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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