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

Abstract

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 fabric-selective dissolution and minor changes in geochemistry producing δ 13 C (−0.61 ± 0.80‰) and δ 18 O (−6.80 ± 0.65‰) values in karst samples close to those of the dolomite matrix (δ 13 C = −0.16 ± 0.78‰; δ 18 O = −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 ΣREE contents. With increasing karst intensity, spongy dissolution dominated the karst system, leading to depletion in δ 18 O 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 ΣREEs 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 δ 13 C (from −0.39 ± 1.02‰ to −1.11 ± 1.01‰) and δ 18 O (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 ΣREEs. 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. • Co-evolution of petrology and isotope–element geochemistry during karstification. • Meteoric water leaching and detrital contamination affect geochemical behaviors. • REY distribution changes from seawater- to riverine water- and clay-like patterns. • Moderate karstification (spongy dissolution) gives rise to high-quality reservoirs.