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Fengshan Pluton: the Break-Up of Dykes and Shearing of Enclaves

Roberto Weinberg, Monash University, Australia

He Bin, Guangzhou Institute of Geochemistry, Guangzhou, China



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


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Evidence for dyke break-up to form enclaves and sheared shapes of enclaves.
The features described here are all from the NW margin of the Fengshan pluton in porphyritic granodiorite. We use the following lines of evidence to conclude that many enclaves were in this region of the pluton were generated by the break-up of dykes
  • Presence of late dykes
  • Ripped endings of enclaves, instead of single pointed tips
  • Semi-constant width throughout and abrupt truncated endings (commonly ripped irregularly point 2)
  • Aspect ratio larger than 5:1
  • Cross-cutting relationships amongst enclaves )

    a) Broken-up dykes


    Figure 1a)  Ripped up abrupt endings of enclave, rather than single pointed ending, note also semi-constant width, specially in the enclave below the lens cap.
    Figure 1a)   The features of this fish-shaped enclave are more subtle. Its width is roughly constant along its main length, and the two terminations (left and right) are abrupt, and semi-planar. This could represent a broken-up piece of a dyke, but only in the context of a pluton preserving many steps of the process. Otherwise, it would more plausibly be interpreted as a sheared magmatic enclave of unknown origin.


    Figure 2a) and b)  This example covers many of the points listed. The diagonal enclave on both photographs are part of the same enclave spaced ~1.5 m. The width is constant, the ending on the right (b) in ripped, and the ending on the left (a) is truncated by a dyke at least 3 m long and 5 cm wide.


    folded enclaves
    Figure 3a)  Folded enclaves: a common feature of elliptical enclaves is an asymmetry of the their tips where one end is broad and rounded and the other is pointy. The pointy end commonly has a small concave indentation filled by the surrounding magmatic rock, forming what is best described as a small fold (Figure below, left enclave). We interpret these enclaves to preserve rootless folds. These can form in a number of ways. One possible way is that they form as a result of the folding of dyke intruded parallel to the shortening direction and then foled and rotated. This is suggested by the two en-echelon enclaves preserving antiformal hinge zones in the photo below. This rock records a conjugate set of sinistral and dextral shearing (C-planes marked in high resolution photo) and all three enclaves shown are oriented parallel to the S-plane common to both sinistral and dextral shearing.



    b) Sheared enclaves


    fish-shaped enclave
    Figure 4a)  Fish-shaped enclave indicative of dextral shearing in accordance to S-C fabric recorded in the granodiorite
    fish-shaped enclave
    Figure 4b)  Two enclaves en echelon. The enclave on the right has a stretched neck more sheared and rotated dextrally towards the C-plane.


    fish-shaped enclave
    Figure 5a)  Sheared enclave forming a fish with sides parallel to the documented S-C fabric on this outcrop. Thus shape asymmerty indicates shear sense. This enclave also shows preference for enclave to break up parallel to the S-plane, which here has been intruded by the surrounding magma. Figures a) and b) here are from an outcrop NE of the pluton, where shearing is dominantly dextral strike slip.
    fish-shaped enclave
    Figure 5b)  Another sheared enclave with similar features.


    sinistral enclave
    Figure 6a) and b)  This pair of enclaves is from the same outcrop and spaces only a couple of metres. The outcrop is characterized by super-imposed dextral shearing where C=100/90 with a lineation 10 to W, and contemporaneous sinistral shearing where C=050/90. This is reflected on the shape of enclaves. a) sinistral enclave asymmetry, C-plane trends 050.
    dextral enclave
    Figure 6b)  dextral asymmetry C-plane trends 100.


    sheared enclave
    Figure 7a)  Dextral shear in granodiorite (notice S-C fabric and deformed feldspar phenocrysts). The tip of the enclave is thinned out and bent into a high strain zone. Otherwise the changes in bulk strain are not visible.
    sheared enclave
    Figure 7b)  Penetrative sinistral S-C fabric in granodiorite with enclave (possible a broken up section of a dyke). Shear becomes localized into narrow fault bands displacing and deforming the tip of the enclave.


    faulted enclave
    Figure 8a)  Fault through enclave.
    faulted enclave
    Figure 8b)  Fault through enclave.


    two crenulations
    Figure 9a)  Fish-shaped enclave indicative of dextral shearing in granodiorite (upper part of the photograph). On the lower part, a 10 cm thick mylonite with sharp boundaries against weakly deformed granodiorite. Mylonite is parallel to the C-plane in the the sheared granodiorite, and asymmetries in the mylonite are indicative of dextral shearing..