Multistage dolomitization in the Qal'eh Dokhtar Formation (Middle‐Upper Jurassic), Central Iran: petrographic and geochemical evidence

Abstract

The late Jurassic Qal'eh Dokhtar Formation lithologically comprises three parts, from bottom to top, a lower sandstone unit, middle shale unit and an upper carbonate unit, which extend in a N–S direction over a wide area to the east of the Shotori Range and west of the Lut Block (Central Iran). This succession was deposited on a mixed carbonate–siliciclastic ramp. Carbonate rocks of the Qal'eh Dokhtar Formation vary from undolomitized, to partly dolomitized, to completely dolomitized. Field observations from two measured sections (the type section, 955 m thick, and the Sorond section, 639 m thick), combined with detailed petrographic and geochemical analyses, revealed the diverse types of dolomite in this formation. Five types of replacement dolomite and one type of dolomite cement were distinguished. Replacement dolomites (RD) consist of: (1) fine crystalline planar‐s (RD1); (2) medium crystalline planar‐s (RD2); (3) medium crystalline planar‐e (RD3); (4) coarse crystalline planar‐s (RD4); and (5) coarse crystalline planar‐e (RD5). Coarse crystalline planar dolomite cements (DC) were observed in low abundance and filling dissolution voids and fractures. Variation in dolomite types is mainly related to early to late diagenetic processes leading to changes in composition of the dolomitizing fluids. Replacement dolomites are non‐stoichiometric (Ca43‐56–Mg34‐45) with Sr, Mn and Fe concentrations of 41–138 ppm, 168–919 ppm and 5000–21000 ppm, respectively. These dolomites are characterized by δ18O values ranging from 0.0 to –11.8 ‰ VPDB and δ13C values of +1.1 to +3.2 ‰ VPDB. These values are depleted in δ18O relative to the postulated values for late Jurassic dolomites precipitated in equilibrium with seawater, while δ13C values are within the range of Jurassic seawater dolomite values. Fluid inclusion data of RD4, RD5 and DC yield homogenization temperatures of 72 to 118 °C. Based on petrographic, fluid inclusion microthermometric data and geochemical results, the replacement dolomites in the Qal'eh Dokhtar Formation are interpreted to have formed in the subsurface at shallow to intermediate burial depths. These dolomites were then recrystallized at increased burial depths and temperatures. Seawater was the major source of Mg2+ for early diagenetic dolomite (DR1), while Mg2+ for late diagenetic dolomites was provided from diagenesis of clay minerals in shales and mechanical compaction of mudstone in the Qal'eh Dokhtar Formation. The dolomite cement is postdated by coarsely crystalline mosaic calcite cement indicating that diagenetic fluids eventually became undersaturated with respect to dolomite and oversaturated with respect to calcite. Copyright © 2016 John Wiley & Sons, Ltd.

Diagenetic Study of Marrón Emperador Ornamental Stone (Upper Cretaceous, SE Spain)

“…Likewise, the presence of limpid cortex indicates the existence of slow replacement phases without the incorporation of impurities from the original sediment or overgrowth phases (syntaxial cementation) on the dolomite cores [60,61]. The δ 18 O values of dolomites reflect the isotopic composition and temperature of dolomitizing fluids [58,[65][66][67][68]. The δ 18 O composition of the ME dolomitic textures is very similar (−2.5-1.0‰ V-PDB), and these values differ from those that would be obtained in dolomites precipitated from Cretaceous marine water (0-3‰, [69]) (Figure 5), so that a primary or early origin for these dolomites can be ruled out.…”

Origin of fabric-selective dolomitization recognizable in the field: two case studies from Anisian carbonate rocks in the western Balkanides

“…It has particularly been suggested that Fe and Mg are released at temperatures >100 °C, presumably up to about 125 °C (Boles and Franks, 1979). Clay min- eral transformations have been interpreted as a major control on dolomitization in argillaceous carbonates (McHargue and Price, 1982;Gregg, 1988;Ali, 1995;Glumac and Walker, 2002) and limestones adjacent to shales (Mattes and Mountjoy, 1980;Gawthorpe, 1987;Srinivasan et al, 1994;Suchý et al, 1996;Sabbagh Bajestani et al, 2018), but also in sandstones (Śliwiński et al, 2016) and volcaniclastic rocks (Zhu et al, 2017). In this case, several lines of evidence support the assumption that illitization of smectite during burial supplied Mg ions for dolomitization of the ribbon rocks, e.g., the selective replacement of argillaceous carbonate layers and the presence of illite (vs. absence of smectite) in the dolostones.…”

Geochemistry characteristics and dolomitization mechanism of the Upper Cambrian dolomite, eastern Ordos Basin, China

“…Adams and Rhodes (1960) showed that seepage-reflux dolomitization resulted in the formation of dolomite in western Texas. In the last 30 years, a series of other dolomitization has been well documented, such as burial dolomitization (Sabbagh Bajestani, Mahboubi, Al-Aasm, Moussavi-Harami, & Nadjafi, 2016;Wierzbicki, Dravis, Al-Aasm, & Harland, 2006) and hydrothermal dolomitization (Qing & Mountjoy, 1994;Smith, 2006). In addition, microbial dolomitization provides a new implication for dolomite formation (Vasconcelos, McKenzie, Bernasconi, Grujic, & Tiens, 1995;Wright, 1999).…”