Earth Science Frontiers ›› 2022, Vol. 29 ›› Issue (4): 385-402.DOI: 10.13745/j.esf.sf.2021.12.2

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Genesis of Carboniferous volcanic rocks in northeastern Junggar Basin: New insights into the Junggar Ocean closure

JIAO Xiaoqin1,2(), ZHANG Guanlong3, NIU Huapeng1,2,*(), WANG Shengzhu3, YU Hongzhou3, XIONG Zhengrong3, ZHOU Jian3, GU Wenlong1,2   

  1. 1. State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum, Beijing 102249, China
    2. College of Geosciences, China University of Petroleum, Beijing 102249, China
    3. Research Institute of Petroleum Exploration and Development, Sinopec Shengli Oilfield Company, Dongying 257015, China
  • Received:2021-07-29 Revised:2021-12-12 Online:2022-07-25 Published:2022-07-28
  • Contact: NIU Huapeng


The Junggar Ocean is an important branch of the northern Paleo-Asian Ocean and part of its stage evolution. However, the subduction and closure time of the Junggar Ocean and the basement property of the Junggar Basin are still unclear. In this study, Carboniferous volcanic rocks including basalts, basaltic andesites, and andesites in the northeastern margin of the Junggar Basin (Wulungu area) were investigated to reveal their magmatic provenance and formation mechanism. Combined with major and trace element and Sr-Nd isotope analysis, the closure time of the Junggar Ocean during the Late Paleozoic was further clarified. Three types of basement volcanic rocks with low TiO2 (0.60%-0.84%) and relatively high alkaline (1.18%-8.59%) contents were identified. The basalts belong to tholeiitic series while andesites to high calc-alkaline series, indicating a volcanic arc association. The low-medium 87Sr/86Sr(i) ratios (0.703250- 0.704559) and relatively depleted εNd(t) (+4.8-+6.8) and tDM2 (483-625 Ma) values suggested all three types formed from fractional crystallization of depleted mantle magmas, with andesites formed later than the others. Trace element and isotope tracing suggested the volcanic rocks originated from mantle wedge metasomatized by slab-derived fluid, with andesites more impacted by crustal contamination. The volcanic rocks were enriched in LILEs (Ba, Sr), Pb, Zr and Hf, and showed strong negative Nb and Ta anomalies. These characteristics are consistent with the tectonic setting of a subduction zone. The rock series showed enrichments of incompatible elements (Th, U, K) and LREE, high Ba/La ratio (30.14-208.86), low TiO2 content (0.60%-0.84%), and low Ce/NB (8.71-12.05) and Th/Nb (0.93-1.74) ratios. These characteristics support the continuing Carboniferous Junggar Ocean subduction along the continental block, which led to island arc accretion, followed by Junggar Ocean closure in the Late Carboniferous (ca. 305.5±4.4 Ma) and subduction-related magma superposition onto the continental crust through island arc. The above results provided new basis for the understanding of basement formation in the Junggar Basin.

Key words: Junggar Basin, Wulungu, Carboniferous, volcanic rocks, island arc, petrogenesis

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