Andreas,
I think it depends on the T and the prior history of the rocks that
are involved for a given shear zone. If you have medium grade
metamorphic rocks that are sheared at 500°C and do not see subsequent
strain, that is fine. At 300°C however, I think there are problems
knowing which part of which grain last equilibrated chemically and in
terms of ages unless grain recovery has coarsened the minerals after
high strain. If the rocks were not previously at higher T, there is a
clear problem of survival of old compositions, and sometimes the old
phases are very fine-grained intergrowths -- they are not always coarse
clasts as one might assume from the literature. We see this problem
extending well up into the biotite zone for metapelites from the Upper
Peninsula of Michigan west of Marquette. One gets low analytical
totals for micas (92-94 wt% oxides) and it is very difficult to get
more than one spot analyzed across the foliation in a single grain. It
is hard to chemically characterize samples with grain size of a 1-3
microns or less unless using AEM and even there there is a serious
sampling and analytical problem. We can argue about the rocks in
between 300-500°C.
eric
On Feb 2, 2005, at 12:50 PM, Andreas Mulch wrote:
> Eric and all,
>
> I wouldn't be that pessimistic. From my experience, in situ dating of
> white
> mica from shear zones can be achieved at rather good spatial resolution
> (down to several tens of microns), a resolution that frequently is
> small
> enough to adequately sample individual microstructures. Once coupled
> with
> adequate compositional (e.g. dense EMP data and X-ray element maps)
> and
> independent isotopic (e.g. delD and del18O) data the distinction
> between
> strained prophyroclasts and newly formed synkinematic grains is not
> that
> much of a miracle. However, the key here is to work with well
> characterized (microstructurally and compositionally) samples. But
> isn't
> that true for any geochron study...?
> The story might be somehow different for biotite given small scale chl
> intergrowths and patchy recrystallization....
>
> Greetings to Michigan,
> Andreas
>
> Certainly, you are lost if you want to distinguish between intergrown
> At
> 12:21 02.02.2005 -0500, you wrote:
>> All,
>> There is also a question of whether the minerals recrystallized in
>> the shear zone or whether porphyroclasts are present that are mixed
>> in. Potentially there is also the possibility of excess Ar and/or Ar
>> loss with fine-grained micas and illites. Until we are able to get
>> single crystal dating with 0.1 micron scales we may be fooling
>> ourselves
>> with a bulk mineral age.
>> eric
>>
>>
>> On Feb 2, 2005, at 12:15 PM, Shoufa Lin wrote:
>>
>>> Hi Eldridge and others,
>>>
>>> A difficulty in dating shear zone fabrics by dating minerals is that
>>> it
>>> is not always easy to determine unambiguously whether minerals that
>>> grew
>>> syn-kinematically cooled through their respective blocking (closure)
>>> temperatures during or after deformation. The cooling ages of such
>>> minerals can potentially be much younger than the host shear zone.
>>>
>>> It is common for mineral ages to gradually decrease towards the
>>> centre of
>>> shear zones and it is often assumed that the youngest age near the
>>> centre
>>> of the zone is the age of the shear zone. However, this is not always
>>> true. The age could be younger than the shear zone for the reason
>>> explained above. It could also be older than the shear zone if rocks
>>> below the "partial retention zone" are not exposed (see Lin 2001 for
>>> a
>>> detailed discussion).
>>>
>>> To overcome these difficulties, it is useful to date different
>>> minerals
>>> with different blocking temperatures and compare their ages. If the
>>> ages
>>> are similar, the rocks must have cooled quickly, more likely during
>>> deformation. It is also useful to consider the age patterns across
>>> the
>>> shear zone, and compare the age pattern with other data set. For
>>> example,
>>> by comparing such an age pattern with pressure pattern (indicating
>>> differential uplift) and kinematic data, Lin (2001) demonstrated
>>> that the
>>> age pattern is a result of differental uplift and the younger age
>>> plateau
>>> defines a reliable age of shear zone deformation. Lin (2001) also
>>> discussed othe possible age patterns associated with differential
>>> uplift.
