Silicic magma can be derived from tonalite or quartz diorite through partial melting when continental rocks are subducted or when they are ponded or intruded by mafic magma. Would the two melts have identical chemistry, assuming 1) identical composition tonalite or quartz diorite in both cases, 2) equal degree of partial melting (say 10%), and 3) no addition of fluids from outer source?
Thank you in anticipation.
I don't think you can assume same composition or degree partial melting in the two scenarios. But you should be able to distinguish between the two: the former would likely be calc-alkaline, the latter would likely have higher Ti and Fe and less marked, if any, Nb anomaly ("continental tholeiites"). If the rocks are less than pristine, Pearce et al (1984) should help, as it relies on the less mobile trace elements rather than majors. If you can demonstrate that majors reflect original composition, then there are many other classifications schemes, see Frost et al. (2001) Journal of Petrology paper and refs therein. Best of luck to you.
My question is fundamentally based on the two assumptions, that the source material and degree of partial melting are the same. If you please allow me these assumptions, woulkd you still say that the partrial melts would have different compositions? Are you implying that the difference in pressure would lead to two different liquids.
Cheers, Qasim
I believe that, all other conditions being equal and your assumptions, the magmatic source depth can affect the composition of magmas. Since the tonalites-quartz diorites are transformed into granulitesin the crust , the partially melted magmas will depend on the presence or absence of garnet, amphibole and plagioclase in the source. Significant HREE depletions and REE fractionation in liquids resulting from granulite melting do require garnet to be stable in sufficient amounts in the residue. Flat REE patterns and negative Eu- anomaly will be characteristic in liquids resulting from source enriched with plagioclase.
Thanks a lot, Roman.
I had overlooked the importance of metamorphism never looked into this angle.
Regards,
Qasim
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