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
> ***************************************************
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