CIPW is a way of estimation the mineralogy, according with chemical composition and normal crystalization sequence.  Variations in volatile composition, water contente na minor elements can interfere in real pressures.

This method allows to estimate the pressure (water pressure, first of all) under which the granitic magma crystallized. It would be better if your rock is equigranular because phenocrysts may shift bulk rock composition aside from the 'right' cotectic line.

Then, an important point is whether magma crystallized under water-saturated conditions because the 'dry' lithostatic pressure and water pressure have a different effect on melting / crystallization. If you are trying to estimate the depth of crystallization, the error is greater in the case of H2O-undersaturated magma.

Then, the addition of Ca has an effect opposite to that of water pressure. Leucogranites expectably reveal better results than granites, granodiorites etc. because they compositionally approach the experimental system Q-Ab-Or.

So, this is like one equation with several variables. For estimates of emplacement depth, additional methods are recommended.

A thorough description of the system Q-Ab-Or has been presented by Johannes and Holtz (1996).

Hi, Petr Tikhomirov 

Thank you very much for constructive advices. I'm studying a magmatic charnockite. According to previous studies, most charnockites crystallized from anhydrous magmas. Therefore, based on your comments above, it is not right to use  the system Q-Ab-Or to estimate crystaalization pressure. Right?

Additionally, I want to estimate the melting pressure in the magma source region. Can you give any good method?

Thanks. 

Lu

Hi Lu,

If your charnockites are voluminous enough (well above 1000 km3), they likely originate from a crustal source. For the continental crust, the estimates on magma generation pressure are, in most cases, quite speculative because the subsequent crystal fractionation and contamination may considerably affect magma composition. Methods based on mineral equilibria and melt inclusions address relatively late stages of magma evolution.

Perhaps you may try to identify the protolith of your charnockites using trace element and / or isotopic criteria - of course, if there is any information about mid and lower crustal complexes in your study area. However, even if you find such presumed protolith, its spatial location in the crust (including the depth) could change since the time of magma generation. Especially, if you deal with Precambrian complexes (sorry, I do not know the age of charnockites you are studying).

I hope somebody else may give a more useful advice.

Best regards,

Petr