The Effects of Disequilibrium and Deformation on the Mineralogical Evolution of Quartz Diorite During Metamorphism in the Eclogite Facies

Abstract

In the Sesia Zone, Western Alps, a large volume of orthogneiss formed as a result of eclogite fades metamorphism and deformation of quartz diorite during early Alpine underthrusting and subduction. Rare lenses of undeformed metaquartz diorite, preserved within the orthogneiss, represent an early stage in the evolution of this latter rock type. The metamorphic and microstructural evolution of the orthogneiss in the eclogite fades has been reconstructed from studies of gradational contacts between undeformed and strongly deformed rocks.High pressure transformations of the original igneous plagioclase + biotite + quartz assemblage to jadeitic pyroxene (Jd 0 . 95 _ 0 . 83 ) + zoisite + quartz + garnet + 2 muscovites developed prior to deformation. Slow intergranular diffusion resulted in a state of disequilibrium between small textural domains in the metaquartz diorite. The compositions of the phases of the undeformed metaquartz diorite do not reflect the bulk rock composition, but were controlled by their position relative to reactant phases. The jadeitic pyroxenes, for example, formed in localized domains which originally consisted of sodic plagioclase whereas omphacite was the equilibrium pyroxene for the bulk rock composition.Mineralogical changes which occurred during subsequent deformation of the metaquartz diorite are interpreted as resulting from a progressive enlargement of equilibrium domains and the partial equilibration of mineral compositions to the bulk rock composition rather than from changes in pressure and temperature. Initially during high-strain deformation, fine-grained aggregates of jadeitic pyroxene + quartz + zoisite (originally pseudomorphing plagioclase) are inferred to have deformed by a mechanism of grain boundary sliding accommodated by diffusive mass transfer. Muscovite and garnet compositions homogenized during the deformation but due to slow intracrystalline diffusion, pyroxene compositions (Jd o . 9J _ 0 . go ) remained metastable. The coarsening of pyroxene eventually terminated deformation by grain boundary sliding and this mineral subsequently deformed by intracrystalline plastidty. This latter process was accompanied by and perhaps catalysed a change in pyroxene composition from metastable jadeite towards omphacite by a reaction involving the resorption of garnet and the nucleation and growth of paragonite. The resulting orthogneiss consists of quartz -I-omphadte + garnet + phengite + paragonite + zoisite. The rock is characterized by a broad range of pyroxene compositions (Jd o . 8 _ O5 ) due to the incomplete equilibration of this mineral to the bulk rock composition and a lack of Fe-Mg exchange equilibrium between pyroxene and garnet. However, in contrast to the undeformed metaquartz diorite, there are no obvious textural indications of disequilibrium between phases in the orthogneiss.

Eclogite-facies shear zones—deep crustal reflectors?

“…Our argument receives further support from the recognition of eclogite-facies overprints on deep crustal rocks in other regions and increased documentation that fluid infiltration and deformation play an important role in the kinetics of the gabbro-eclogite transition (e.g. Blattner, 1976;Compagnoni et al, 1977;Koons et al, 1987;Philippot, 1987;Sanders, 1988;Wilkerson et al, 1988;Biino and Pognante, 1989;Philippot and Kienast, 1989;Lardeaux and Spalla, 1991;Philippot and Selverstone, 1991;Ellis and Maboko, 1992;Selverstone et al, 1992;Indares, 1993).…”

Late Devonian subduction and early‐orogenic exhumation of eclogite‐facies rocks from the Champtoceaux Complex (Variscan belt, France)

“…Garnet coronas around biotite in undeformed pods were originally ascribed to a granulite-facies event (Lasnier et al 1973;Vidal et al 1980), but later studies have emphasized their similarity with coronas developed in jadeite-bearing metagranites (e.g. Sesia Zone, western Alps (Compagnoni and Maffeo 1973;Koons et al 1986); Dora-Maira, western Alps (Biino and Compagnoni 1992); Malpica-Tui Unit, Iberian Massif (Gil Ibarguchi 1995)). Although jadeite has not been found in the plagioclase pseudomorphs, the coronas are interpreted as recording partial re-equilibration during the high-pressure event (Balle Ávre et al 1989).…”

Pre-Caledonian granulite and gabbro enclaves in the Western Gneiss Region, Norway: indications of incomplete transition at high pressure

“…Similar relationships are now being recognized in the Western Gneiss Region (Engvik, Austrheim & Andersen, 2000, and this paper). Outside the Caledonides, metastability at high pressure has been documented in the Zermatt-Saas Fee Zone (Wayte et al 1989) and in the Sesia Zone (Koons, Rubie & Frueh-Green, 1987), both in the Alps.…”