are alumina‐rich claystones and graywackes with low maturity, implying near‐source sedimentation. The chemical index of alteration (CIA) values of these samples range from 60 to 80, indicating that the protoliths underwent moderate weathering. The sediment source was mainly felsic components of the upper crust, possibly with the addition of ancient sediments. Various tectonic environment discrimination diagrams combined with previous results on metamorphism and isotope chronology of these metasedimentary rocks in the JLJB indicate that the rocks in the study area originated from an active continental margin with an evolving island arc. Abstract : Through detailed field geological surveys and comprehensive laboratory research, many high‐grade metapelitic rocks are identified in the metasedimentary rock series of the South Liaohe Group. The preliminary results from petrography, mineral phase transformations, and temperature and pressure calculations show that these high‐grade meta‐argillaceous rocks record a clockwise P–T path of approximately isothermal decompression that related to a collisional orogeny. A comprehensive study shows that the associated high‐grade metapelitic rocks are mainly formed in an active continental margin with an evolving island arc.