LAUSR

Abstract Interpretations of depositional environments of Precambrian carbonates are commonly difficult because of their monotonous appearance and paucity of biological components. The REE + Y signatures of Precambrian carbonates can, however, provide insights into their original depositional environments, terrestrial contamination, and/or diagenetic modifications. This is exemplified by consideration of the REE + Y signatures from strata in the Mesoproterozoic Gaoyuzhuang Formation (~1543 m thick in Jixian region, China) that is formed of dolomite, calcite, silica cement, detrital quartz, and silicate grains. The basal members of the formation (1, 2, and 3-L) are formed of dolostone-dominated successions that contain variable amounts of quartz and chert cement and detrital quartz, K-feldspar, and mica grains. In contrast, the upper members (3-M, 3-T, and 4) are dominated by intercalated limestone-dolostone successions that are locally characterized by quartz and chert cements. The low Th concentration and little evidence of siliciclastic components from petrographic and XRD analyses indicate that the REE + Y values in samples from members 3-M to 4 were inherited from the coeval seawater. Their chondrite-normalized REE + Y patterns are characterized by a slight enrichment of LREE ([Nd/Yb]C = 1.5–3.9; [Sm/Yb]C = 0.9–2.1) and a superchondritic Y/Ho ratio (>38.3). These primary REE + Y features are different from those found in carbonates from other eras, suggesting that the Mesoproterozoic seawater may have had a different REE + Y composition. If siliciclastic components are present in the carbonates, as in some parts of members 1 to 3-L, the REE + Y profiles are characterized by higher LREE and MREE enrichments ([Nd/Yb]C = 3.4–8.9; [Sm/Yb]C = 1.9–4.6) and a less positive Y anomaly (Y/Ho = 27.2–35.6) than the primary Mesoproterozoic REE + Y values evident in the carbonate members 3-M to 4. From the basal members to the upper members, the increase in the Y/Ho ratio and decrease in [Nd/Yb]C and [Sm/Yb]C reflect a decrease in the amount of terrestrial detrital material that was transported into the depositional basin while the carbonate sediments accumulated on the seafloor.