Multistage dolomitization and fluid evolution of the late Ediacaran cap carbonates, Hormuz complex, Paskhand salt diapir, southern Iran: Insights into the dolomite problem

Abstract

The Late Ediacaran to early Cambrian Hormuz dolomites were originally deposited on a shallow marine platform within the southern proto-Tethys Ocean, undergoing significant dolomitization during this period. The origins of different types of dolomites and their original fluids were studied in this work through petrological, mineralogical and geochemical data, including carbon, oxygen and clumped isotopes. The laminated micritic dolomite (LMD) (very finely to finely crystalline planar-e/s dolomite with preserving fabrics) and matrix dolomite (MD) including D1 (very finely to finely crystalline planar-e/s dolomite), the D2 (finely to medium crystalline planar-s/a dolomite) and D3 (medium to coarsely crystalline nonplanar-a dolomite) were precipitated from the combination of hydrothermal fluids and hydrologically cycled seawater in the subsurface to shallow burial stage. Fluctuation of the temperatures and changes in the characteristics of dolomitizing fluids formed different textures and distributions in the dolomites. Due to changes in temperature and Mg²⁺ concentrations in the hydrothermal fluids, the precipitation of cemented dolomite (CD1) and cemented dolomite (CD2 or SD) exhibited different textures and filling fracture system. In the final phase of hydrothermal alteration, late-stage calcite precipitates at low temperatures as a result of decreasing Mg²⁺ concentrations. Hydrothermal fluids, depleted in Mg²⁺ and enriched in silica, could result from the dissolution of siliciclastic sediments. The silica could be sourced from igneous rocks driven by the strongly extensional faults in the back-arc in Cadomian margin during the late Ediacaran time. The isotopic and petrographic analysis of the dolostones within the caprock of the Paskhand salt diapir indicate that their dolomitization occurred under hydrothermal and shallow burial conditions within the extensional back-arc Hormuz basin during late Ediacaran time. Dolomites exhibiting negative δ¹³C values in the cap rock of the Paskhand salt diapir likely formed during the late Ediacaran period. This isotopic signature suggests that hydrothermal fluids interacted with organic-rich sediments, incorporating isotopically light carbon into the dolomite structure. This study provides new insights into the evolution of dolomitizing fluids responsible for multiple dolomitization events and the formation of hydrothermal dolomite in the Hormuz Basin and other regions worldwide.