Geochemistry of limestones deposited in various plate tectonic settings

Abstract

Limestone, a major part of the global sedimentary succession, susceptible to post-depositional diagenesis. Studies of limestone geochemistry are essential in the discrimination of tectonic settings of basins in which the limestones were deposited. Six Late Mesozoic and one Tertiary limestone successions of Tibet, western China, that were deposited in oceanic plateau, passive continental margin, active continental margin (fore-arc basin, back-arc basin and foreland basin) and continental inland freshwater basins were analyzed for their major, trace and rare earth element (REE) composition. This geochemical dataset, in combination with the Deep Sea Drilling Project and Ocean Drilling Program (DSDP and ODP) literature geochemical data regarding limestones deposited in open ocean environments, permitted delineation of the geochemical characteristics of limestones accumulated in these various plate tectonic settings. Major elements (e.g., Fe2O3 and MnO, except for CaO) of these limestone successions show large variations but are positively correlated with Al2O3. The REE and trace element abundances for the inland and margin limestones show a distinct positive correlation with Al2O3 whereas REEs and trace elements of the open ocean limestones are positively correlated with MnO. There is a systematic increase in the magnitude of Ce anomalies of open ocean floor limestones away from spreading ridges to open ocean highs, to passive margins, and to active margins and inland freshwater basins. Open ocean limestones display a narrow range of (La/Sm)n (0.46–0.96), (Sm/Yb)n (0.25–1.96), and (La/Yb)n (0.23–1.38) but high (La/Ce)n (>1.5) whereas the inland+margins limestones display a much larger range ((La/Sm)n (0.43–2.18), (Sm/Yb)n (0.6–2.98) and (La/Yb)n (0.7–2.25) but low (La/Ce)n (<2). The inland+margins limestones are influenced geochemically upon terrigenous clasts while geochemistry of open ocean limestones is more dependent upon the flux of the hydrothermal Fe–Mn-oxyhydroxides. The control of the tectonic environments of the basins on the limestone geochemistry permits development of proxies for the discrimination of depositional regimes. The REE ratios (i.e., (La/Ce)n, Ce/Ce*) along with other immobile elemental ratios (e.g., Zr/Ti, La/Sc) of limestones provide the best means for the geochemical resolution of all four depositional regimes. A Rb–Sr–Ba triangular diagram is also useful for distinguishing between four tectonic settings. Applications of the immobile geochemical proxies to 39 literature limestone successions demonstrate their validity, independent of diagenetic modification, metamorphism and high siliciclastic content (≤40wt%).