ALBITE VEIN FORMATION DURING EXHUMATION OF HIGH-PRESSURE TERRANES: A CASE STUDY FROM ALPINE CORSICA

Abstract

The formation of late-stage veins can yield valuable information about the movement and composition of fluids during uplift and exhumation of high-pressure terranes. Albite veins are especially suited to this purpose because they are ubiquitously associated with the greenschist facies overprint in high-pressure rocks. Albite veins in retrogressed metabasic rocks from high-pressure ophiolitic units of Alpine Corsica (France) are nearly monomineralic, and have distinct alteration haloes composed of actinolite + epidote + chlorite + albite. Estimated P-T conditions of albite vein formation are 478 ± 31 °C and 0.37 ± 0.14 GPa. The P-T estimates and petrographic constraints indicate that the albite veins formed after the regional greenschist facies retrogression, in response to continued decompression and exhumation of the terrane. Stable isotope geochemistry of the albite veins, their associated alteration haloes and unaltered hostrocks indicates that the vein-forming fluid was derived from the ophiolite units and probably from the metabasalts within each ophiolite slice. That the vein-forming fluid was locally derived means that a viable source of fluid to form the veins was retained in the rocks during highpressure metamorphism, indicating that the rocks did not completely dehydrate. This conclusion is supported by the observation of abundant lawsonite at the highest metamorphic grades. Fluids were liberated during retrogression via decompression dehydration reactions such as those that break down hydrous high-pressure minerals like lawsonite. Albite precipitation into veins is sensitive to the solubility and speciation of Al, which is more pressure sensitive than other factors which might influence albite vein formation such as silica saturation or Na:K fluid ratios. Hydraulic fracturing in response to fluid generation during decompression was probably the main mechanism of vein formation. The associated pressure decrease with fracturing and fluid decompression may also have been sufficient to change the solubility of Al and drive albite precipitation in fracture systems. © 2006 Blackwell Publishing Ltd.