It depends what rock it is, what you are trying to date. If you want to date the crystallisation of a felsic magmatic protolith, you can extract zircons from the deformed rock, examine their internal structure using cathodoluminescence on an electron microscope, and use Laser Ablation ICP-MS or SHRIMP/SIMS do date the crystallisation with U-Pb. This is usually pretty robust. If your rock underwent granulite facies metamorphism (quite extreme "deformation") then you might get several zircon populations, with a protolith age, a metamorphic age and mixed ages.

It is also possible to date fracturing in some cases, if your fracturing results in the formation of illite in a fault gouge. Illite can be isolated using centrifuges and dated with K-Ar for example. We do this quite routinely.

A more detailed description of what you are trying to date and the deformation the rock underwent will be helpful in providing a more detailed answer.

Dear Dr. Latha,

the decisive question is "Do you want to date the formation of the rock affected by the alteration/deformation or the processes of alteration and deformation ?". Fault gouge and argillic alteration zones are a case in point to demonstrate in the best way  the problem you might be faced with. You can find a wide range of K-bearing micas different in grain size, morphology, structure and chemistry indicative of the orginal mica which developed in the host rock through to the newly formed mica-type minerals heralding the youngest stages of deformation and argillic alteration. We investigated this issue in vein-type U deposits where we had all systems at hands from U-Pb, Rb-Sr though K-Ar. This is not to discourage you during your work but it seems to be fair and honest to refer also to the problems you might be confronted with because not each isotope ratio can be called a "geological age".

I wish you much success

H.G.Dill

The rocks are granitoids both intrusive and older basement rocks.

To differentiate these two age data may be useful in problematic areas as mineralization is associated with a particular set of granites..

the zircon population is very scarce  and we don't have the facility of Laser Ablation ICP-MS or SHRIMP.

we usually do Rb-Sr or U-Pb method.

usually alteration involves elemental tranfer.

but in case of crushing fracturing, granulation will there be any elemental inflow/ outflow?

Important thing is what are you dating, infact, all ages are cooling ages, clock starts ticking when a mineral cooled down a certain temperature. So you are dating the time when that mineral attained its closure temperature. And stopped gain or loss of either parent or daughter in or out of crystal. And this relates to the concept of diffusion. Also certain minerals may record resetting like the mineral was subjected to elevated temperture again and then cooled down below its closure T.  Zircon can grow at a variety of temperatures from more than 1000C to below 400. For an igneous zircon several episodes of deformation or alteraion can not disturb its closed system. Only older zircons get metamictized and you get Pb loss.This can be easily identified by Concordia plots and zircon morphology. Zircon precipitated from hydrothermal fluids  (as this is the case with many Au deposits) are spongy and lacks typical magmatic zoning and may be differentiated from magmatic zircon based on their Hf and O isotopes composition. Time constrained micro analyses on monazite can tell you the deformation story more clearly. For example newly grown rim which formed during decompressing melting will be enriched in elements released by garnet breakdown to coiedirite.

Hello Dr Latha,

I presume from what you described that you have a U-Pb TIMS lab capable of low blank (sub pg common Pb), single zircon work and supported by decent mineral separation facilities and zircon prep.  The issue to consider with fracturing and deformation of the host rocks is mainly lead loss from zircon surface domains and your ability to remove those by say chemical abrasion, which post dates my experience with air abrasion.   An additional problem in your case, distinguishing the ages of basement and intrusive granitoids, is inheritance.   I cannot say much about the Rb-Sr systematics from limited experience except to say I have seem protolith ages preserved in domains within granulite facies gneisses.

Where I would start is to classify the granitoids by composition and in particular I vs S type.  With I type you might also find titantite, which would help confirm lower end ages and with S type, monazite or maybe xenotime, also useful for direct igneous crystallization ages, metamorphic ages and major hydrothermal events.  S type granitoids are more susceptible to zircon inheritance although this has to be suspected in any case.   I would also look out for clastic sedimentary rocks in the basement as sources of older detrital zircon and the proximity of both basement and later granitoids. Cathodoluminescence  may help with 

You mentioned that the zircons are very scarce so that is another reason to look for monazite or titanite.  However, you should also check your mineral separation facilities if you suspect that you are getting low recoveries of zircon.  Smaller zircons can be washed out in the light fraction on a Wilfley table if it is not operating at the right speed.  Or you might be losing them in inefficient magnetite separations if you are dealing with e.g. gabbros and have a lot of magnetite or a propylitically altered rock and have a lot of pyrite.   On the other hand a scarcity of zircons is not a reason to consider the rocks unsuitable for U-Pb dating.  You may only need a few zircon grains.

The problem in your situation also depends on the age, and age difference of the basement and intrusive, mineralized granitoids and therefore the precision you need to distinguish them.  If they are both Archean you may be fine with just the zircons so long as you do not have significant lead loss events.  With younger .e.g. Mesozoic granitoids I would back up the zircons with monazite or titanite.  

From my understanding of your geological problem, Rb/Sr will not be the most reliable method in this case. It might give reliable results (if you can get isochrons with 4 or more points) but really, U-Pb is the way to go.

I would try what Hardolph recommends and look for more zircons first. Granite should have plenty.

Secondly, if you cannot have access to an LA-ICP-MS lab or a shrimp lab, single grain Chemical Abrasion (CA-) ID-TIMS will probably give you good results, so long as your zircons do not have lots of inherited cores and rim overgrowths.

Nevertheless, time is money and CA-ID-TIMS is very time consuming. You can date 100 to 200 zircons a day with LA-ICP-MS, but a dozen or two per week is more typical with TIMS, which is a fantastic tool but better suited for very niche applications.