The transition from a passive to an active continental margin in the Jiamusi Block: Constraints from Late Paleozoic sedimentary rocks

摘要

The eastern margin of the Jiamusi Block records the tectonic transition from a passive to an active continental margin during the Late Paleozoic- Early Mesozoic. Late Paleozoic sedimentary rocks from the Heitai and Zhenzishan formations along the eastern margin of the Jiamusi Block are a key to resolving these events. Major and trace element data (including REE) for 18 samples from late Paleozoic sedimentary rocks, with the characteristics of high SiO2, Al2O3, Na2O and K2O, low CaO, MgO and TiO2, enrichment in LREE and LILE, depletion in HFSE, and negative Eu anomalies, were plotted on various discrimination diagrams, indicating that the Heitai Formation formed in a passive continental margin setting, whereas the Zhenzishan Formation formed in an active continental margin setting. LA-ICPMS U-Pb dating of detrital zircon from two samples of the Heitai Formation and two samples from the Zhenzishan Formation indicates a range in age from 2735 +/- 19 to 308 +/- 3 Ma, showing similar populations to the Jiamusi Block. These detrital zircons not only indicate that the sedimentary material was derived from the Jiamusi Block, but also limit the depositional ages of the Heitai and Zhenzishan formations. The youngest zircon population of 374-384 Ma, with a peak at 382 Ma, from the Heitai Formation, together with evidence from paleontological studies, indicates the depositionol age of the Heitai Formation is Devonian (Givetian- Famennian), whereas the 308-318 Ma population, with a peak at 315 Ma, from the Zhenzishan Formation indicates that the depositionol age was late Carboniferous (post- Moscovian). This indicates that the transition from a passive to an active continental margin in the Jiamusi Block occurred between the Late Devonian and the late Carboniferous (380-310Ma) and was more likely related to the Mongol Okhotsk oceanic subduction-accretion or Panthalassa Ocean, and not to the Paleo-Pacific Ocean or to the Paleo-Asian Ocean as previous considered.