The Liaohe Group: An insight into the Paleoproterozoic tectonic evolution of the Jiao–Liao–Ji Belt, North China Craton

Abstract

The Jiao–Liao–Ji Belt represents one of the largest exposed regions of Paleoproterozoic rocks along the eastern margin of the North China Craton. Paleoproterozoic basement material in this area is dominated by deformed and metamorphosed volcano-sedimentary successions with minor amounts of granitic and mafic intrusions. This study presents the results of a systematic zircon U–Pb geochronological, whole-rock geochemical and Sm–Nd isotopic analysis of the volcano-sedimentary successions within the Liaohe Group of the Jiao–Liao–Ji Belt. These data provide insights into the multiple Paleoproterozoic tectonothermal events that have affected the belt as well as the petrogenesis of the Paleoproterozoic meta-volcanics and metasedimentary rocks on the Liaodong Peninsula. Accurate U–Pb age and trace element determinations of zircons from the meta-volcanics in this region using laser ablation-inductively coupled plasma-mass spectrometry indicate that these units formed at ca. 2.2 Ga and were metamorphosed at ca. 1.9 Ga, respectively. These data indicate that the Paleoproterozoic geochronological framework of this region is dominated by 2.2–2.0 Ga magmatic, 1.98–1.90 Ga sedimentary, ca. 1.90 Ga metamorphic, and 1.87–1.82 Ga post-tectonic magmatic events. The meta-volcanics can be split into basalt, basaltic andesite, andesite, dacite, and rhyolite units that define a continuous calc-alkaline-dominated magmatic sequence that is similar to the sequences commonly found within Phanerozoic magmatic arcs. The meta-basalts are enriched in the light rare earth elements, Cr, Co, and Ni, and are depleted in the heavy rare earth elements and the high field strength elements (e.g., Nb, Ta, and Ti), indicating that they formed from magmas generated by the partial melting of lithospheric mantle material that was previously metasomatized by subduction zone-derived fluids/melts. The large geochemical and isotopic variations within the metamorphosed andesite, dacite, and rhyolite units all provide evidence of magma mixing between felsic melts generated by the partial melting of Paleoarchean–Neoarchean basement material and the mafic melts that may also formed the meta-basalts within the Liaohe Group or evolve via fractional crystallization prior to mixing. The geochemistry of the metasedimentary rocks indicate that the pre-metamorphism sedimentary units ranged from being compositionally mature to being immature and poorly sorted, and were weakly to moderately weathered. These sediments were derived primarily from metamorphosed felsic volcanics within the Liaohe Group as well as the Paleoproterozoic Liaoji granites. They also contain Archean continental crustal material that is derived mainly from ancient intermediate–mafic basement units. The combined geochronological, geochemical, and geological data indicate that a Paleoproterozoic arc–continent collision model can well explain the nature and formation of the Jiao–Liao–Ji Belt.