During our analysis of U-Pb zircon dating, the intercept age is found to be 69.86±0.31 Ma which is discordant age while mean age is 69.83±0.39 Ma. The reviewer preferred mean age rather than the intercept age. So, my confusion is which age does convincingly constrain the crystallization age of a rock and why does so?
Thank you Prof. Leonid. You have exactly mentioned the case which are in our results although the ages are quite close to each other. In such case, can we show the mean age of the weighted average plot instead of intercept age, but showing both the diagrams (weighted average plus concordia)?
Dear Nongmaitheng: I agree with Leonid´s explanation, you should check for the points in the discordia line an see if they can really represent the truth. It would be nice if you could annex the concordia-discordia diagram which you obtained for a better analysis. With a Late Cretaceous age it looks to me that you are dealing with pre-Himalayan batholits formed by subduction of the Thetys Ocean beneath the active margin of southern Asia. Am I right? Regards, Sebastián.
Thank you and yes prof Sebastian its before the main collision between Indian and Eurasian continent. And i will try to annex the plot soon.
Dear Nongmaitheng: yes, I was almost sure that was the case, I think these Late-K batholiths are called Transhimalayan, and are totally different (supra-Benioff zone and orogenic) from the Central Himayalan postcollisional leucogranitic batholiths of Oligocene age. I have a copy of the Journal of Metamorphic Geology, a Special Issue called "Himalayan Metamorphism", vol. 7 (1), january 1989, which illustrates all the complexities of the mighty mountain range. I suggest you read it, if you haven't done it already.
Awaiting the concordia graph!
With regards, Sebastián.
Thank you Prof. Sebastian and I will go through the mentioned paper. I have few other small queries. How do we choose either normal plot or inverse plot respective of their ages? and Can we use normal plot for those which can also be plotted in inverse or vice versa? In short, are these plot fixed to the particular age ranges? Graphs are enclosed herewith.
Thank you Jorge, but in our case, we could not get the upper intercept age in some of the samples and we found the lower intercept age as the crystallization age. Please explain more about upper and lower intercept age.
Dear Nongmaitheng: Jorge's citation seems to stem from any good text of Isotopee Geology, like the Fauré and the like. Upper concordia-discordia intersect, if well defined, is taken as the age of crystallization, the lower intercept in some cases can be attributed to post-magmatic thermal events, including metamorhism. But I don't think your granitoids have any metamorphism, do they? Regards, Sebastián.
Thank you Dear Sebastian Grande
The studied rock types are rhyolite and andesite and yes, I could not see any feature due to metamorphism.
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