Dear Mr. Chatterjee,

Quartzites often contain apatite, monazite and / or zircon crystals. If you find in your quartzites grains that are at one tenth of a millimeter in diameter, and contain at least 10 ppm uranium, then you should try the fission track dating methods.

Please compare two of my publications dealing with the apatite fission track dating method.

Best regards,

Guenter Grundmann

Article Zircon Fission track thermochronology of the southeastern pa...

Article The Young Uplift and Thermal History of the Central Eastern ...

A few more options:  

1) Exactly how big are the white micas?  If they are in the 50 micron range, then they can (with difficulty) be mineral picked for conventional step-heating Ar-Ar dating.  

See the published works of Christian Teyssier (University of Minnesota) & students/colleagues.  They have done a very nice job dating & determining kinematics in mylonites from many localities and different tectonic settings using a variety of techniques.

Dear colleagues:

The question raised in this Q&A , refers to constrain the age of ductile deformation in mylonitic quartzite. The minerals mentioned in this case and proposed as a potential target of age dating have been tested first and foremost as to their position relative to the kinematic processes. Apatite, monazite and zircon are common allochthonous heavy minerals in arenaceous rocks and as such frequently are transferred from the parent rock into the quartzitic rock which underwent regional metamorphism. They have to be checked as to whether they are prekinematic or synkinematic to the structural disturbances; only in the latter case they can be used. There is another question which has not been addressed so far. What was the P-T regime like during metamorphism of the pre-existing rocks, resulting in the formation of the quartzite and what was the P-T- regime during mylonitization ? I am in doubt that kinematic processes like those can be chronological constrained by these so-called heavy minerals.

Mica has often been used (see inter alia Bonhomme and coworkers) but it needs a lot of very careful work to distinguish relic mica originating from the wall rocks from newly formed mica ("fault gauge") in the mylonitic zone. It is not only the type and the grain size of the mica that counts but also the issue to what extent the system was open or closed during the mylonitization.

We do not know anything about the origin of the quartzite which can cover the full range from late diagenesis to granulite facies. I agree with most of the recommendations. There is a wide range of methods available to carry out fine age dating, but the geological issue is still rather obscure so as to make far-reaching suggestions in view of the plethora of uncertainties only touched here.

Prior to any dating of samples, a very precise investigation has to be carried out from macro- to micro scale. And the selection of the most appropriate specimen can only be done based on field work and microscopic studies. The latter is key before embarking on any highly-sophisticated method aimed at creating age data rather than "street and house numbers", which are meaningless as to the structural disturbances.

Best regards

H.G.Dill

I would definitely go for the micas. Since dynamic recrystallization of quartz happens above ~250-300ºC, mica should have equilibrated at least for the K-Ar system during mylonitization. Closure temperature of white mica is slightly above this temperature o even a bit lower if mica crystals are very small, which means that you get a cooling age if T was significantly higher during mylonitization. You can check for T looking at feldspar (if there is some). Is feldspar also recrystallized then temperature was above ~450-500ºC if it reacts fragil T was lower. If you observe textures like porphyroclasts with white mica in pressure shadows you can be pretty sure the mica grew during mylonitization. Then you can try also the Rb-Sr method. Closure temperature for white mica is higher for the Rb-Sr system (~500ºC). If you can texturally proof that mica grew during mylonitization (and is not detrital) you will get the age of deformation, supposed only quartz not feldspar is recrystallized.

Regards

Bodo Weber 

I would agree with prof. Dill's answer.

Careful consideration of the dating method is required after paying attention to field relationships and microstructural analysis in order to date quartzite mylonites. Determination of pre and post-deformation P-T-t conditions would dictate the thermochronometric system to be used; but one should always consider that thermochronometers tell us something about the temperature-time history (not necessarily the deformation history). Temperature histories are often complex with multiple heating-cooling events and would require coupled thermochronology and thermal modeling to decipher the full history.

I would go first with cross-cutting relationships if there are any available. Dating deformed and undeformed igneous rocks would give the the best constrains for the timing of deformation. Any thermochronometric techniques should be calibrated against cross cutting relationships.

I hope this helps

Alex