Diagenetic modification of dolomite in middle Ordovician carbonates, Taiyuan City area, China

Abstract

Patchy dolomite occurs in the middle Ordovician limestones of the Taiyuan City area, China. The dolomite is finely-crystalline, subhedral to euhedral and is considered to have formed initially by syndepositional, subtidal dolomitization in evaporative or modified marine water, as indicated by the close association of supratidal gypsum deposits with subtidal dolomite, by the widespread distribution of dolomite and by the close association of dolomite with burrows. The much depleted δ 18O and δ 13C values of both dolomite and coexisting limestone relative to middle Ordovician marine carbonates suggest intensive diagenetic modifications. The average δ 18O and δ 13C values of present-day dolomite are −6.0% and −1.2%, enriched by about 3% and 1%, respectively, when compared with the coexisting limestone (−8.8% δ 18O and −2.4% δ 13C on average). This is consistent with the known fractionation between dolomite and calcite and may indicate that both the dolomite and coexisting limestone were finally modified, or stabilized, in the same diagenetic fluid. The trace element (Sr, Fe and Mn) concentration difference between the dolomite and coexisting limestone also supports a simultaneous modification or stabilization. Although the diagenetic modification of both dolomite and coexisting limestone by seawater, modified seawater or basin-derived water is possible, the simultaneous modification by meteoric water seems most likely.