Contrasting Lu–Hf isotopes in zircon from Precambrian metamorphic rocks in the Jiaodong Peninsula: Constraints on the tectonic suture between North China and South China

摘要

Zircon Lu–Hf isotopes, together with zircon U–Pb ages, trace elements and O isotopes were analyzed for various types of metamorphic rocks from the Jiaodong and Jiaobei terranes in the Jiaodong Peninsula, east-central China. The results show contrasts in the crustal nature between the two terranes and thus provide constraints on the tectonic suture between the North China Craton and the South China Block in this region. Zircon U–Pb dating for the Jiaodong terrane indicates protolith emplacement mainly in the middle Neoproterozoic and subordinately in the middle Paleoproterozoic. The protolith zircon of Neoproterozoic U–Pb ages exhibits a wide range of ε_(Hf) (t) values from ?16.8 ± 1.0 to 5.9 ± 1.0, indicating reworking of both ancient and juvenile crustal rocks during the Neoproterozoic magmatism. Low zircon δ~(18) Ovalues(<5‰)are a diagnostic feature of the Neoproterozoic protolith in the Jiaodong terrane. Growth of the juvenile crust in the middle Paleoproterozoic is also recorded by a few samples. In contrast, zircon U–Pb dating for the Jiaobei terrane yields three episodes of magmatism at ～2.9, ～2.7 and～2.5 Ga, respectively. Zircon Hf isotope compositions suggest growth of the juvenile crust at the two earlier episodes but reworking at the later episode. The crustal reworking is particularly intensive in the middle Paleoproterozoic, resulting in the diagnostic high-pressure granulite-facies metamorphism at 1.90–1.85 Ga and migmatization at 1.75 Ga. At the same time, remelting of the Archean crust gave rise to the 1.90–1.85 Ga S-type granitic rocks. The zircon of Paleoproterozoic U–Pb ages usually exhibits high δ~(18) O values(7–10‰).Taken together,the crustal nature of the Jiaodong and Jiaobei terranes are generally similar to that of the South China Block and the North China Craton, respectively. This lends support to the contention that the suture between the two collided continents is located along the Wulian–Yantai fault marking the boundary between the Jiaodong and Jiaobei terranes.