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Uncommon Magmatic Structures in Granites of the Borborema Province

Roberto Weinberg, Monash University, Australia

 

 

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.

 

Borborema Province map
Map of South America. White box: central part of the Borborema Province
K-rich granites of the Borborema Province in NE Brazil present a large number of uncommon magmatic structures which convey the dynamic history of the magma chamber. Some of these structures were of relatively simple interpretation (Weinberg et al., 2001). However, there are many other enigmatic features which are presented here.

These structures are more than curiosities. They tell us something about chamber dynamics. The purpose here is to illustrate the structures and trigger discussions, ideas and potential collaborations. If you want to help expand this site or connect your own related page to this one please let me know.

Weinberg, R.F., Sial, A.N., and Pessoa, R.R. 2001, Magma flow within the Tavares Pluton, NE Brazil: Compositional and thermal convection. Geol. Soc. Am. Bull. 113, 508-520 + cover image.

CLICK ON ANY PHOTO FOR HIGH RESOLUTION VERSION

 

 

Magmatic Structures

 

  • Feldspar Aggregate Structures

    • Random aggregate patches in homogeneous granite
    Aggregate layers: unidirectional protrusions
    Ellipsoidal structure
  • Feldspar Aggregates and Mafic Enclaves

    • Aggregates at enclave margins
    Aggregate spheroid and mafic enclave dyke
    Other aggregate-mafic association
  • Granite Layering
  • Ellipsoids and Snails
  • Diapirs
  • Ladder Dykes
  • Complex Dykes
  • Flow Structures
  • Interstitial Melt Extraction
  • Feldspar Deformation: Magmatic and Solid-State
  • Dendritic Feldspar

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    Feldspar Aggregate Structures

    Random aggregate patches

     

    A) Boqueirao Pluton

     

     

    B) Itaporanga Pluton

     

     

     

    Aggregate Layers: Unidirectional Structures

     

    A) K-feldspar megacryst aggregate forming a 20m long band (to right the photograph). with a flat straight bottom and a hilly top. The country rock is a megacrystic granite/granodiorite, Tavares Pluton.

     

    K-feldspar megacryst aggregate

    B) More mafic granitic batch (at the bottom) associated with feldspar aggregate, Campina Grande pluton. See also Aggregates at enclave margins, in particular Fig. F

     

    K-feldspar megacryst aggregate

     

    C) Unidirectional mounds on a K-feldspar aggregate layer, Campina Grande pluton.

     

    K-feldspar megacryst aggregate

    D) Same outcrop as C) and same orientation, Campina Grande pluton

     

    . K-feldspar megacryst aggregate

     

    Ellipsoid Aggregate Structure

     

    Aggregate of K-feldspar forming an ellipsoidal structure. This structure is related to other ellipsoidal structures and to ladder dykes. Conceicao das Creoulas pluton.

     

    K-feldspar megacryst aggregate

     

    Feldspar Aggregates Spheroid and Mafic Enclave Dyke

    Feldspar megacryst spheroid linked to disaggregated mafic dyke

     

    A) Aggregate spheroid related to a curved mafic enclave dyke

     

    K-feldspar megacryst aggregate

     

    B) Aggregate spheroid

     

    K-feldspar megacryst aggregate

     

    C) Detail inside aggregate

     

    K-feldspar megacryst aggregate

     

     

     

    Aggregates at enclave margins

     

    A) Megacryst aggregate filter pressed where mafic pillows separated

     

    K-feldspar megacryst aggregate

     

    B) Curved schlieren indicating magma flow in between enclaves

     

    curved schlieren

     

    C) Megacryst aggregate around mafic pillow

     

    K-feldspar megacryst aggregate

     

     

    D) Helicoid: mafic enclave and nest of megacrysts rotated by magma flow

     

    K-feldspar megacryst aggregate

     

    E) Mafic enclaves surrounded by a nest of megacrysts, stretched by magma flow

    K-feldspar megacryst aggregate

     

    F) Mafic enclaves surrounded by a nest of megacrysts, stretched by magma flow

    K-feldspar megacryst aggregate

     

     

     

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