Abstract

Geochemical and petrographic studies of the Triassic Portoro limestone of the Tuscannappe in Liguria, Italy, indicate that fluid flow was channeled through interlayered bedding-parallel shear zones during Miocene shearing and low-grade metamorphism. Carbon, oxygen, and strontium isotopic compositions and trace element concentrations in the Portoro indicate that it was precipitated from normal marine waters. In sheared and unsheared layers these isotopic compositions are indistinguishable, yet sheared layers of microspar contain less than half the amount of strontium preserved in undeformed layers. Wavy grain boundaries and a dimensional preferred orientation of elongated grains indicate that calcite within sheared zones was dynamically recrystallized.

On the basis of these observations we suggest that during burial, extraformational fluids were buffered into oxygen isotopic, but not strontium-concentration, equilibrium with the Portoro. These syndeformational fluids were channeled through discrete 1- to 15-cm-thick shear zones in which strontium was expelled from calcite and incorporated into grain-boundary fluids during dynamic recrystallization.

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