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

摘要

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 177 Ma and formed scattered volcanic rocks in the region. In contrast, the large-scale Late Yanshanian (Cretaceous) volcanism, with multiple eruptive stages between 140 and 88 Ma, formed the widespread lower and upper volcanic series. Two volcanic hiatuses were clearly identified at 120-110 Ma between the lower and upper volcanic series and at 128-122 Ma 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 1 of the lower series (140-128 Ma) occurred in the largest scale, which is much larger than the cycle II (122-120 Ma) as well as the whole upper series (110-88 Ma). 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 122 Ma. The analyzed zircons of the Early Yanshanian volcanic rocks show crustal model ages (rDM2) mainly of 2.00-2.16 Ga, and weighted mean ε_(Hf)(t) value of -13.4. The ε_(Hf)O and T_(DM)2 values are ca. -10 and 1.67-2.00 Ga for zircons from the cycle I volcanic rocks, -11.4 to -1.4 and 1.27-1.90 Ga for those from the cycle II volcanic rocks, -9.3 to -2.7 and 1.33-1.78 Ga 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 128-122 Ma and then dramatically changed to rapid high-angle rollback subduction at the second hiatus of 120-110 Ma.