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El Pichao Ultramylonites, Argentina

 

diapirs

Roberto Weinberg
Monash University, Australia

 

 

Ultramylonites from El Pichao: isolated theta- and delta-type porphyroclasts of feldspar and garnet in a uniformly fine-grained biotite-rich matrix, porphyroclasts lack pressure shadows.
Granitic protolith with xenoliths of Punco-Viscana from ultramylonites to mylonites
Figure 1A: Granitic protolith with xenoliths of Punco-Viscana source rocks. Figure 1B: Wall showing strain gradation from ultramylonites to mylonites marked by an increase in the number of porphyroclasts. Notice also large clasts derived from pegmatites.
variation in porphyroclast content
Figure 1C: Detail of variations in modal content of porphyroclasts.
gradation into ultramylonite breakdown of pegmatite into ultramylonite
Figure 1D: Gradation into ultramylonite in block. Figure 1E: Detail of breakdown of pegmatite into ultramylonite zone.
granitic protolith rotation of pegmatite clasts in ultramylonite
Figure 1F: Ultramylonite: granitic protolith intruded by pegmatite. Foliation dips 45 degrees into the photographed surface. Down plunge of the stretching lineation. Figure 1G: Detail of 1A. Rotation of the large pegmatite clasts caused considerable circumcentric strain marked by stretched feldspar clasts.
porphyroclasts in ultramylonite porphyroclasts in ultramylonite
Figure 1H: Detail of 1F. Notice tourmaline-bearing clasts of pegmatite. Interesting contrast between comet-shaped K-feldspar and other clasts with no tails. Figure 1I: Detail of 1F.

 

porphyroclasts in ultramylonite porphyroclasts in ultramylonite
Figure 1J. Same surface as Fig. 1F. Foliation dips 45 degrees into the photographed surface. Down plunge of the stretching lineation. Figure 1K. Detail of 1J showing rotation of the large K-feldspar.
Mantled K-feldspar clasts in ultramylonite
Figure 1L.Detail of 1J. Mantled K-feldspar clasts perpendicular to the stretching lineation.

 

Naked garnet and K-feldspars in ultramylonite Naked garnet and K-feldspars in ultramylonite
Figure 2A. Naked garnet and K-feldspars in ultramylonite. Figure 2B. Naked garnet and K-feldspars in ultramylonite.
asymmetric shearing in ultramylonite asymmetric shearing in ultramylonite
Figure 3A. Thrusting: asymmetric stretching of Tml-pegmatite in mylonite, parallel to lineation, Arbolar. Figure 3B. Thrusting: asymmetric stretching and a naked clast of Tml-pegmatite in ultramylonite, parallel to lineation, and very cloase to Fig. 3C, Arbolar.
asymmetric shearing in ultramylonite asymmetric shearing in ultramylonite
Figure 3C. Thrusting: , parallel to lineation, Arbolar. Figure 3D. Thrusting: , parallel to lineation, and very cloase to Fig. 3C, Arbolar.
ultramylonitie complex and sigmat clasts in ultramylonite
Figure 3E. Same as 3C showing variation in modal amount of clasts. Parallel to lineation, Arbolar. Figure 3F. Thrusting: complex and sigma clasts of K-feldspar, parallel to lineation, Arbolar.
asymmetric shearing in ultramylonite delta clasts in ultramylonite
Figure 3G. Thrusting: asymmetric stretching of Tml-pegmatite in mylonite, parallel to lineation, Arbolar. Figure 3H. Thrusting: delta Tml-pegmatite clasts, and much smallel K-feldspar sigma clast, parallel to lineation, Arbolar.
Folded ultramylonite
Figure 3I. Folded ultramylonite block, scale: 2m in horizontal Anchillo.
Pseudotachylite
Figure 4. Pseudotachylite.