Petrogenesis and Significance of Volcanic Rocks of the Northern Songliao Basin: Constraints of Geochronology, Elements Geochemistry and Sr-Nd-Pb-Hf Isotopes

Keywords: Songliao Basin; Basement Volcanic Rocks; Elements Geochemistry ;Petrogenesis ;Sr-Nd-Pb-Hf Isotopes The Songliao Basin is located in the central part of Northeast China, in the eastern part of the Central Asian orogenic belt. It is the largest terrestrial Cretaceous basin in China and also the site of the final closure of the ancient Asian Ocean. The HT-1 well is located in the northern part of the Songliao Basin, drilling to the basement volcanic rock. Studying this basement volcanic rock is of great significance for exploring the basement age, structural evolution, and closure process of the ancient Asian Ocean in the Songliao Basin.This work carried out systematic petrology, zircon U-Pb chronology, and elemental and isotopic geochemistry research. The zircon U-Pb dating results show that the age of the basement volcanic rocks is 262-277 Ma, which was formed during the Permian period, indicating a significant volcanic activity in the region during the Permian period. This set of volcanics is enriched in light rare earth elements, with high fractionation of light and heavy rare earth elements, presenting a right leaning rare earth distribution pattern. It is enriched in large ion lithophile elements such as Rb, Ba, U, K, and lacks high field strength elements such as Nb, Ta, P, and Yb, exhibiting negative Eu anomalies. The isotopic characteristics are characterized by low (87Sr/86Sr) i (0.70376~0.70474) and high ε Nd (t) (-0.13~+3.09), initial values of high Pb isotopes (average values of 208Pb/204Pb , 207Pb/204Pb and 206Pb/204Pb are 38.59, 15.56, 18.66, respectively), and high εHf (t) (+8.70~+15.37) is a characteristic. Based on the geochemical characteristics of the above elements and isotopes, it is indicated that the magma source area of the basement volcanic rocks in the Songliao Basin has the characteristics of a mixed source area of newly formed crust and depleted mantle. The potential development area of the basement volcanic rocks was in an active continental margin tectonic environment during the Permian, which may have formed in the tectonic background of subduction of ancient Asian ocean basins. Fig 1 Chondrite-normalized REE patterns and primitive mantle-normalized trace elements spidergram of the basement volcanics(normalized values from Sun and McDonough,1989) Fig 2 Tectonic discriminative diagrams for the basement volcanics(after Gorton et al.,2000) Fig 3 (87Sr/86Sr)i-εNd(t) diagramso for the basement volcanics(after Jahn et al.,1999; Zhang et al.,2021) References: [1] Sun S S, Mcdonough W F. 1989. Chemical and isotopic systematics of oceanicbasalts: implications for mantle composition and process. Geological Society, London, Special Publications, 42(1):313~345. [2] Gorton M P, Schandl E S. 2000. From continents to island arcs: a geochemical index of tectonic setting for arc-related and within-plate felsic to intermediate volcanic rocks. The Canadian Mineralogist, 38(5):1065~1073. [3] Zhang Lishi, Sun Fengyue, Li Bile, et al. 2021. Petrogenesis and tectonic setting of granitoids in the Fu anpu molybdenum deposit, Lesser Xing an Zhangguangcai range metallogenic belt: constraints from element geochemistry, zircon U-Pb geochronology and Sr-Nd-Hf isotopes. Acta Geologica Sinica, 95(8): 2471~2492.