A MAGMATIC SHEAR ZONE IN THE KIMBERLEYS (WESTERN AUSTRALIA)

Copyright 1999 by Patrice Rey. All right reserved.
Unlimited permission to copy or use is hereby granted for non-profit driven enterprise subject to inclusion of this copyright notice and my World Wide Web URL: www.geosci.usyd.edu.au/users/prey/.  An email stating where this material is going to be used would be very appreciated.  Thanks PR


The Kimberley plateau in northwest Australia is one of the most remote place on earth.  This plateau is formed of Proterozoic sediments deposited on an Archaean craton.  This craton is wrapped around by a paleao-proterozoic mountain belt composed of the King Leopold ranges and the Halls Creek ranges.  This belt includes mafic and felsic intrusions, and metamorphic rocks include granulites and migmatites.  A great deal of deformation occurred during the emplacement of both felsic and mafic magmas.  This virtual field-trip is a walk along Spring Creek (on the way to the Bungle Bungle National Park, off the main highway) and the Ord River illustrating the finite product of the interation between felsic and mafic intrusions with shearing.
 Enjoy...
The rheological behaviour of magmas (mixture of crystals and silicate melt) is quite complex and depends on a large number of parameters including in addition to temperature: (1) the SiO2 content, (2) the amount of dissolved volatiles as well as the relative proportion between H2O and C02, (3) fO2: the oxygene fugacity, and (4) the % of silicate melt (or alternatively the % of crystal).  This list is far from being exhaustive and other parameters in particular the strain rate are as important as "chemical" parameters in determining the mechanical behavior of magmas.
During crystallisation, the rheology of magma changes drastically.  At low crystal fraction ~<0.3 (First Rheological Treshold), the behavior is close to that of a Newtonian fluid (linear relationship between strain rate and applied stress). At crystal fraction>0.3 but <0.7 (the Second Rheological Treshold), a visco-plastic behavior may develop whereby the magma can sustain stresses. Above the SRT, magmas may behave as rigid bodies and therefore are capable of fracturaction.  It is important to keep is mind that strain rate plays a key role in the rheology of magmas.  For example at very low strain rate, magma even with a large crystal fraction (in excess to 0.85) may behave has a Newtonian fluid through melt-enhanced deformation processes.  However at higher strain rate the same magma will react has a rigid body and will be capable of fracturing.  This point is illustrated here...

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The first three slides show magmatic flow structures within a tonalitic magma (quarz-plagioclase±hornblende±biotite with little to no K-felspar) intruded by a mafic dike.  a1/ Magmatic flow is responsible for variation in the thickness of the mafic dike, the elongation of schlierens (aggregates of dark minerals) and for the shearing of the dike.  a2-3/ Close up on a magmatic shear responsible for the apparent sinistral offset of the mafic dike.  Note that there is no plastic deformation in shear zone, there is no grain size reduction  and the texture in both felsic and mafic magma is equigranular. The shear zone is magmatic and strain diffuses away in the felsic magma.  a4-5/  A mafic dike intrusive in the felsic magma.  Felsic veins of same composition than the host rock cut the dike (back-veining). This suggests that the mafic dikes were intrusive in the felsic magma at a time the felsic magma it was beyond the second rheological treshold.  This supports the fact that felsic magmas may be brittle.


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b1-to b5 show the morphology of mafic dikes intrusive in felsic magma.  Criteria include cuspate dike's wall (b2, b3), felsic viens cutting back the dike (b4), and (b3-b5) "flames" of mafic magma escaping into the felsic magma.


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c1/ Thin sheets of mafic magma escape from a dike, they run parallel to the magmatic foliation.  c2/ Close-up on the felsic magma showing a magmatic fabric. c3/ Felsic veins cutting back into a mafic dike. c/4 Fractures cutting across a mafic dike.  The fractures, filled with felsic magma, do not seems to affect the felsic host rock.


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d1 and d2/ Isolated mafic enclaves, most likely end product of a dismembered mafic dyke.  d/3 Another magmatic shear zone.


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e1/ Boudinaged mafic dike, the stretching is nearly parallel to the dike.  e/3 and 3/4 Close-up showing that the stretching of the mafic dike occured during the magmatic flow of the felsic magma. e4/ and e4/ Close-up on the fabric of the felsic magma, note the absence of plastic deformation and recrystallisation.


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f1/ Another boudinaged mafic dike.  Note that the boudins are stretched in a direction nearly perpendicular to the direction of the dike.   A magmatic shearing perpendicular to the mafic dike could explain this feature.  f2 and f3/ Segments of mafic dikes overlaping each other, this suggests the shortening of the mafic dike.  f4/  Close-up of f3 showing the interboudin zone.  Note the coarse grained equigranular texture of the felsic material in the "pressure shadow" of the overlap zone.


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g1/ Boudinaged mafic dike, note the accumulation of felsic melt the bouding necks.  g2 and g3/  Another boudinaged mafic dike, the stretching is here quite large.  g4/ Close-up of the felsic host.  Note the absence of platic deformation around the feldspars and amphiboles nicely aligned parallel to each other.


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h1/ View of an outcrop showing a large proportion of mafic material.  h2 and h3/ In this zone, mafic dikes are isoclinaly folded. h4/ Parasitic folds in the hinge zone of an isoclinal fold.



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i1/ A small scale isoclinal fold.  i2 and i3/ Brecciated mafic dike.


A few shots from the Kimberleys...

More granite pictures here:  Wilsons Promontory Field Trip
Did you know that Granite are like ice cream? Check this out:
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