Multistage hydrothermal dolomites in the Middle Devonian (Givetian) carbonates from the Guilin area, South China

Abstract

Pervasive dolomites occur preferentially in the stromatoporoid biostromal (or reefal) facies in the basal Devonian (Givetian) carbonate rocks in the Guilin area, South China. The amount of dolomites, however, decreases sharply in the overlying Frasnian carbonate rocks. Dolostones are dominated by replacement dolomites with minor dolomite cements. Replacement dolomites include: (1) fine to medium, planar-e floating dolomite rhombs (Rd1); (2) medium to coarse, planar-s patchy/mosaic dolomites (Rd2); and (3) medium to very coarse nonplanar anhedral mosaic dolomites (Rd3). They post-date early submarine cements and overlap with stylolites. Two types of dolomite cements were identified: planar coarse euhedral dolomite cements (Cd1) and non-planar (saddle) dolomite cements (Cd2); they post-date replacement dolomites and predate late-stage calcite cements that line mouldic vugs and fractures. The replacement dolomites have d 18 O values from )13AE7 to )9AE7& VPDB, d 13 C values from )2AE7 to + 1AE5& VPDB and 87 Sr/ 86 Sr ratios from 0AE7082 to 0AE7114. Fluid inclusion data of Rd3 dolomites yield homogenization temperatures (T h ) of 136-149°C and salinities of 7AE2-11AE2 wt% NaCl equivalent. These data suggest that the replacive dolomitization could have occurred from slightly modified sea water and/or saline basinal fluids at relatively high temperatures, probably related to hydrothermal activities during the latest Givetian-middle Fammenian and Early Carboniferous times. Compared with replacement dolomites, Cd2 cements yield lower d 18 O values ()14AE2 to )9AE3& VPDB), lower d 13 C values ()3AE0 to )0AE7& VPDB), higher 87 Sr/ 86 Sr ratios ( 0AE7100) and higher T h values (171-209°C), which correspond to trapping temperatures (T r ) between 260 and 300°C after pressure corrections. These data suggest that the dolomite cements precipitated from higher temperature hydrothermal fluids, derived from underlying siliciclastic deposits, and were associated with more intense hydrothermal events during Permian-Early Triassic time, when the host dolostones were deeply buried. The petrographic similarities between some replacement dolomites and Cd2 dolomite cements and the partial overlap in 87 Sr/ 86 Sr and d 18 O values suggest neomorphism of early formed replacement dolomites that were exposed to later dolomitizing fluids. However, the dolomitization was finally stopped through invasion of meteoric water as a result of basin uplift induced by the Indosinian Orogeny from the early Middle Triassic, as indicated by the decrease in salinities in the dolomite cements in veins (5AE1-0AE4 wt% NaCl equivalent). Calcite cements generally yield the lowest d 18 O values ()18AE5 to )14AE3& VPDB), variable d 13 C values ()11AE3 to )1AE2& VPDB) and high T h values (145-170°C) and low salinities (0-0AE2 wt% NaCl equivalent), indicating an origin of high-temperature, dilute fluids recharged by meteoric water in the course of basin uplift during the Indosinian Orogeny. Faults were Sedimentology

Formation of saddle dolomites in Upper Cambrian carbonates, western Tarim Basin (northwest China): Implications for fault-related fluid flow

“…First, for the matrix dolomites, 87 Sr/ 86 Sr ratio decreases from the RD1 (n ¼ 4, mean ¼ 0.70986, s ¼ 0.000404) (0.709307-0.710297) to RD2 (n ¼ 2, mean ¼ 0.709235, s ¼ 0.000066) (0.709188-0.709281) dolomites ( (Burke et al, 1982;Nicholas, 1996;Montañ ez et al, 2000;Sial et al, 2008). In contrast, the values of Well Tong 1 are greater, suggesting that these dolomites may be affected by burial diagenesis or secondary alteration, because the 87 Sr/ 86 Sr ratio can increase with time and burial depth (e.g., Mountjoy et al, 1992;Qing, 1998;Gregg et al, 2001;Duggan et al, 2001;Chen et al, 2004). The 87 Sr/ 86 Sr ratios of the RD2 dolomites are lower than those of the RD1 dolomites, indicating recrystallization of the RD2 dolomites by fluids with lower 87 Sr/ 86 Sr ratios (Durocher and Al-Aasm, 1997;Machel, 1997) in equilibrium with the underlying Middle Cambrian evaporites because Middle Cambrian water has low 87 Sr/ 86 Sr ratios (Burke et al, 1982;Nicholas, 1996;Montañ ez et al, 2000;Sial et al, 2008) (Fig.…”

“…In general, the 87 Sr/ 86 Sr ratios and d 13 C values of the RD1 dolomites indicate that the dolomitization fluid is Late Cambrian seawater. Their depleted d 18 O values and slightly high 87 Sr/ 86 Sr ratios (greater than that of the Late Cambrian seawater) (e.g., in Well Tong 1) may be result of high temperatures due to deep burial setting (e.g., Mountjoy et al, 1992;Qing, 1998;Gregg et al, 2001;Duggan et al, 2001;Chen et al, 2004).…”

Polyphase dolomitization of the Wuchiapingian Zechstein Limestone (Ca1) isolated reefs (Wolsztyn Palaeo-Ridge, Fore-Sudetic Monocline, SW Poland)

“…War ren, 2000). In ad di tion, the for ma tion of nu mer ous do lo mite bod ies was driven not by one, but by sev eral mech a nisms of dolomitisation (e.g., Nader et al, 2004;Chen et al, 2004;Gar cia-Fresca et al, 2012).…”

Multistage dolomitization in the cretaceous carbonates of the east Shahmirzad area, north Semnan, central Alborz, Iran

“…In the fully dolomitized carbonates, also the earliest diagenetic features (e.g. submarine cements) are obliterated by dolomitization (Chen et al 2004). The dolomicrites (Rd1) were formed shortly after deposition of the Cretaceous succession.…”

“…The distinction was done also on the base of the classifications made by Friedman (1965), Gregg and Sibley (1984), Sibley and Gregg (1987), Gregg and Shelton (1990), Mazzullo (1992), Chen et al (2004) and Nader et al (2004). The dolomite types we have defined are (1) very fine-crystalline, nonplanar, xenotopic-a (Rd1); (2) fine-to medium-crystalline, planar-e dolomite rhombs, idiotopic-e (Rd2); (3) fine-to medium-crystalline, planar-e (porphyrotopic), idiotopic-p (Rd3); (4) medium-crystalline, planar-s, subhedral to Fig.…”