Zircon U–Pb geochronology and geochemistry of the post-collisional volcanic rocks in eastern Xinjiang Province, NW China: implications for the tectonic evolution of the Junggar terrane

ABSTRACT

We report zircon U–Pb geochronologic and geochemical data for the post-collisional volcanic rocks from the Batamayineishan (BS) Formation in the Shuangjingzi area, northwestern China. The zircon U–Pb ages of seven volcanic samples from the BS Formation show that the magmatic activity in the study area occurred during 342–304 Ma in the Carboniferous. The ages also indicate that the Palaeo-Karamaili Ocean had already closed by 342 Ma. Moreover, the volcanic rocks also contained 10 inherited zircons with ages ranging from 565 to 2626 Ma, indicating that Precambrian continental crust or microcontinents with accretionary arcs are two possible interpretations for the basement underlying the East Junggar terrane. The sampled mafic-intermediate rocks belong to the medium-K to high-K calc-alkaline and shoshonitic series, and the formation of these rocks involved fractional crystallization with little crustal contamination. These Carboniferous mafic-intermediate rocks show depletions in Nb and Ta and enrichments in large ion lithophile elements (e.g. Rb, Ba, U, and Th) and light rare earth elements. The low initial ⁸⁷Sr/⁸⁶Sr values (0.7034–0.7042) and positive ε_Nd(t) values (+2.63 to +6.46) of these rocks suggest that they formed from depleted mantle material. The mafic-intermediate rocks were most likely generated by 5–10% partial melting of a mantle source composed primarily of spinel lherzolite with minor garnet lherzolite that had been metasomatized by slab-derived fluids and minor slab melts. In contrast, the felsic rocks in the BS Formation are A-type rhyolites with positive ε_Nd(t) values and young model ages. These rocks are interpreted to be derived from the partial melting of juvenile basaltic lower crustal material. Taken together, the mafic-intermediate rocks formed in a post-collisional extensional setting generated by slap breakoff in the early Carboniferous (342–330 Ma) and the A-type rhyolites formed in a post-collisional extensional setting triggered by the upwelling asthenosphere in the late Carboniferous (330–304 Ma).

KEYWORDS:

• Post-collisional volcanic rocks
• late Carboniferous
• Junggar terrane
• basement
• slap breakoff

Acknowledgements

The authors thank the staff of the State Key Laboratory of Continental Dynamics, Northwest University, Xi’an, China, for their help with zircon U–Pb dating. We appreciate the Jinan Supervision and Inspection Center of Mineral Resources, the Ministry of Land and Resources, China, for their assistance in the geochemistry analysis. We are also grateful to the State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences for help with collecting the bulk rock Sr–Nd isotopic data.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplemental material

Supplemental data for this article can be accessed here.

Additional information

Funding

This study was financially supported by the National Natural Science Foundation of China [41372108 and 41602110], the Research Fund for the Doctoral Programme of Higher Education of China [20133718130001], the Qingdao Postdoctoral Applied Research Project [2015193], and the SDUST Research Fund [2015TDJH101].