Secondary ilvaite in a dolerite dyke from Rogaland, SW Norway

Abstract

A BST R ACT. llvaite is an alteration product of carbonate, which itself replaced clinopyroxene, in a Precambrian tholeiitic dyke. Additional secondary minerals are ferroactinolite, cummingtonite, chlorite, biotite, prehnite, and epidote. Microprobe analyses demonstrate that the ilvaite is close to the ideal end-member composition [CaFe22" Fe3+Si2Os(OH)] and that only cummingtonites, Mg/(Mg+Fe 2+) 0.77-0.39, and carbonates (solid solutions between calcite and dolomite, dolomite and ankerite, magnesite and siderite) show significant compositional variations. It is estimated that ilvaite formed at temperatures < 470 ~ and at pressures < 2 kbar, probably during a low-grade metamorphic event (or events) associated with the Caledonian orogeny. The fluids involved in the alteration process contained both CO2 and HzO, initial fluids probably being richer in CO2 than later ones. Ilvaite may be more common in hydrothermally altered igneous rocks than previously supposed.

Alpine tectono‐metamorphic evolution of the Corsica basement

“…Further evidence of fluid–rock interaction is provided by the occurrence of secondary Ca‐rich mineral assemblages (Adr‐rich garnet, epidote, Ca‐amphibole, ilvaite and late carbonates), which indicate intensive alteration of the early magmatic assemblages (Cpx, Aln, Mag) in the basement rocks by the chemically reactive infiltrating fluids and variable redox conditions (e.g. Barton & Bergen, 1984; Martín & Delgado Soleri Gil, 2010). Similarly to other studies dealing with formation of microstructures during mylonitization of granitoid precursors, the fluid–rock interactions, reaction weakening and partitioning of deformation are recognized key elements for strain softening and localization during shear zone nucleation and development (e.g.…”

An occurrence of ilvaite layers in the Cinco Villas metasomatic rocks, Western Pyrenees (Spain)

“…alteration product in some basic rocks (Naslung et al, 1983;Barton and Bergen, 1984). The first occurrence of ilvaite in Spain was reported from the Burguillos del Cerro skarn southwest of Badajoz as containing magnetite and vonsenite (Ruiz, 1976).…”

Asymmetrical magnetic fabrics in the Egersund doleritic dike swarm (SW Norway) reveal sinistral oblique rifting before the opening of the Iapetus

“…Below is a synthetic petrographic description based on a transmitted and reflected light microscope study of our samples. More information on the petrography can be found in Antun (1955), Barton and van Bergen (1984), Venhuis and Barton (1986), Bingen et al (1998), and Bingen and Demaiffe (1999).…”

“…Most dikes have been affected by hydrothermal alteration during prehnite-pumpellyite metamorphism related to Caledonian overthrusting (Barton and van Bergen, 1984;Venhuis and Barton, 1986). This alteration ( Fig.…”