ABSTRACT

Major, trace element and Sr-Nd isotopic compositions of Cenozoic sedimentary rocks from the Dongying Sag and the Zhanhua Sag, Jiyang Depression, provide evidence for provenance evolution and paleogeographic reconstruction. Compared with the average composition of post-Archean Australian shales (PAAS), the Paleogene sedimentary rocks of Shahejie formation (Es) from the Dongying Sag have similar SiO₂, Al₂O₃ contents, lower K₂O, Na₂O contents with K₂O/Na₂O ranging from 0.18 to 4.58, relatively lower contents of V, Cr, Co, Ni, Nb, Ta, Y, Rb, and Cs, and higher contents of Sr, Ba, Zr and Hf. All samples show chondrite-normalized REE patterns similar to that of PAAS, with significant light REE enrichment, flat heavy REE and negative Eu anomalies (Eu/Eu* = 0.71-0.99, average of 0.80). The Sr-Nd isotopic compositions of the Paleogene sedimentary rocks are different from those of the Neogene sedimentary rocks which generally have higher ⁸⁷Sr/⁸⁶Sr and lower ¹⁴³Nd/¹⁴⁴Nd. In the geochemical discrimination diagrams (e.g., La/Th-Hf, La-Th-Sc, and Th-Hf-Co), the studied samples were plotted in the area of felsic rocks rather than mafic rocks, which indicated that the source rocks are mainly composed of post-Archean felsic igneous rocks and recycled sedimentary rocks. This is also supported by interelement ratios such as La/Co, La/Th, Th/Co, and Th/Cr. The Sr-Nd isotopic compositions vary from the Paleogene to Neogene sedimentary rocks further suggesting a change in provenance, during which contemporaneous volcanic lava supplied some clasts with higher ¹⁴³Nd/¹⁴⁴Nd and lower ⁸⁷Sr/⁸⁶Sr ratios for deposition of Paleogene Es sedimentary rocks, whereas the contribution of volcanic detritus became much less during the Neogene time. The geochemistry of sedimentary rocks from the Jiyang depression possibly implies that the Paleozoic sedimentary rocks covering the old metamorphic crystalline basement rocks may not be totally denuded at the surrounding uplifts and highs during the Paleogene time.