Just a quick provisional check, the rock wasn't crushed in a tungsten carbide tema mill was it, this will give erroneous Nb and Ta?

Cheers

John

Hi H-J. I suspect that this stock can be an A-type granite. What about the content of fluorine? In Sierra de Velasco (Argentina) there is a very similar granite stock named La Chinchilla (76 wt% SiO2, 0.08 wt% TiO2,

Hi H.J. Forster

I've experienced with evolved granites wich presents Nb up to 183 ppm and Ta up to 15 ppm (a Nb/Ta ratios of 12 more or less)

They were alkaline granites, from an alkaline complex along the cameroun line where a complete alkaline serie from basalt to alkali feldspar granites was exposed.

Most evolved granites show a Nb/Ta similar to alkali-basalt outcropping in the complex.

Thus indicating maybe a cogenetic origin?

for the paper look at:

Njonfang, Emmanuel, et al. "Petrogenesis of the Sabongari alkaline complex, cameroon line (central Africa): Preliminary petrological and geochemical constraints." Journal of African Earth Sciences 83 (2013): 25-54.

Hi John,

good point! But the samples were powdered in an achate mill, so artificial disturbance of the Nb and Ta concentrations could be excluded.

Hi Lucca,

thanks for your answer and the references listed.

Indeed, the granite could be regarded as aluminous A-type, even if the concentrations of not all A-type specific elements are elevated. For instance, F is below 0.05 wt%. Yes, the accessory minerals have still to be determined and their compositions measured. However, even the term A-type nor the accessory minerals are striking tracers of how the source rock looked like.

Cheers, HJ

Hi Lucci,

yes, if the basalts and highly fractionated granites show corresponding Nb/Ta ratios, their cogenetic origin is quite likely. And because this series has a strong mantle-imput, elevated Nb/Ta ratios could be readily explained.

And thanks for the reference!

Good luck,

HJ

Maybe not much help, but: my work on very-highly evolved Egyptian Younger Granites (ca Pan-African age), indicated "post-kinematic" but maybe not purely Anorogenic bodies. These are very Si rich and mafic poor. However RB/Sr and Sr ratios seem to indicate derivation from very primitive, depleated "crustal" or newly cratonized material. From what I have seen in the wider literature there is a great variety of possible chemical affinities, mostly representing source materials and % of partial melting derivation.

Hi, H.J. Forster:

Just check the petrography and see if hornblende or / and titanite were present as fractionation or residual phases. Hornblende with DNb/Ta > 1 (Tiepolo et al., 2000; Liu et al., 2013) and Ta is preferentially partitioned into titanite relative to Nb. Therefore, fractionation of the two mineral phases or their presence in the residue could increase Nb/Ta ratios in the equilibrating melt. I think it is more likely for titanite. Titanite likely is present in a reduced source. The highly-evolved A-type granites tend to stem from the same reduced source. Just try start from the reduced source composition to accommodate the evolved A-type geochemical signatures and elevated Nb/Ta rations.

The following are some papers from my group and Tiepolo. Perhaps, you can find some clues to your problem.

Liu, W., Liu, X.J., Liu, L.J., 2013. Underplating generated A- and I-type granitoids of the East Junggar from the lower and the upper oceanic crust with mixing of mafic magma: Insights from integrated zircon U–Pb ages, petrography, geochemistry and Nd–Sr–Hf isotopes. Lithos, 179: 293–319

Tiepolo, M., Vannucci, R., Oberti, R., Foley, S.F., Bottazzi, P., Zanetti, A., 2000. Nb and Ta incorporation and fractionation in titanian pargasite and kaersutite: crystal-chemical constraints and implications for natural systems. Earth and Planetary Science Letters 176 (2), 185–201.

M. Tiepolo et al., 2002. Chemical Geology 191: 105– 119.

Liu, W., Siebel, W., Li, X.-J., Pan, X.-F., 2005. Petrogenesis of the Linxi granitoids, northern Inner Mongolia of China: constraints on basaltic underplating. Chemical Geology, 219: 5–35

Hi Wei Liu,

I appreciate your interesting suggestions to clarify the matter. Hornblende is definitely absent in the granite in question, but titanite may eventually occur, thought both Ca and Ti are very low. Investigation of the critical mineral phases will start next week, so we will soon know the hosts of Nb and Ta in this particular granite. If interested, I would let you know the outcome.

Greetings and regards,

HJ

Dear Jeffrey,

thanks for your contribution. We have studied younger Pan-African granites from Jordan, but these follow the "normal" trend of melt evolution, i.e., Nb/Ta decreases with progressive fractionation, and granites of Si content comparable to those from the granite in question display Nb/Ta ratios below 10. Anyway, "juvenile" granites tend do possess lower Nb/ta ratios compared to crustal granites at about the same level of differentiation.

