Episodic eruptions of the Late Mesozoic volcanic sequences in southeastern Zhejiang, SE China: Petrogenesis and implications for the geodynamics of paleo-Pacific subduction

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Zircon U–Pb and Hf isotope compositions have been systematically measured to constrain the timing and petrogenesis of the Late Mesozoic volcanic sequences in southeastern Zhejiang, SE China. The Early Yanshanian (Jurassic) volcanism took place at 177Ma and formed scattered volcanic rocks in the region. In contrast, the large-scale Late Yanshanian (Cretaceous) volcanism, with multiple eruptive stages between 140 and 88Ma, formed the widespread lower and upper volcanic series. Two volcanic hiatuses were clearly identified at 120–110Ma between the lower and upper volcanic series and at 128–122Ma between the two subdivided cycles within the lower series. In addition, the magnitude of the two volcanic cycles in the lower series is significantly different. The cycle I of the lower series (140–128Ma) occurred in the largest scale, which is much larger than the cycle II (122–120Ma) as well as the whole upper series (110–88Ma). Petrogenetically, the widespread Late Mesozoic volcanic rocks were mainly derived from the partial melting of the Paleoproterozoic crustal basement materials, with obvious addition of juvenile materials after 122Ma. The analyzed zircons of the Early Yanshanian volcanic rocks show crustal model ages (TDM2) mainly of 2.00–2.16Ga, and weighted mean εHf(t) value of −13.4. The εHf(t) and TDM2 values are ca. −10 and 1.67–2.00Ga for zircons from the cycle I volcanic rocks, −11.4 to −1.4 and 1.27–1.90Ga for those from the cycle II volcanic rocks, −9.3 to −2.7 and 1.33–1.78Ga for those from the acidic rocks of the upper series, respectively. The characteristics of the Late Mesozoic volcanic sequences indicate the varying influence of the subduction of the paleo-Pacific plate. Our study supports that the subduction style changed from low-angle to intermediate-angle forward subduction at the first hiatus of 128–122Ma and then dramatically changed to rapid high-angle rollback subduction at the second hiatus of 120–110Ma.