Zircon, xenotime and garnet can be responsible of HREE depletion in magmas. Of the three possible cases above you would expect, respectively: Ce, Hf, and Zr depletion and Eu anomaly decrease with differentiation (zircon fractionation);  a P depletion with differentiation (xenotime fractionation); and a LREE increase if garnet is fractionated.

http://www.elementsmagazine.org/archivearticles/e3_1/hanchar.pdf

I would expect Amphibole fractionation in a chondrite normalized REE 'spoon-shaped' pattern.

Different HREE contents in co-genetic magmas could be explained by accesory minerals fractionation

https://www.cambridge.org/core/journals/earth-and-environmental-science-transactions-of-royal-society-of-edinburgh/article/controls-on-the-trace-element-composition-of-crustal-melts/9E20C0139567C67ECA013F4A062C5C2F

You can explore de different partition coefficients in this link:

https://earthref.org/KDD/

Hope it helps.

It really depends on how differentiated your rocks are. If  LREE, Zr, P, etc are not strongly depleted, you are likely looking at garnet, probably as a restite phase. In that case, a ratio such as Sr/Y can sometimes indicate whether you might have a high-P melting regime in the garnet stable/plag unstable field. If you are looking at very highly differentiated rocks with low Zr and P for example, trace phase fractionation could be important. Look at all of the elements, not just the REE. The fact that you have a  a strong positive Eu anomaly may indicate either preferential plag melting at the source or plagioclase accumulation in the rock. In the former case, it is possible that high-P plays a role in limiting plagioclase stability.

Hi Jackeline,

To your question:

Which processes can cause different HREE content in a co genetic magma series?

Some authors suggest that this could be explained by partial melting of a heterogeneous source. For example, the wide range of the REE (and HREE) composition of the c. 1.87 Ga Ngualla K-granites, in the Ubendian belt, is indicative of the involvement of a heterogeneous crustal source (Tulibonywa et al., 2017). Similarly, according to Jayananda et al. (2006), the heterogeneous chemical compositions of ~2.61 Ga potassic granites from Dharwar craton, indicate involvement of various sources and also significantly different depths of partial melting.

However, I wouldn’t say that they are co-genetic.

Many of these rocks could be spatial-related, co-temporal and also originated by the same thermal anomaly,

but if they are the melting product of different sources, they are not co-genetic.

Good luck on your research,

Felipe  

References:

Jayananda, M. Chardon, D., Peucat, J.J., Capdevila, R., 2006. 2.61 Ga potassic granites and crustal reworking in the western Dharwar craton, southern India: Tectonic, geochronologic and geochemical constraints. Precambrian Research, 150, 1-26.

Tulibonywa, T., Manya, S., Torssander, P., Maboko, M.A.H., 2017. Geochemistry of the Palaeoproterozoic volcanic and associated potassic granitic rocks of the Ngualla area of the Ubendian Belt, SW Tanzania. Journal of African Earth Sciences, 129, 291-306.