I think I do not completely understand your question. Zircon is a mineral with the formula ZrSiO4; iron oxy-hydroxide is something like FeO(OH). So that is rather a clear difference, I'd say.

Iam asking about somthing related closely to zircon morphology . You can find old generation of zircon core associated with iron oxides. and sometimes you only find iron oxides in the core. What is the difference between the two cases? Is that related to the changes of physico-chemicalc parametars during crystalization?

Is this a general observation, or are you talking about a particular set of zircons? I haven't noticed that old cores are commonly associated with iron(hydr)oxides, so it might be something particular to the set of zircons you are working on. But perhaps other members have made similar observations...

Is your picture illustrating an old core? It's generally a good idea to annotate such pictures, or provide some sort of caption, since you are the expert on this picture, whereas no one else on ResearchGate is likely to have a clue what you want to illustrate. It could just as well be some other random inclusion in zircon. I see some very BSE-bright spots as well, which may be Fe-oxides, but could also be uraninite. You know - I don't.

In general, the preservation of old cores is related to 1. the fact that they have to be there in the first place (due to assimilation or remelting); 2. the temperature and chemistry of the melt they are incorporated into, as well as the time they spent in this melt.