Hi Dadayakkarage, I would say that your estimates do generally apply for crystalline detrital zrn in your metasediments. A ~100 µm-diameter, crystalline model zircon has a U closure temperature (Tc) of roughly 1100-1300°C at a cooling rate of 10°C/Myr, with Pb diffusion (loss) starting at lower Tc, roughly 900°C as you state (these calculated Tc values slightly vary from model to model, dependent on grain/diffusion domain size and on the cooling rate). For details see papers by D. Cherniak and colleagues, as well as Lee et al. 1997. In your case, it might appear more likely that the zrn would reflect their original crystallization age if there is petrologic evidence for lower metamorphic facies. However, if the zircon subjected to metamorphism was partly or even largely metamict (radiation-damaged due to high initial U contents), the Tc for resetting can be (much) lower and could reach temperatures as low as ~400°C over a heating duration of ~1 Myr. To gain further insights into this, it might be interesting to additionally employ other dating techniques on co-existing detrital minerals with different specific Tc, such as mica or feldspars using 40Ar/39Ar, for example. These would rather reflect the ~200-400°C temperature range for Tc (again, depending on diffusion domain size). Also, if metamorphism reached higher T (900°C is quite a high T, higher hornfels to granulite facies!), you would certainly see this in the mineral assemblage of your rock and also the zircon texture (e.g., metamorphic rims and overgrowths as revealed in SEM-CL). See, e.g., "Atlas of zircon textures" by F. Corfu 2003. Do you know the metamorphic grade of your rocks?

Dear Martin, thank you very much for the explanation. My samples are high grade metamorphic rocks. Orthopyroxene bearing assemblages are common. So all samples are in granulite facies. Peak condition has marked as UHT metamorphism. However my samples do not have any evidences of them except spinel + cordierite assemblage which may seems like remains of UHT assemblages. However, the terrain is almost 2-3 Ga old but most age dating data highlight the 550Ma. Those age dating values are under arguments due to the possibility of more stages of metamorphism in during and after those events.

Dear Ddayakkarage, if your samples are sedimentary, you analysed detrital zircons which means limitations of sedimentation. For example if your youngest zircon age is 300 Ma, sedimentation has to be after 300 Ma. If they are effected by metamorphism, all of them sholud have metamorphic rims. you can also date them by using LA-ICP-MS and calculate the metamorphic age. It is also possible to determine metamorphic zircons by using their CL images, morphology and Th/U ratio. Petrographic investigation may help you, also.

Dear Zera, Thank you very much for the suggestion. In my study I first focus on detail classification on zircon internal textures to filter out the different domains. I am widely using BSI and CL from EMPA.

Dear Juan. Thank you for your excellent guess. I just finished with exactly the same conclusions... and proposed two source areas due to the detrital core morphology. then two step overgrowth and newly grown grains with inherited cores by UHT peak metamorphism

Dear Zehar ,

As you mentioned that the Th/U ratio can differentiate the origin of zircon ( either metamorphic or volcanic) so can you please let me know range of value for volcanic , metamorphic. I am also working on Detrital zircon of foreland basin so your answer will me much more helpful for me.

Dear Dadayakkarage

As far as I know the dating method might also differ as your interest (as crystallization age or deposition age). If you did U-Pb dating then it gives you the crystallization age and if you conduct Thermochornology then I guess it give the depositional age.

Thermochronology of zircons in a high grade metamorphic rock will NOT provide you any information on the depositional age of the protolith, as some have suggested! But will provide you with the history of post-metamorphic cooling, which can postdate metamorphism by hundreds of millions of years (!)

The correct procedure would be to carefully analyse the CL textures of the zircons and date cores and rims - if you find a coherent, younger age population from sector zoned zircon rims, and these ages correspond to low Th/U ratios (e.g.

Thanks for your answer. I agree with you, and high grade metamorphic processes MAY result in new overgrowths around old cores but yes, the closure temperature of zircon can be higher than the typical metamorphic temperatures reached, unless the zircon is metamict as Martin pointed out.

Dear Sanjaya,

Most of the above answers give a cross section of current thinking on zircon dating. If you are arriving at conclusions following the same lines of thinking your work will definitely be accepted for publication, however, you may not be making a significant contribution towards development of new ideas. So try to think different and may be you can break the barriers of conventional thinking. Good luck!