Two Sources of Magnesium for Dolomitization in the Permian Changxing and Triassic Feixianguan Formations in the Northeastern Sichuan Basin, China

Porous dolostones of the Permian Changxing (P2c) and the Triassic Feixianguan (T1f) formations are mainly developed in the platform edge reefs and shoals in the east and west of the liangping-kaijiang trough (northwest to southeast trending) in the northeastern Sichuan Basin. Both crystalized and fabric-residual dolostone can be found in the west and east. Under cathodoluminescence, the P2c and T1f dolostones in the east display dull red luminance, whereas the P2c dolostones in the west display light red luminance. The δ13C and δ18O values of dolostones in the west and east are from +2.00 to +5.00‰ (VPDB), and from –6.00 to –3.00‰ (VPDB), respectively. The isotopic compositions imply that the dolostones were formed in shallow burial environment (the calculated burial depth is 400 to 1200 m). In addition, the δ18O values of dolostones in the east are 2.00‰ heavier than that of P2c dolostones in the west. The 87Sr/86Sr values of the P2c dolostones in the west (0.707155–0.707544, average 0.707366) are similar to that of the P2c sea water (0.7072). Whereas, the 87Sr/86Sr values of the P2c and T1f dolostones in the east (0.706755–0.708167, average 0.707526) are heavier than that of the P2c dolostones in the west, but approximate that of the late T1f sea water (0.7076). The characteristics of cathodoluminescence, δ13C and δ18O values, and 87Sr/86Sr values suggest that the P2c and T1f dolostones in the east have similar dolomitizing fluids, which are different from the dolomitizing fluids in the west. The east platform edge is adjacent to an evaporated platform in the T1f. In this way, the magnesium-rich fluid can migrate from the evaporated platform laterally to the platform edge. However, no similar evaporated platform was developed in the west in P2c, the magnesium for dolomitizaiton are most probably derived from the mudstone, limy-mudstone and shaly-limestone in the slope and the trough during shallow burial and compaction processes. Porous dolostones are mainly developed in the platform edge where the porosity related to early sub-aerial exposure and dissolution can form easily. This can demonstrate that the early porosity is needed for sufficient magnesium-rich fluids accumulation to ensure that the replacement processed gradually, whether the magnesium-rich fluids were migrated from laterally evaporated platform or adjacent strata during shallow burial processes.