GENESIS OF REPLACIVE BURIAL DOLOMITE AND OF DISPLACIVE ZEBRA AND BRECCIA VEINS VIA THE INDUCED STRESS: A PARADIGM FOR METASOMATISM

Burial dolomitization exemplifies several possible consequences – displacement, replacement, and fracturing – of the induced stress in metasomatism. The dolomite grows in limestone and exerts a local induced stress. The induced stress then displaces, pressure-dissolves, or fractures the host rock. Each of these consequences modifies differently the induced stress that causes it. Each can be modeled linking mineral growth kinetics and the rheology of the host rock. The induced stress thus differs from Ramberg's force of crystallization.

Zebra and breccia dolomites consist of displacive veins, not passive cements filling supposed hydrofractures or dessication cracks as in the prevailing view. Evidence of the displacive nature of the veins includes a) minor stylolites in or outside the rhythmites, b) “weeding” by pressure-solution of “too-close-for-comfort” incipient veins – this weeding produces the regular spacing of the rhythmites, and c) small transform faults in the breccias. The stylolites, transforms, and weeding all result from the induced stress generated by the growth of the displacive veins. Displacement of the host requires it to have low-enough viscosity.

Replacement (by pressure solution driven by dolomite growth) takes place where the viscosity is too high for displacement. The induced stress self-adjusts so as to equalize the rates of growth of dolomite and pressure-solution of calcite: this coupling explains why replacement always preserves volume. Replacement has profound consequences for the modeling of dolomitization or any other case of metasomatism or metamorphism: 1) replacements should be adjusted on volume. This complies with petrographic evidence and kinetic-rheological theory, and often provides new insight into the actual reaction. Dolomite replacing calcite in dolomitization is 1.75calc + Mg⁺⁺ + 0.25CO₃^--=dolo + 0.75Ca⁺⁺, not 2calc + Mg⁺⁺=dolo + Ca⁺⁺. 2) The coefficients of the volume-adjusted replacement multiply source terms in the continuity equations of reaction-transport.