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

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.