>>>
>>> Shoufa Lin
>>>
>>> Lin, S. 2001. 40Ar/39Ar age pattern associated with differential
>>> uplift
>>> along the Eastern Highlands shear zone, Cape Breton Island, Canadian
>>> Appalachians. Journal of Structural Geology, v. 23, p. 1031-1042.
>>>
>>> Abstract The Eastern Highlands shear zone in Cape Breton Island is a
>>> crustal scale thrust. It is characterized by an amphibolite-facies
>>> deformation zone ~5 km wide formed deep in the crust that is
>>> overprinted
>>> by a greenschist-facies mylonite zone ~1 km wide that formed at a
>>> more
>>> shallow level. Hornblende 40Ar/39Ar plateau ages on the hanging wall
>>> decrease towards the centre of the shear zone. In the older zone
>>> (over
>>> 7.8 km from the centre), the ages are between ~565 and ~545 Ma; in
>>> the
>>> younger zone (within 4.5 km of the centre), they are between ~425 and
>>> ~415 Ma; and in the transitional zone in between, they decrease
>>> abruptly
>>> from ~545 to ~425 Ma. Pressures of crystallization of plutons in the
>>> hanging wall, based on the Al-in-hornblende barometer and
>>> corresponding
>>> to depth of emplacement, increase towards the centre of the shear
>>> zone
>>> and indicate a differential uplift of up to ~28 km associated with
>>> movement along the shear zone. The age pattern is interpreted to have
>>> resulted from the differential uplift. The pressure data show that
>>> rocks
>>> exposed in the younger zone were buried deep in the crust and did not
>>> cool through the hornblende Ar blocking temperature (~500EC) until
>>> differential uplift occurred. The 40Ar/39Ar ages in the zone
>>> (~425-415
>>> Ma) thus date shear zone movement or the last stage of it. In
>>> contrast,
>>> rocks in the older zone were more shallowly buried before
>>> differential
>>> uplift and cooled through the blocking temperature soon after the
>>> emplacement of ~565-555 Ma plutons in the area, long before shear
>>> zone
>>> movement. The transitional zone corresponds to the Ar partial
>>> retention
>>> zone before differential uplift. The lain 40Ar/39Ar age pattern thus
>>> reflects a Neoproterozoic to Silurian cooling profile that was
>>> exposed as
>>> a result of differential uplift related to movement along the shear
>>> zone.
>>> A similar K-Ar age pattern has been reported for the Alpine fault in
>>> New
>>> Zealand. It is suggested that such isotopic age patterns can be used
>>> to
>>> help constrain the ages, kinematics, displacements and depth of
>>> penetration of shear zones.
>>>
>>>
>>> ***************************************************
>>> Shoufa Lin
>>> Associate Professor and Graduate Officer
>>> Department of Earth Sciences
>>> University of Waterloo
>>> 200 University Avenue West
>>> Waterloo, Ontario, Canada N2L 3G1
>>> Tel: +1 (519) 888-4567, ext. 6557
>>> Fax: +1 (519) 746-7484
>>> E-mail: [log in to unmask]
>>> <http://www.sci.uwaterloo.ca/earth/about/people/facdir/lin/
>>> index.html>http://www.sci.uwaterloo.ca/earth/about/people/facdir/
>>> lin/index.html
>>>
>>> ***************************************************
>> </blockquote></x-html>
>
> -----------------------------------------------------------------------
> ------------------------------------------------
> Andreas Mulch
> Research Associate
>
> Geology and Geophysics
> University of Minnesota, Minneapolis
> Pillsbury Hall
> 310 Pillsbury Drive SE
> Minneapolis, MN 55455
> Phone +1 (612) 626 9805
> http://www.geo.umn.edu/people/researchers/mulch.html
>
> and
>
> Geological and Environmental Sciences
> Stanford University
> 450 Serra Mall
> Stanford, CA 94305
> USA
>
>
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