| This migmatite outcrop close to the town of Sertania illustrates several steps from melt segregation to its extraction through composite dykes. All structures come from an area of ~ 100 x 100m. Deformation was active at the time of melting. Maximum shortening was, in this 2D outcrop, perpendicular to dominant layering in migmatitic felsic and mafic gneisses, and gave rise to melt-lubricated conjugate shear zones and boudins. This page is divided into four parts: a) depicts regions with relatively little residual melt, cut by shear zones filled with leucosomes interpreted to represent melt products, and which accommodated block rotation, notice that all shear zones are sinistral (in 2D) and accommodated clockwise rotation of blocks, and consequently shortening perpendicular to the dominant orientation of layering; b) boudinage examples of the competent mafic gneiss, including Fig. 7 where boudinage, shearing and melt migration all interact; c) shows cases where boudins have been broken up along their necks by either melt channels or melt-filled shear zones, causing disruption of the original layering; d) shows evolved stages of melt extraction from this outcrop, where melt segregated into dykes at high angles to the dominant gneissosity, and produced, schlieren-rich composite dykes. Garnet is only locally present in this outcrop. |
a) Regions of melt extraction associated with melt
lubricated shear
zones accommodating block rotation.
Figure 1. |
 Figure 2. |
 Figure 3. |
Figure 4. |
Figure 5a. |
 Figure 5b. |
b) Boudinage
 Figure. 6a. Boudinaged competent mafic layer with garnet-bearing leucosome in boudin neck. |
 Figure 6b. Detail of a). |
 Figure 7. A boudin neck formed by conjugate melt-filled shear zones (middle of photograph), and larger melt-filled sinistral shear zone on the right-hand-side. Melt in this shear zone is continuous with leucosome layers in the gneiss. |
c) Break-up of boudin necks
 Figure 8. Melt channelway at high angles to main foliation/banding using boudin necks as preferential paths across the more competent mafic layers. |
 Figure 9. Boudinaged competent layer, displaced by melt lubricated shear zones. |
 Figure 10. Complex pattern of shearing in the presence of melt and disruption of a mafic layer. |
d) Melt channelways: dyke sources
Figure 11. Heterogeneous dyke, with mafic schlieren cutting across layered mafic-felsic gneisses on the right-hand-side but connected to a zone of complexx interaction with the mafic and felsic gneisses. Notice the development of melanosome close to the pegmatitic, irregular dykes. |
 Figure 12. Same heterogeneous dyke furhter to the right, with mafic schlieren cutting across layered mafic-felsic gneisses. Notice in the upper, central area, a band of pegmatite leaving the dyke and linking with the parallel, irregural dyke on the upper part of the photograph. |
 Figure.13. Heterogeneous, banded, pegmatite, granitic dyke cutting across the gneissic layers. |