Discovery and geological implications of Neoproterozoic metagranite in Jiaozhou Bay

Jiaozhou Bay is a semi-enclosed bay in Qingdao City in Shandong Peninsula, East China. Understanding the main geological frame of the basement rocks in the bay is crucial for large engineering projects and future urban planning in Qingdao. In this study, Neoproterozoic metamorphic granites (ca. 772 Ma) was discovered for the first time from drilling cores in Jiaozhou Bay .The basement rock is contemporaneous with Neoproterozoic granites that widely distributed in the Sulu orogenic belt. The lower part of the Neoproterozoic metamorphic granites in drilling cores was intruded by Early Cretaceous granites in age of ca. 120 Ma, which is consistent with the ages of adjacent Laoshan granites. The trace element distribution patterns of zircons in the Early Cretaceous granites and Neoproterozoic metamorphic granites in the drilling cores of Jiaozhou Bay are similar but showing a narrower range. The Early Cretaceous granite magma in the Jiaozhou Bay area might be derived from the remelting of Neoproterozoic granitic rocks. The crystallization temperatures of the Neoproterozoic metamorphic granites and Early Cretaceous granites determined by using zircon-Ti thermometer are both higher than 750°C, indicating that the source magma were water-undersaturated. This results suggest that the dynamic mechanisms of the two magmatic events may not be related to the oceanic subduction-related "Big mantle wedge" model.