CHANGES IN BURIAL TEMPERATURE AND FLUID SALINITIES DURING DOLOMITIZATION OF THE GREAT BANK OF GUIZHOU (SOUTH CHINA)

The Great Bank of Guizhou consists of a Triassic isolated carbonate platform (2.5 km-thick) that is located within the Nanpanjiang Basin of south China. Complete exposures of partially dolomitized Early to Middle Triassic carbonates are exposed within a steeply dipping syncline. Dolomitization is most pervasive in Lower Triassic portions of the platform, slope, and basin deposits. A wide variety of analytical tools has placed constraints on diagenesis. Data suggest multiple modes of dolomitization based upon petrography, fluid-inclusion and clumped isotope geothermometry, Sr-isotope data, and stable isotope compositions. This was further supported by U-Pb LA-ICPMS age dating of dolomite. Most vug and fracture-filling calcite postdates dolomite in pervasively dolomitized samples.

Fluid-inclusion data for primary assemblages reveal that dolomitization occurred over a range of temperatures from 90°C to over 200°C, increasing with time, in the presence of 4 to 19.3 wt. % brine. Individual crystals and growth zones exhibit much narrower temperature ranges. Some crystals reveal a temporal trend showing increasing temperatures and decreasing salinities during crystal growth, indicating a change in fluid compositions near the end of dolomitization. Calcite entrapment temperatures for primary fluid-inclusion assemblages varied from <~50°C to 108°C, with secondary FIAs preserving entrapment temperatures up to 180°C. U-Pb LA-ICPMS age dating yields a variety of ages for dolomite ranging from the early to late Triassic, with later fracture-filling calcite yielding an early Jurassic age.

A model that best fits existing field, petrographic, and geochemical data involves at least two stages. First, early evaporative-reflux dolomitization in the platform interior likely dominated the dolomite volumetrically. Next, existing dolomite was overprinted by recrystallization and additional cementation as burial progressed to at least 4-6 km. Progressive temperature increases preserved in the growth zones of many dolomite crystals, as well as spatial temperature gradients preserved near dolomitization fronts at the field scale support the interpretation of late burial dolomitizing fluids invading higher permeability units.