Migmatite Deformation, Yalgoo Dome, Youanmi Terrane

The Yalgoo dome is a large (70 x 30km) Archean granite-gneiss dome in the Youanme Province. The core of this granite is comprised of tonalitic migmatites and amphibolites and narrow bands of BIF defining elliptical folds, clearly visible in aeromagnetic images. This page records some of the features of the tonalitic migmatite related to migration of the anatectic magma flow during folding.

 

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Migmatite self-organization during folding.

(outcrop IXZBDJ509 Badjan Homestead)

 

In this outcrop, a layered tonalitic migmatite with an early formed layer-parallel leucosomes (S1) is overprinted by asymmetric folds with leucosomes along axial planes (S2). In many places layer-parallel leucosomes and axial planar leucosomes are continuous with one another, but in some places S2 leucosomes cross-cut S1 leucosomes and pegmatites (Fig. 1h, i, j, k). The axial planar leucosomes generally has diffuse margins. S2 is typically N-S trending and vertical.

Fig 1a.	Fig 1b.
Fig 1c. Fold train with axial planar leucosome. Notice a late dyke cutting across the fold.	Fig 1d.
Fig 1e.	Fig 1f. Granite truncating the folds with axial planar leucosome.
Fig 1g.	Fig 1h. Detail of Fig. 1g. Axial planar leucosome has fuzzy boundaries whereas the layer parallel has sharp boundaries.
Fig 1i. Neosome cross-cutting, disruption and dragging older leucocratic and melanocratic layers.	Fig 1j. Early pegmatitic intrusion with large K-feldspars broken up and rotated by the folds and axial planar leucosomes
Fig 1k. Layer-parallel granite is folded and cut across by leucosomes showing that migmatization post-dates some of the layer parallel granites.

 

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Fold Interference Pattern.

Outcrop IXZBDJ942 (301 JC)

 

In this outcrop a well-developed fold interference pattern between co-axial folds with axial planes at right angles to each other. F1 axial planes trend 100-110 and is refolded by F2 trends 020-030 almost at right angles to F1. In this outcrop lineation is vertical and parallel to both F1 and F2 fold axis. Note leucosomes in D2 structures and that we have documented melting of amphibolite with S2 leucosomes with peritectic Hbl) Fig. 2d. Detail of 1c. Fig. 2h. Boudinaged layered migmatite sequence. One boudin neck is cut across by a planar dyke. The lower part of the sequence is cut across by a fault.

Fig. 2a. Fold interference pattern between F1 and F2 marked by lines. The folds are detailed in the following photographs. Outcrop IXZBDJ942.     Fig. 2b. Left side of Fig. 1a.
Fig. 2c.
Fig. 2e. Detail of 1c.	Fig. 2f. I wonder if this was an early thrust plane that has been involved in this interference pattern. The thrust is visible by the truncation along the plane from upper left to lower right. Above the thrust plane there are asymmetric folds that could be drag folds.
Fig. 2g. Fold interference pattern.

 

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Dyke disruption in granite

 

Fig. 3a.	Fig. 3b. Disrupted enclaves in biotite granite. Notice that the immediate surroundings of the enclaves is leucocratic.
Fig. 3c. Enclaves frozen in in the process of disaggregation. Notice circumcentric schlieren around the enclaves.	Fig. 3d. Continuation of Fig. 3c.
Fig. 3e. Enclave broken up by surrounding magma into asymmetric enclaves. Notice that enclaves have melanocratic rims. The very small enclaves are entirely made up of coarser biotite.

 

 

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