Wei Sun

The attributes of Late Paleozoic magmatic events are of paramount significance in elucidating the tectonic evolution of the Ulanhot region, which is located in the middle of the Hegenshan–Heihe tectonic belt (HHTB). This study undertook a comprehensive investigation of the petrography, LA–ICP–MS zircon U–Pb dating, whole rock geochemistry, and zircon Hf isotopes of the Early Carboniferous volcanic rocks. The volcanic rocks are predominantly composed of andesite, schist (which protolith is rhyolitic tuff), and rhyolitic tuff. The results of zircon U–Pb dating reveal that the formation ages of volcanic rocks are Early Carboniferous (343–347.4 Ma). Geochemical characteristics indicate that the andesites possess a comparatively elevated concentration of Al₂O₃, alongside diminished levels of MgO and TiO₂, belonging to the high-K calc-alkaline series. The zircon εHf(t) of the andesites range from −13 to 9.4, while the two-stage Hf model ages span from 697 to 1937 Ma. The felsic volcanic rocks have high contents of SiO₂ and Na₂O + K₂O, low contents of MgO and TiO₂, and belong to high-K to normal calc-alkaline series. The zircon εHf(t) values of the felsic volcanic rocks range from −12.8 to 10, while the two-stage Hf model ages span from 693 to 2158 Ma. The Early Carboniferous volcanic rocks exhibit a notable enrichment in large ion lithophile elements (LILEs, such as Rb, K, Ba) and light rare earth elements (LREEs), depletion in high-field-strength elements (HFSEs, including Nb, Ta, Ti, Hf), as well as heavy rare earth elements (HREEs). The distribution patterns of the rare earth elements (REEs) demonstrate a conspicuous right-leaning tendency, accompanied by weak negative Eu anomalies. These characteristics indicate that the andesites represent products of multistage mixing and interaction between crustal and mantle materials in a subduction zone setting. The felsic volcanic rocks originated from the partial melting of crustal materials. Early Carboniferous igneous rocks formed in a volcanic arc setting are characteristic of an active continental margin. The identification of Early Carboniferous arc volcanic rocks in the Central Great Xing’an Range suggests that this region was under the subduction background of the oceanic plate subduction before the collision and amalgamation of the Erguna–Xing’an Block and the Songnen Block in the Early Carboniferous.

The Late Paleozoic tectonic evolution of the Xing’an block in the eastern Central Asian orogenic belt has long been the subject of debate. In this paper, a comprehensive study of U-Pb zircon ages, Lu-Hf isotopes and whole-rock elemental analyses was carried out on Hadayang schists. Representative samples of the epidote-biotite-albite schist and biotite-albite schist yielded the weighted mean ²⁰⁶Pb/²³⁸U ages of 360 ± 2 Ma and 355 ± 3 Ma, respectively. This indicated the presence of Late Devonian–Early Carboniferous intermediate-basic rocks in the eastern Xing’an block. The Hadayang schists exhibited a Na-rich, tholeiitic and calc-alkaline affinity in composition with low Mg# (35.2–53.0), Cr (23.7–86.5 ppm), Ni (21.1–40.0 ppm) and Co (12.1–30.6 ppm). They were characterized by enrichment of LILEs, depletion of HFSEs and highly positive zircon ε_Hf(t) values (the average values were +8.93 and +9.29, respectively). The magma source of the Hadayang schists was a mantle that consisted of both spinel and garnet lherzolite, with a partial melting degree of 1%–5%, and it had undergone fractional crystallization of olivine, orthopyroxene and plagioclase. The Hadayang schists, together with other Late Devonian–Early Carboniferous intermediate-basic magmatic rocks in the eastern Xing’an block, were formed in an intracontinental extension tectonic setting similar to that of the North American Basin and Range basalt. Moreover, Late Devonian–Early Carboniferous ophiolite under a similar tectonic background in the western Xing’an block has been reported. We believe that the Xing’an block would have been in the stage of intracontinental extension during the Late Devonian–Early Carboniferous.

《地球科学》(ISSN 1000-2383;CN42-1874/P) (原刊名《地球科学——中国地质大学学报》ISSN 1000-2383;CN42-1233/P)，创刊于1957年，是由教育部主管、中国地质大学主办的综合性地学学术理论期刊，月刊，2015年改名为《地球科学》,她以反映中国地质大学教学与科研最新的、高水平的基础地质、应用地质、资源与环境地质及地学工程技术研究成果为主要任务。