The rare earth elements and yttrium (REY) geochemistry of the upper Carnian (Upper Triassic) carbonates from northwestern Sichuan Basin (South China)

Abstract

Rare earth elements and yttrium (REY) serve as valuable tracers of the geochemical properties of paleo-seawater and diagenetic processes. Four different carbonate components, ooids, microbalites, cements and brachiopods, from upper Carnian (Upper Triassic) of the northwestern Sichuan Basin were analyzed for REY geochemistry. Petrographic and cathodoluminescence analyses were employed to identify diagenetic features. The bright luminescent microspar calcite cement, ooids and the microbialites from the lower part of Qingyangou (HWQ) section exhibit bell-shaped REY_SN patterns. Microbialites from the upper part of HWQ section have relatively high REY concentrations with a flat REY_SN pattern. The high content of ΣREY and terrigenous elements (e.g., Al, Zr, Th) in ooids and microbialites reflect the input of terrigenous clastic in the northwestern Sichuan Basin during Late Carnian. Articulated terebratulid brachiopods in the study area display a generally modern seawater-like REY_SN pattern. However, this pattern may be influenced by the presence of sparry cement that has infilled the cracks and punctae of the brachiopod shells. The ooids, microbalites, cements and brachiopods are unsuitable for reconstructing the REY characteristics of Carnian seawater in the northwestern Sichuan Basin due to diagenetic alteration and terrigenous input. This study also highlights the necessity of thoroughly evaluating carbonate components for diagenetic effects and terrigenous contamination when employing them for REY characteristic reconstruction.