boudinage

Boudinage refers to structures deformed by extension in ductile shear zones. Boudinage structures contain a rigid tabular body that has been stretched and deformed where embedded within more deformable (less competent) rocks.

Competent tabular bodies that are susceptible to boudinage include veins and strata such as sandstones. Where conditions favor brittle fracture rather than ductile deformation, imbricate (overlapping) fracturing occurs.


In boudinage, the competent bed break ups into sausage-shaped boudins – forming structures such as ribbon-like boudins or chocolate-tablet boudins (depending upon the axis and isotropy of extension).


[links: images: Amphibolite boudins in gneisses; formations: Zoroaster Veining, boudins composed of quartz and plagioclase, boudin of metagabbro (HP mafic granulite) in tonalitic gneiss]

| 0 Guide-Glossary

vein

In geology, a vein is often defined as a long, regularly shaped occurrence of an ore (lode). However, more generally, a vein is a finite volume within a rock, which is filled with crystals of minerals precipitated from an (aqueous) fluid. The mineral-laden fluids, often of hydrothermal origin, circulated hydraulically before depositing the minerals by open-space filling or crack-seal growth.

Open space filling occurs at relatively low pressure epithermal vein systems, such as stockworks, in greisens, or in some skarn environments. Crack-seal filling occurs at higher pressures, with reopening of the vein fracture by progressive deposition of minerals on the growth surface.

In a process called boudinage, veins may be pinched and distorted into sausage-shaped bodies called boudins. (image above left - click to enlarge - boudinaged quartz vein in dextral shear foliation, Fortnum Gold Mine, Western Australia.)

An accretion vein is formed by the repeated filling of channels, followed by their opening by pressure-related fracturing in the zone undergoing mineralization.

An asymmetrical vein is a crustified vein of geologic material with unlike layers on each side.

A banded, or ribbon vein is composed of layers of different minerals lying parallel to the walls.

Barren vein matter, or a pinch in a vein, assumed to overlie an ore is called a cap rock.

[links: images: Amphibolite boudins in gneisses; formations: Zoroaster Veining, boudins composed of quartz and plagioclase, boudin of metagabbro (HP mafic granulite) in tonalitic gneiss]