HYDROTHERMAL DOLOMITIZATION AND RECRYSTALLIZATION OF DOLOMITE BRECCIAS FROM THE MIOCENE MONTEREY FORMATION, TEPUSQUET AREA, CALIFORNIA

Dolomite breccias from the Miocene Monterey Formation, Tepusquet area, California are composed of dolomitic siliceous mudstones that are ext ensively fractured and filled with white, coarse-grained saddle dolomi tes, Fracturing and brecciation are much more extensive and intense in the Tepusquet area than in most other out-crops of the Monterey Forma tion. Despite the intensity of brecciation and its potential importanc e as an analog to Monterey fractured hydrocarbon reservoirs, no detail ed petrographic, crystallographic, or geochemical analyses have been p erformed on the breccias from the Tepusquet area. In the-present study , petrographic, crystallographic, and geochemical analyses show that t he vein filling dolomites were precipitated from hydrothermal fluids t hat were associated with hydrocarbon migration, and that the early dia genetic matrix dolomites have been recrystallized, resetting their geo chemical and crystallographic properties. Recrystallization of matrix dolomites is indicated by the uniformly negative delta(18)O compositio ns ((x) over bar = -7.1 parts per thousand), low Sr contents ((x) over bar = 230 ppm), low Na contents ((x) over bar = 364 ppm), contracted unit cells ((x) over bar: a = 4.812 Angstrom, c = 16.058 Angstrom), hi gh degree of cation order, high Mg content ((x) over bar = 46.1 mol% M gCO3) as compared to most Monterey dolomites, and increasing Sr-87/Sr- 86 ratios with decreasing Sr content. Vein dolomites have large variat ion in stoichiometry (41.7-49.0 mol % MgCO3), Sr content (124-414 ppm) , unit-cell dimensions (a: 4.806-4.829 Angstrom, c: 16.013-16.178 Angs trom), and cation order (Ca site occupancy: 0.73-0.93, Mg site: 0.68-0 .94), Recrystallization of some vein dolomites is indicated by the cov ariance of mol% MgCO3 with Sr, delta(18)O, cation order, and unit-cell parameters. The covariance of delta(18)O with mol% MgCO3 is the inver se of the trend expected for the recrystallization of dolomite during burial diagenesis, possibly because of recrystallization during uplift . Vein dolomites have higher Sr-87/Sr-86 values than matrix dolomites. The least radiogenic matrix dolomites (the least recrystallized as in ferred from the delta(18)O composition) have Sr-87/Sr-86 apparent ages (13.0 and 15.5 Ma +/- 0.3) that agree well with the previously mapped age of the brecciated unit. The more recrystallized matrix dolomites have higher Sr-87/Sr-86 ratios (apparent ages too young for the lower Monterey Formation), thus recording the evolution of Sr isotopic compo sition of the pore fluid. The close chemical similarity of the vein do lomites and the most recrystallized matrix dolomites, the episodic ass ociation between hydrocarbons and the vein dolomites, and the recrysta llization trends in the matrix dolomites, all indicate that evolved fo rmation waters were the source of the hydrothermal fluids that precipi tated the vein dolomites.