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A structure in which beds set in a softer matrix are divided by cross fractures into segments resembling pillows.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



division of rock strata or veins into separate blocks or lenses extending in the direction of the stratum or vein. It is formed during the crushing of a relatively more competent, flat geological body (stratum or vein) between two relatively more fluid layers under the influence of forces perpendicular to the layers. Such conditions arise, for example, on the limbs of strongly compressed folds, where the more plastic layers, being crushed and spreading in the direction of the axes of the folds, stretch the harder layers locked between them. The harder layers, being unable to flow with the same speed, at first divide into pinches—necks and swellings—lenses (lensing stage of boudinage), and then separate into pieces—boudins (boudinage proper). In the process of further deformation these pieces move farther and farther apart and the spaces between them fill up with material from adjacent plastic layers or with new mineral material (quartz, pegmatite). Boudinage is observed predominantly in metamorphic rocks.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
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This transitional rheological context, which was contemporary to gold deposition, is supported by field evidence including brittle-ductile auriferous faults (with low-T mylonitization) as well as cross-cutting relationships between pure tensile gold-bearing veins and ductile features (folding on various scales and boudinage).
The upper Cretaceous compression from the south caused refolding, scaling and boudinage of the nappe structures (Birkenmajer, 1974).
These features include conspicuous variations in the orientation of the intersection lineation, rootless folds that are cross-cut by cleavage, and boudinage and fragmentation of siltstone layers, all absent in the overlying and underlying members.
The resulting boudinage show that ductile deformation went on after formation of iron oxides (hematite) and dolomitic-marble host rock (Figure 3(b)).
Pilot Gold reports that the Western Flank area hosts numerous features that are similar to the geology at the Long Canyon deposit located approximately 75 kilometres (47 miles) to the north, including evidence of potential boudinage of a 100 metre-thick dolomite horizon and focusing of gold mineralization in and around this boudin neck area, which strikes north-northeast.
However, viscous flow is not completely disregarded because of the presence of boudinage. The emplacement process of this pluton will fit better a description of emplacement-ascent driven by buoyancy and regional stress as a viscoelastic diapir in the sense of Miller and Paterson (1999), where the rheologic behavior of the country rock varies both temporally and spatially from brittle to ductile.
In trap sites in the hinges of the folds associated with the boudinage and hydrothermal fracture zones, vanadium-bearing green grossular (tsavorite) formed; over considerable time the grossular reacted to form calcite, quartz and vanadium-bearing zoisite (tanzanite).