LA-ICP-MS U-Pb zircon ages of the Carboniferous volcanic and sedimentary rocks of the Songliao block, NE China: implications for the tectonic evolution of the eastern segment of the Central Asia Orogenic Belt

Abstract

The Songliao block is in the centre of NE China, located at the eastern segment of the Central Asia Orogenic Belt (CAOB). This block has been generally considered to suture with the Xing’an block during the Paleozoic along the Hegenshan-Heihe belt, which marked the closure of the Paleo-Asian Ocean and the amalgamation of NE China. However, the closure timing and the tectonic setting has not been fully elucidated due to the lack of the precise age dating for the volcanic and sedimentary rocks in the Songliao block. In this study, the LAICP-MS U-Pb zircon age analyses were conducted for a suite of volcanic and sedimentary rocks collected from the Carboniferous stratigraphic formations in the northern border of the Songliao block, in order to shed new light on the tectonic evolution of this area. Both the basaltic sample from the Baishan formation and the rhyolitic sample from the Hetaoshan formation show porphyritic textures with aphanitic groundmass and subhedral to angular plagioclase being porphyroblast. The magmatic zircons with clear oscillatory zonings from the basaltic and rhyolitic samples yield mean 206Pb/238U ages of 301±6 Ma and 318±9 Ma respectively, which can be considered as the volcanic eruption ages. The sedimentary rocks from the Kunaerhe formation are mainly composed of quartz, lithic fragments, and rounded plagioclase, poorly sorted and cemented by the clayey groundmass, indicating the relatively short transportation distance. Detrital zircons of the sedimentary rocks are various both in morphology (prismatic, columnar, rounded) and in structure (oscillatory zoning, concentric zoning, nebulous rim, metamorphic rim, structureless), indicating their varied provenances. All the isotopic analyses for detrital zircons generate a relatively tight age-cluster, ranging from 316 Ma to 513 Ma. The youngest zircon age zircon age is 316±6 Ma (the apparent 206Pb/238U age), which is consistent to the volcanic eruption age. Considering all data together, it can be proposed that (1) a volcanic event took place during the Late Carboniferous along the northern border of the Songliao block, which is most likely incurred by the subduction of the Paleo-Asian-Ocean; (2) the sedimentary rocks were formed in the volcanic arc environment and accepted the depositional materials directly from the arcs. Therefore, it can be concluded that the eastern segment of CAOB underwent oceanic crust subduction and formed arc-related basins during the Late Carboniferous period.