Dynamic paleokarst geochemistry within 130 Myr in the Middle Ordovician Shanganning carbonate platform, North China

Reconstructing the geochemical evolution of paleokarst is essential for understanding the fundamental chemical behaviors and mechanisms associated with meteoric alteration during karstification. Here we report a rare example of a prolonged (130 Myr) period of karstification in the Middle Ordovician carbonate succession of North China. During the initial subaerial exposure, short-term leaching by meteoric water resulted in fabricselective dissolution and minor changes in geochemistry producing d13C ( 0.61 +/- 0.80%) and d18O ( 6.80 +/- 0.65%) values in karst samples close to those of the dolomite matrix (d13C = 0.16 +/- 0.78%; d18O = 6.92 +/- 0.67%), extremely low contents of detritus-associated lithophile elements (e.g., Al, Rb, La, Ti, and Sc), low Mn-Fe and high Na-K contents, and a slight decrease in SREE contents. With increasing karst intensity, spongy dissolution dominated the karst system, leading to depletion in d18O values (from 7.09 +/- 0.61% to 7.66 +/- 0.45%) and elevated Mn-Fe and decreased Na-K contents. Intermittent inputs of terrigenous materials during this stage also resulted in elevated contents of detritus-associated elements and SREEs as well as riverine water-like convex (MREE-enriched) REY patterns. In the late meteoric stage, breccias and mixed infillings were formed, and the breccia-related samples yield the highest degree of negative excursions in d13C (from 0.39 +/- 1.02% to 1.11 +/- 1.01%) and d18O (from 6.89 +/- 1.27% to 7.85 +/- 0.76%) values, the lowest Y/Ho ratios and Na-K contents, clay-like flat REY patterns, and the highest contents of Mn-Fe, detritus-associated elements, and SREEs. Our data suggest that the isotopic and elemental evolution was affected collectively by meteoric water leaching and detrital contamination. The detailed records of the stepwise changes in petrography, isotopes, and major and trace element compositions (including REY patterns) under different karst intensities enable us to decipher the variable geochemical behaviors at different stages of the prolonged process of karstification. The results provide a better understanding and interpretation of the petrological-geochemical signals of subaerially exposed carbonates, and they indicate that exploration for high-quality paleokarst reservoirs could be concentrated on areas with moderate rather than strong karstification.

Automated summary

This study reports a prolonged period of karstification in the Middle Ordovician carbonate succession of North China and reveals the geochemical evolution at different stages through isotopic and elemental analysis.