Rare earth elements in dolostones and limestones from the Mesoproterozoic Gaoyuzhuang Formation, North China: Implications for penecontemporaneous dolomitization

Abstract

The Mesoproterozoic Gaoyuzhuang Formation (1.54–1.6 Ga), which is up to 1543 m thick, is a dolostone-dominated succession that originated in the Yanshan-liaoxi rift zone. In ascending order, this formation comprises Members 1 to 4. Member 1 is formed of clastic rocks and dolostones, whereas Members 2 and 4 are formed largely of dolostones. Member 3, which is divided in ascending order into three units (Members 3-L, 3-M, 3-T), is a succession of intercalated limestone and dolostone that have total rare earth elements (ΣREE) ranging from 0.5 to 116 ppm and variable Post-Archean Australian Shale (PAAS) normalized profiles. Carbonates from Member 3-M have REE profiles characterized by positive La and Y anomalies, a negative Ce anomaly, and a weak positive Eu anomaly. Relative to Member 3-M, carbonates from Member 2 and Member 3-L have flat REE profiles characterized by enriched light REEs, negative La and Ce anomalies, a positive Eu anomaly, and a lower Y/Ho ratio. Carbonates from Member 3-T and Member 4, relative to those from Member 3-M, are slightly more depleted in light REEs, lack a La anomaly, are enriched in Eu, have less depletion in Ce, and have similar Y/Ho ratios. Correlations between the dolomite content of samples and their [Pr/Sm]PAAS, [La/La*]PAAS, [Ce/Ce*]PAAS, and [Gd/Gd*]PAAS ratios indicate that dolomite-forming processes were probably associated with REE fractionation that reflects mixing between seawater and freshwater. Therefore, the REEs in Mesoproterozoic dolostones have the potential of being used as proxies for constraining the diagenetic environment under which the dolomites formed.