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Part 3: 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:


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.


Magmatic Structures Continued

  • Ladder Dykes
  • Complex Dykes
  • Flow Structures
  • Interstitial Melt Extraction

    Ladder Dykes


    Weinberg et al. (2001) explained these structures as marking the stem of a rising plume or diapir where the source moved in relation to the exposed surface, so it marks the path of the source, like a hot spot. There are other less well-developed examples of ladder dykes in other sections of this website (e.g., Complex Dykes below). Ladder dykes and snails are thought to be formed by similar processes and Ellipsoids and Diapirs to represent the leading head of the rising magma batch on top of the stem.

    A) Long ladder dyke, Tavares pluton

    ladder dyke/dike


    B) Detail of A

    ladder dyke/dike


    E) Begining of dyke in A), Tavares pluton

    ladder dyke/dike

    D) Detail of dyke in A), Tavares pluton

    ladder dyke/dike

    E) Poorly developed ladder dyke, Conceicao das Creoulas pluton

    ladder dyke/dike




    Complex Dykes


    These are a group of dykes mostly from the Tavares pluton with complex internal structures. Some of which develop into ladder dykes (C and D), some are split along strike in one mafic part and a felsic part (E and F, also D), G) has irregular schlieren roughly parallel to the walls, and H) is a composite dyke formed by an early darker rock intruded by a later whiteish dyke, which cuts across the early dyke from right to left, upwards in the photo. At present most of these features remain unexplained.


    A) Dyke breccia, Itaporanga pluton

    dyke/dike breccia


    B) Several interlinked complex dykes

    complex dyke/dike


    C) Ladder dyke going into K-feldspar megacryst rich magma

    ladder dyke/dike


    D) Mafic and felsic dyke at the base grade upwards into ladder dyke

    complex dyke/dike



    E) Mafic dyke turns to felsic halfway

    complex dyke/dike


    F) Mafic and felsic complex layering in dyke

    complex dyke/dike


    G) Schlieren in dyke with non-planar walls

    complex dyke/dike


    H) Composite dyke (later felsic dyke) cutting across layered granite

    complex dyke/dike




    Flow structures


    Magmas in the Borborema plutons have a number of internal flow markers, such as the diapirs above, delineated by schlieren, other mafic schlieren marking flow between enclaves, and also as shown here in A) magmatic layering marks flow around a rigid block, and enclaves mark the flow of different internal magma batches (B and C). poor magma batches.

    A) Magmatic flow around an angular autoclave of granite, Tavares pluton

    angular magmatic xenolith


    B) Internal contact between magma batches, Itapetim pluton

    internal magmatic contacts



    C) Deformed enclaves forming an arc at the margins of an internal flow (white arrows), same outcrop as B), Itapetim Pluton

    deforming enclaves forming arc




    Interstitial Melt Extraction


    Segregation structures are thought to represent filter pressing of interstitial melt from a crystallizing magma with a solid interconnected network, effectively behaving like an isotropic migmatite. All structures are from Tavares pluton

    A) Felsic melt flowed into an opening in rigid magma



    B) Felsic interstitial melt diapir, extracted from crystallizing magma



    C) Liquefaction structure in an isotropic granitic magma causing separation of mafic minerals from felsic melt and horizontal layering

    magma liquefaction and layering



    Generally homogeneous granite becomes heterogeneous with mafic-rich bands in the vicinity of an irregular patch of leucocratic granite, suggesting it formed from local extraction of interstitial evolved melt, as magma crystallized. In its entirety, these extraction structures represent a filter pressing step in magma fractionation.

    D) Irregular 3D extraction structure

    extraction structure


    E) What is going on here?

    extraction structure


    F) Residual material (mafic) from extraction of interstitial melt frozen in pockets (leucosome). Extraction channelways? Same outcrop as B,C and D.

    Residual material



    Complex extraction features?



    magma extraction features


    H) Detail of G)

    magma extraction features




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