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Syn-Magmatic Deformation and Melt Extraction, Alagoinhas Pluton, Borborema Province, BrazilRoberto Weinberg, Monash University, Australia Gorki Mariano, Federal University of Pernambuco, Brazil |
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2004-2011 by Roberto Weinberg. All rights reserved. Unlimited
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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.
CLICK ON ANY PHOTO FOR HIGH RESOLUTION VERSION |
Figure 1a. |
Figure 1b. |
Figure 1c. In the bouding neck there is a higher concentration of felsic minerals including feldspar phenocrysts. |
Figure 3. |
Figure 4a. Boudinage of felsic layer |
Figure 4b. Detail of a) showing felsic concentration in neck and bending phenocrysts at margin of boudin. |
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. |