5. CARBONATE VEINS FROM THE DÉCOLLEMENT ZONE AT THE TOE OF THE NORTHERN BARBADOS ACCRETIONARY PRISM: MICROSTRUCTURE, MINERALOGY, GEOCHEMISTRY, AND RELATIONS WITH PRISM STRUCTURES AND FLUID REGIME ¹

P. Labaume,² M. Kastner,³ A. Trave,⁴ and P. Henry ⁵

ABSTRACT

Carbonate veins occur in and above the décollement zone at the toe of the northern Barbados accretionary prism, most of them spatially associated with fault zones. Four vein types were identified on the basis of their structural relations with the clayey host-sediment and their internal microstructures. In all veins, the carbonate consists mostly of rhodochrosite plus Mg-kutnohorite, in spheroidal crystals that did not precipitate in open fractures as classical veins, but grew as a cement within a millimeter-thick band of sediment along the fractures. This distinctive growth feature was probably caused by the high porosity of the clayey host-sediment. In one vein type, the sediment cemented by the first generation of carbonate became rigid enough to fracture, and for these fractures to be held open during later episodes of vein formation. Vein microstructures indicate that vein formation corresponds to dilatancy episodes during which high pore pressure favored hydraulic fracturing and inhibited compactional shear strain in the fault zones.

In those veins featuring several generations of carbonates, the chemical (Sr concentrations) and isotopic (Sr, C, O) data indicate that the carbonates have precipitated from a fluid distinct from the present-day in situ pore water or from the contem-poraneous seawater. This fluid was injected from a deeper and hotter arcward source; the extent of mixing between this exotic fluid and the in situ pore water decreased from the earliest to the latest carbonate generation, which corresponds to small amounts of a mixed dolomite plus Mg-kutnohorite. The difference between the latest vein fluid and the present-day pore water, which is highly overprinted by in situ diagenetic reactions with volcanic glass, suggests that a major fluid-flow episode has not occurred recently.

Both the microstructures and chemical features thus indicate that carbonate vein formation is controlled by episodic injection of overpressured fluid originating from deeper horizons along the fault zones, inhibiting compactional shear strain and causing hydrofracturing, permeability increase, and mineralization.

¹ Shipley, T.H., Ogawa, Y., Blum, P., and Bahr, J.M. (Eds.), 1997. Proc. ODP, Sci. Results,156: College Station, TX (Ocean Drilling Program).
² Laboratoire de Géophysique Interne et Tectonophysique, CNRS-Université Joseph Fourier, BP 53X, 38041 Grenoble Cedex 9, France. pierre.labaume@obs.ujf-grenoble.fr.
³ Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92093-0212, U.S.A.
⁴ Géofluides, Bassins, Eau, CNRS-Université Montpellier II, 34095 Montpellier Cedex 5, France.
⁵ Laboratoire de Géologie, CNRS-Ecole Normale Superieure, 24 Rue Lhomond, 75231 Paris Cedex 5, France.