Petrogenesis of early Carboniferous bimodal-type volcanic rocks from the Junggar Basin (NW China) with implications for Phanerozoic crustal growth in Central Asian Orogenic Belt

Jian Wang, Yuping Su*, Jianping Zheng*, E. A. Belousova, Ming Chen, Hongkun Dai, Xiang Zhou

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract

The Central Asian Orogenic Belt (CAOB) is one of the world's largest and longest-lived accretionary orogens that has been regarded as an important area of crustal growth during the Phanerozoic, yet the growth mechanism is still under debate. A combined study of petrology, geochronology and geochemistry for drilling-sampled early Carboniferous volcanic rocks from the Junggar Basin (NW China), southwestern CAOB, aims to constrain their petrogenesis and implications for Phanerozoic crustal growth. The volcanics show a bimodal-type suite, dominated by basalts, basaltic andesites and subordinate rhyolites, and have consistent zircon U–Pb ages of 334–336 Ma. The mafic rocks exhibit arc-like trace-element distribution patterns characterized by enrichment of LILEs (e.g., Pb) and LREEs but depletion of HFSEs (e.g., Nb, Ta and Ti). They have juvenile isotopic signatures of zircon ɛHf(t) (+11.4 to +15.4) and whole-rock ɛNd(t) (+7.8 to +8.5), with initial (87Sr/86Sr)i of 0.7040–0.7054 and (206Pb/204Pb)i of 17.681–17.817. These geochemical features, together with their variable Ba/La (4.28–44.2) but low (Tb/Yb)N (1.24–1.85) and Sm/Yb (1.65–3.44) ratios, suggest that the mafic volcanics could be derived from a main spinel-bearing mantle source metasomatized by subduction-related fluids. In contrast, the felsic samples are geochemically equivalent to A2-type granite, with high contents of SiO2, Zr, Ga, and show strong depletions in Eu, Sr and Ti. They have zircon Hf (ɛHf(t) = +12.0 to +15.3) and whole-rock Sr-Nd-Pb isotopic compositions ((87Sr/86Sr)i = 0.7032–0.7038; ɛNd(t) = +7.8 to +8.1; (206Pb/204Pb)i = 17.973–18.004) similar to the coeval mafic rocks. These observations, along with the model calculations using MELTS, favor a derivation of felsic rocks from the mafic magma by fractional crystallization. We suggest that the early Carboniferous bimodal-type volcanism formed in a localized back-arc extension within an overall convergent setting, presumably triggered by slab roll-back of the subducting Paleo-Junggar Ocean plate. This bimodal-type magmatism recorded a significant vertical crustal growth event in the Junggar Basin during the Phanerozoic. We propose that oceanic subduction (flare-ups) not only leads to the lateral accretion of arcs, but also results in the vertical growth of mantle-derived materials and subsequent magma differentiation during slab roll-back (flare-lulls); this may considerably change our view of the models of crustal growth in CAOB.

Original languageEnglish
Pages (from-to)220-237
Number of pages18
JournalGondwana Research
Volume89
DOIs
Publication statusPublished - Jan 2021

Keywords

  • Petrogenesis
  • Bimodal-type volcanism
  • Junggar Basin
  • Phanerozoic crustal growth
  • Central Asian Orogenic Belt

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