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Extreme Strain Localization into Dyke: Prata Pluton, Borborema Province, Brazil

Roberto Weinberg, Monash University, Australia

Gorki Mariano, Universidade Federal de Pernambuco

Carlos Archanjo, University of Sao Paulo, Brazil

 

 

Copyright 2004-2011 by Roberto Weinberg. All rights reserved. Unlimited permission to copy or use is hereby granted for non-profit driven enterprise, subject to inclusion of this copyright notice and acknowledgment of the source URL: users.monash.edu.au/~weinberg.

 

I would very much appreciate an email stating how this material will be used: Roberto Weinberg, Monash University, Australia. Thanks, RW.

 

DISCLAIMER. The material on this website has not undergone the scrutiny of Monash University and does not conform to its corporate web design. It is entirely based on a free-spritied, curiosity-driven research effort by the author, and therefore in no way expresses the official position of the University.

 

This web page documents a case of extreme strain localization and partitioning during late, syn-emplacement deformation of the 530Ma Prata Pluton in the Borborema Province. All photos are from a single 10m long dyke trending 100/sv, cutting across deformed plutonic rocks, with a simple shear foliation at high angle to the dyke orientation trending 025/sv (point 30, 24M 0712654E 9133326N). Here, the dyke is deformed contemporaneously with the surrounding pluton, but provide a weakness plane at a different orientation which is used to take up dextral shearing parallel to the dyke, producing a mylonitic foliation, S-C fabric and shearing of phenocrysts.

 

 

magma mingling, Prata Pluton

Mingled diorite and granite outcrop of the Prata Pluton including in the forefront a migmatite country rock xenolith. This mingled rock hosts the porphyitic dacite dyke.

 

strain localization in dykes
Figure 1a: Dacite dyke (plagioclase phenocrysts)cutting a mingled and foliated diorite with a shear foliation at high angle to dyke trend. Dyke itself is sheared dextrally as indicated by drag folds, S-C fabric and sheared phenocrysts. Outcrop is nearly horizontal and lineation is subhorizontal. East is on the left, west on the right.
 strain localization in dykes
Figure 1b. Same dyke as in a). Note here more clearly than in a) how the foliation in the dyke is continuous with that in the drag folds. East is to the left.

 

strain localization in dykes

Detail of Figure 1b, dextral shearing  indicated by sheared phenocrysts.

 

strain localization in dykes

Same dyke showing irregular margins and rotation of the mylonitic foliation running parallel to the dyke's margins towards the orientation of foliation outside the dyke. This indicates that the deformation that produced the foliation in the pluton at a high angle to the dyke was ongoing after dyke intrusion. The dyke provided a weakness plane that was reactivated by dextral shearing.  East is on the left, west on the right.

 

strain localization in dykes dyke4
This part of the dyke is less intensely sheared than others. Notice that the layering in the country diorite only shows very faint dextral drag folding. The dyke parallel mylonitic foliation and S-C fabric is limited to the core of the dyke. East is on the left, west on the right.

 

strain localization in dykes

In this part of the dyke, the mylonitic foliation is close to the northern boundary (lower part of the photo). Note that the dyke septa are intensely foliated parallel to the dyke margin and truncate layering and foliation in the coun try rock..East is on the left, west on the right.

 

strain localization in dykes strain localization in dykes
Mingled and folded dyke host. Note strain localization into two shear bands trending 020 (horizontally across the photo), and segregation of fractionated melt (quartz and feldspar) into straight veins trending 050, and interpreted to represent late formed tension gashes. Northeast is to the right. Detail showing strain localization into a dextral shear band trending 020. Note the sigma-shaped granitic enclave indicating early pervasive dextral shearing (feldspar grains in enclave are also sheared). Note also the diagonal (upper left to lower right) felsic veinlets interpreted as late formed tension gashes. Northeast is to the right.

 

strain localization in dykes strain localization in dykes
Detail. NE is to the right. Detail of shear zone. NE is to the right.

 

The dextral shearing on a plane trending 100 indicates shortening oriented approximately NW-SE, which is a regionally important orientation during the Brasiliano in this Borborema Province. More specifically this orientation is important around the Prata Pluton and controls emplacement of NW-trending porphyritic datice dykes in the gneissic country rocks and the NE-elongation of the pluton. However, it does not match the inferred shortening axis derived from the shearing of the host diorite-granite mingled body. Structures here indicate dextral shearing trending NNE-SSW nearly parallel to syn-magmatic axial plane of folds. Two separate strain phasse could be inferred from this. However, the drag folds in and around the late dyke indicates continuity between the NNE-trending foliation in the dyke and in the host rock. So currently there is no clear answer. The most likely in my view is that the current NNE trend of dextral shearing in the host magmatic rocks has been rotated from its original orientation towards the maximum shortening plane.