Dear Mr. Xaba,

to cut it short. The paleosome, also called melanosome ,  contains the rather immobile, mostly dark minerals ("melano") of the mgimatite. It may be amphibole or biotite. The neosome or better called leucosome is white to gray, as already inticated by the Greek prefix "leuco". It is made up of quartz and feldspar, coming close in its composition to a finer-grained granite. The basic book to read is MEHNERT (1968).

Best regards

H.G.Dill

Thank you for your input. A book called Atlas of Migmatites written by Sawyer (2008) differentiate these zones whereby its stated that the Paleosome is the part which did not undergo partial melting, the part that represents the original rock. He also says that the neosome is the newly formed part of the migmatite representing the melanosome and leucosome. It now becomes a bit difficult to understand if both the paleosome and melanosome are one and the same thing.

This is misleading. Follow the way I recommended. It is based on Mehnert´s work.

Good luck

Harald G. Dill

Mr Xaba,

in the Atlas of migmatites the terms of the migmatite parts are well defined.Melanosome  is the dark coloured part of the neosome.Paleosome is the residual metamorphic rock which had survived from tha partial melting that formed the neosome(leucosome+melanosome)

How do you precisely differentiate the melanosome from the paleosome mineralogically in the the field ? Do we need another term for , e.g., black schorl in the felsic mobilizate ? I have seen many felsic mobilizates, showing different stages of evolutions with some of them ending up as aploids or pegmatoids (DILL, H.G. (2015) Pegmatites and aplites: Their genetic and applied ore geology.- Ore Geology Reviews 69: 417-561). Please download in case of need from the RG server the publications of the CMS classification scheme, showing the transition from the initial stages of mobilization up to a pegmatite or even granitic pegmatite.

Looking forward to a precise definition of these terms and their applicability in field geology.

Harald G. Dill

Mr. Xaba.

I never would like to work with migmatites. For me its  more complicate do classify the diferentes type of  migmatite strutures. When i iniciate field geology in Brazil ( mainly at Minas Gerais state) i had to classify migmatites, because was the current method of to study, work with this type of rock. But today we can classify this rocks as bimodal gneiss suíte: example:  leucocratic grey gneisses interfolded with melanocratic amphibolite. Do you undesrtand? If you take the book : Precambrian Tectonics Illustrated ( A. Kroner  & R. Greilling, 1984) you can read that this structures are boarded in diferente way. There are terms like agmatites, that coulb be interpreted as : a magmatic stage of granitic intrusion was sometimes accompanied by the explosive fragmentation of a supracrustal sequences. As the definion of mgmatite: in all migmatites there is  clearly a reaction betwen the host rock and the invading material, and migmatite may develop a totally new structure  of is own, distroying in the process all traces of the original fabric. So, migmatites generally pass through a plastic stage , with the developmente of flow ( folds - ptygmatic folding).    Tanks for ask me about this theme. OK.    

Dear Mr. Xaba:

4 terms need to be distinguished: Paleosome, neosome, melanosome, and leucosome. As the prefix indicates paleo is old, neo new, leuco light, and melano dark. Whereas paleo and neo is timely, leuco and melano is the color. In practice this means that the neosome is the product of anatexis that can be formed by reactions that produce for example graitic melt + garnet during fluid absent melting. In this case the quartzofeldspatic part is the leucosome, together with garnet forming the neosome. If you don't have an additional melanocratic phase like garnet or cordierite during anatexis, leucosome and neosome is the same. On the other hand the melanosome is the restite that did not melt and is formed by dark minerals, whereas the paleosome is the original rock of which the melt was extracted. The latter two terms are very difficult to distinguish is field and often chemical analyses is necessary.

Thank you everyone for your input. What i would also like to ask is the difference between the paleosome and leucosome chemically or in terms of the types of minerals present that one would expect, especially in the paleosome.

The separation between paleosome and leucosome can be made by mineral, textural or chemical analysis, where the paleosome is the migmatite part that does not melt, showing the original metamorphic mineral and chemical composition; and the leucossome is quite light coloured (leuco), mainly composed of quartz and feldspars with igneous textures (due to its source being the melt), therefore showing a more evolved chemical composition (enriched in incompatible elements).

Perhaps one thing that confuses all this is the fact that the 'melanosome', which is supposed to be the 'restite' can react back with the melt in the leucosome if the two remain in contact during cooling. So even if you formed garnet and/or cordierite during the melting reaction, these anhydrous minerals will react back with the hydrous melt (to biotite, for instance) when they remain in contact during cooling.

Mr. Xaba.

Migmatites are better judged from meter-scale rather than hand-specimen; however, if you only have that, well, better than nothing.

When a rock begins its heated enough a fraction of it will undergo partial melting. This one is the neosome. The rest of the rock that did not melt is names PALEOSOME. It is nothing more than the protolith or the parent rock. In the case of metased rxs you may be able to distinguish the paelosome by the conservation of structural and stratigraphic features that were present in the rock before it underwent melting. The more heat, the more melting, and the more destroyed the original features of the rock will be. If the rock is plutonic igneous you may have more difficulties in separating the neosome from the paleosome.

The neosome is composed of leucosome and melanosome, but any of both may not be present, more often the melanosome than the leucosome. The leucosom usually is felsic, but no always, generally composed of K-fdp and Qz. Not always, but it may be accompanied by a surrounding blackish halo, usually of biotite. If the halo is not in sharp contact then it would be the MELANOSOME. I disagree with someone above who said " paleosome, also called melanosome" that is incorrect.

Apart, the book of MEHNERT (1968) should be avoided despite that the Metamorphic comission still suggested following it (in 2007). Personally, I think it should be phased out. The work of Sawyer (2008) on his Atlas of Migmatites is brilliant as it moves from non-genetical meaningless descriptions to genetical linking. This is going to be the new classification in the future, I tell you. Just give it time.

Also it gives me the impression that Marlina is mixing terms? What she calles melanosome I think is meant to be the selvedge in Sawyers' classification.

Paleosome are the older rock which can be called the preexisting rock, while the neosome are the younger rock which are granitic or granitiod in ccomposution.