DOLOMITIZATION OF THE LEDUC FORMATION (UPPER DEVONIAN), SOUTHERN RIMBEY-MEADOWBROOK REEF TREND, ALBERTA

Upper Devonian Leduc carbonate buildups along the southern part of the Rimbey-Meadowbrook reef trend in Alberta (buried at depths of 2300-35 00 m) have been extensively dolomitized. Dolomitization occurred first by pervasive replacement and later by minor cementation. In general, replacement dolomites are composed of fabric-destructive, fine to medi um (60-250 mu m) planar subhedral to anhedral crystals. Replacement do lomitization postdates submarine cementation and deposition of shales and carbonates from the overlying Ireton Formation, and overlaps stylo litization. It predates dolomite cementation that is interpreted to ha ve originated from hydrothermal fluids in a shallow burial environment prior to basin tilting. This, combined with the delta(18)O (mean = -4 .5 +/- 0.4 parts per thousand PDB) and Sr-87/(86) (mean = 0.70853) val ues in the replacement dolomites, suggests that replacement dolomitiza tion occurred also prior to basin tilting, probably during the Antler Orogeny in the subsurface at a temperature of about 45 degrees C and a depth of about 500 m, from slightly modified Late Devonian seawater. Two generations of dolomite cements postdate replacement dolomites: a coarse and planar-e(s) cement (Dolomite-C1), and a later coarser nonpl anar cement (Dolomite-C2). Dolomite-C1 has oxygen (mean delta(18)O = - 4.73 parts per thousand PDB) and strontium (mean Sr-87/Sr-86 = 0.70855 ) isotopic compositions similar to replacement dolomites. These simila rities, combined with the local association of Dolomite-C1 with stylol ites, suggest that some Dolomite-C1 precipitated from fluids whose com position reflects pressure solution of the preexisting replacement dol omites. The composition of fluid inclusions in Dolomite-C1 (mean T-h = 117 degrees C; mean salinity = 19 wt % NaCl equivalent) indicates tha t most Dolomite-C1 precipitated from warm hydrothermal brines. Little variation in fluid-inclusion homogenization temperatures occur with bu rial depth, suggesting that Dolomite-C1 precipitated prior to basin ti lting, and may be related to Antler orogenic events, Dolomite-C2 postd ates Dolomite-C1, is relatively more depleted in O-18 (mean delta(18)O = -9.06 parts per thousand PDB), and has a more radiogenic Sr-87/Sr-8 6 ratio (0.71087). Preliminary fluid inclusion data from the Crimson F ield suggest precipitation temperatures of about 150 degrees C. Thus, Dolomite-C2 originated from fluids of higher temperatures and differen t composition than Dolomite-C1. The petrographic, paragenetic, and geo chemical characteristics of replacement dolomites in this study are si milar to those of other Leduc buildups along the reef trend and of oth er Devonian dolostones of western Canada, supporting a model of large- scale basinwide fluid flow. The pervasive distribution of replacement dolomites along the Rimbey-Meadowbrook reef trend, and the similar pet rographic and geochemical character of dolmites from both the Leduc an d Cooking Lake Formations, is consistent with the hypothesis that the underlying Cooking Lake platform acted as a subsurface conduit system for the dolomitizing fluids. A potential regional fluid source may hav e been formation fluids (including residual evaporite brines) expelled tectonically during the Antler orogeny.