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Syn-Magmatic Deformation and Melt Extraction, Alagoinhas Pluton, Borborema Province, Brazil

Roberto Weinberg, Monash University, Australia

Gorki Mariano, Federal University of Pernambuco, 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.

 

The Alagoinhas Pluton has several features indicative of deformation and magma extraction from the pores of a crystallizing mush. The features also indicate the shortening direction during crystallization, and unlike many other localities, melt is extracted preferentially along the maximum shortening axis, an orientation similar to that expected for dyking. 

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local house in sertao
The outcrop, point 16, 24L 0744256E 9063062N, just south of the village of Alagoinha, south of the Pernambuco Shear Zone. Photographs below show nearly horizontal expsoure but vertical exposures were locally available.

 

a) Boudinage of porphyritic granite

syn-magmatic boudins with melt seggregation
Figure 1a.
syn-magmatic boudins with melt seggregation
Figure 1b.
syn-magmatic boudins with melt seggregation
Figure 1c. In the bouding neck there is a higher concentration of felsic minerals including feldspar phenocrysts.

 

syn-magmatic boudins with melt seggregation
Figure 3.
syn-magmatic boudins with melt seggregation
Figure 4a. Boudinage of felsic layer
syn-magmatic boudins with melt seggregation
Figure 4b. Detail of a) showing felsic concentration in neck and bending phenocrysts at margin of boudin.

 

b) Boudinage and rotation of porphyritic granite in shear zones

 

syn-magmatic boudins with melt seggregation
Figure 5a. Boudinage of a granitic band trending 060, with dextral shear bands striking E-W in the boudin necks. These allow anticlockwise rotation of the boudins towards 045 (compare to sinistral shearing in Fig. 7) Notice increase in felsic minerals in the boudin neck. Magmatic phenocrysts are oriented parrallel to the shear zone, indicating syn-magmatic shearing. NE is to the rightof the photograph.
syn-magmatic boudins with melt seggregation
Figure 5b. Detail of a). Note how the more felsic shear band on the right-hand-side extends beyond the limit of the granitic layer and merges with another granitic band in the upper part of the photograph. This indicates that the shear band was once mobile and therefore rich in melt (see also Fig. 6 for another example).
syn-magmatic boudins
Figure 6. Boudinage of foliated diorite. Boudin neck filled with more felsic magmatic rock, interpreted to represent local segregation out of the pores of a crystallizing mush. The boudin neck is an incipient vein that extends beyond the boudin neck area..


bouding necks in granite
Figure 6. Two boundin necks, one adextral shear band trending E-W and the other a simple leucocratic neck with no obvious record of shearing trending 125. This change is interpreted to result from an original curvature in the trend of the granitic band.
syn-magmatic sheared boudin
Figure 7a. Boudinage of a granitic band trending 350. Boudin necks are sinistral shear zones allowing clockwise rotation of the boudins towards 045 (compare to dextral shearing in Fig. 5). Notice increase in felsic minerals in the boudin neck. NE is to the left of the photograph.
syn-magmatic sheared boudin
Figure 7b.Detail of a)

 

c) Shearing of porphyritic granite, syn-magmatic?

 

sheared phenocrysts in granite
Figure 7a. Dextral shearing of porphyritic granite (horizontal exposure). The shear plane is parallel to the granite-diorite contact close to the coin, trending 020, and otherwise the phenocrysts are oriented diagonally across the phtotograph (from lower left to upper right) in a northerly trend.
sheared phenocrysts in granite
Figure 7b. Detail of a) showing the dextral shearing (on 020/sv) of the phenocrysts forming S-C fabric.
sheared phenocrysts in granite
Figure 8. Phenocrysts arranged in two preferred orientations suggesting dextral shearing.

 

d) Preferential orientation of irregular leucocratic veinlets and dykes

 

late magma seggregation
Figure 9. Irregular leucocratic melt extraction ("leucosomes") with a preferential E-W orientation and irregural fuzzy boundaries. Arrow in scale indicates the main trend of the foliation, and arrow points towards 225 (SW).
late magma seggregation
Figure 10a. Boudinaged granitic bands on the left-hand-side, parallel to the main foliation (045), and leucocratic bands trending ~E-W, with fuzzy boundaries with the surrounding diorite. Arrow in scale pointing W.
late magma seggregation
Figure 10b. Detail of a). Note also leucocratic veins in the upper left-hand-side parallel to the main foliation in diorite. Arrow in scale indicates the main trend of the foliation, and arrow points towards 225 (SW)

 

 

The structures recorded in this outcrop are: a) boudinage of more competent porphyritic granite trending 045 with leucocratic granite in the necks, b) porphyritic granite bands trending originally  more northerly rotate on sinistral shear bands in a clockwise fashion (Fig. 2), those oriented more towards the east rotate on dextral faults anticlockwise (Fig. 5), c) extraction veinlets/dykes trending preferentially 090.  In combination all of these features suggest deformation during magma crystallization, melt extraction into boudin necke and leucocratic veins, and maximum shortening axis trending NW-SE. This is the dominant shortening direction in this broader region and explains the NE-SW elongation of the pluton and its dextral shearing in the north along the E-W trending Pernambuco Shear Zone.