EARLY CRETACEOUS BASIN FRAMEWORK IN NORTHWEST JIAOBEI REGION: EVIDENCE FROM SHRIMP ZIRCON U-PB DATING FOR"PENGLAI GROUP"AT QIMUDAO

It is traditionally believed that the bedrocks exposed at the Qimudao of the Jiaobei region belong to the Penglai Group of Sinian and Early Paleozoic. However, the newly acquired SHRIMP zircon U-Pb ages from the siltstone and volcanic tuff of the mapping unit of "Penglai Group" tells a different story. 12 analyses of zircon grains from the tuff yielded a crystallization age of 124.2 ± 2.3 Ma, indicating that the rock succession at the Qimudao was deposited during Early Cretaceous, rather than Sinian. Detrital zircon U-Pb age data of fifty zircon grains, moreover, from the siltstone can be grouped into five age populations: 430 Ma, 1.1 Ga, 1.6 Ga, 1.8 Ga and 2.45 Ga. The 430 Ma peak comes from two magmatic zircons with oscillatory zonation, and could be sourced from the eroded Triassic Xiachangzhuang magnetite amphibolite in the Luxi region. Other age peaks from the siltstone are rather consistent with the detrital zircon peaks of the quartzite of the Penglai Group found in the nearby Changdao, suggesting a possible recyclic origin. In addition, these peak populations are conspicuously different from the detrital zircon U-Pb populations of the coeval Laiyang Group in the Jiaolai Basin to the south of the study area, implying proximal and varying source regions for discrete Cretaceous basins in the region. Together with normal faulting activities, it is inferred that there was an Early Cretaceous extensional basin developed in the region.