Greetings and regards,

HJ

Dear H.-J. Förster:

Nb, Ta contents of titanite is crucial. If some restite can be found, the Nb, Ta of titanite residual phase would be more important. Rarity or abundance magnetite in the granite may also help to determine the redox condition. Yes, your study is quite interesting. I am waiting for your results.

Hi Wei Liu,

If we observe titanite, its Nb and ta contents will certainly be determined. Granites of this type are normally ilmenite-series rocks, i.e., reduced. But we will see.

HJ

For those who are interested in the preliminary results of the microprobe studies of the granite in question, the major hosts of Nb are columbite, fergusonite-(Y) and aeschynite-(Y). Nb has also been substituted to a lesser extent in thorite and Ti-minerals. As anticipated, titanite is absent among the mineral assemblage. The major Fe-mineral is magnetite. No trace of ilmenite. Full quantitative mineral data will be aquired in June.

Thanks H.-J. Forster for informing us the interesting results.

Hello, just add to the comments of Wei Liu about the roles of hornblende and titanite, ilmenite can also incorporate very high concentrations of Nb and Ta, as found in my highly evolved felsic rocks.

Hello Chao,

thanks for that info about elevated Nb and Ta concentrations in Ilmenite. Unfortunately, ilmenite did not occur in the granite in question. The main Nb-sequestering ordinary Ti mineral in our granite is rutile, which has incorporated up to 3 wt% Nb2O3

Oh rutile, I am interested in why the TiO2 activity is so high there. Is it crystalline in melt or residue?

Dear Wei Liu Sir and Chao Zhang Sir,

I have read a number of papers by you. I feel honoured that I am being able to clarify my doubts from you'll. My granite samples also show high Nb/Ta(~10 to 19) ratios and EPMA analysis have shown that they contain quite a good amount of magnetite, ilmenite and rutile as opaque mineral phases and they are highly evolved granites as well. I want to know why is thw Nb/Ta ratio so high?

Unfortunately, we are not really progressing in explaining the non-chondritic elevated Nb/Ta ratios in highly evolved granite. They are likely not controlled by extended magma fractionation, but instead the conditions of partial melting or the nature of the source rock.

H.-J. Förster yes I agree with you nature of the source and degree of partial melting also control the Nb/Ta ratios. Those using Nb/Ta ratio as proxy for degree of fractionation need to take that into consideration.

That's correct. Nb/Ta ratio alone may be misleading.

Dear H-J Förster and Swayoma Bose,

The Nb-Ta composition of your granites is quite interesting. What I can say from my own granite whole-rock compilation is that such Nb-Ta composition (e.g. Nb/Ta ~17.5, Nb~55 ppm, Ta ~3 ppm) is generally only reached by mantle-derived A1-type peralkaline to metaluminous granites.

If you think that this granite is purely crustal in origin. It could suggest that it formed from the melting of either a residual crust or, for example, mantle-derived andesite rocks characterized by high Nb/Ta ratios. If, for example, sediments are submitted to a first event of partial melting at low temperature (until muscovite breakdown), the accumulation of biotite and ilmenite can induce an increase of the Nb/Ta value in the residue. If this residual crust is then submitted to a second partial melting event at high temperature (above biotite breakdown condition), the generated granitic melt can reach a high Nb/Ta value. See Stepanov et al. (2014) for some discussion about such processes.

Regards,

CB

Stepanov, A., Mavrogenes, J.A., Meffre, S., Davidson, P., 2014. The key role of mica during igneous concentration of tantalum. Contrib. to Mineral. Petrol. 167, 1–8. https://doi.org/10.1007/s00410-014-1009-3

Dear Christophe,

I really appreciate your scientifically sound comments. I also realized that you have published a respective paper in the near past, which I have missed. So far, there is no strong indication for a mentle-departure of the granite batch nor its generation by partial melting at high T. The granite body is pretty small and the melt probably has not much moved from its destination depth. B isotopes are in line with significant interaction with sedimentary material.

I will study your paper in more detail and perhaps get a clue.

Again, many thanks for your contribution,

Hans

Hi dear Sanaz.for your question refer to this article below:

Contrasting behaviour of Nb/Ta and Zr/Hf ratios in a peraluminous granitic pluton (Nova Scotia, Canada).

Cheers.

Hi Ismael,

could you provide the full reference for this paper?

HJ Förster

H.-J. Förster I think this is what Ismail was mentioning: https://www.sciencedirect.com/science/article/abs/pii/S0009254199001138

Correct! And many thanks.

HJF