Synneusis of zircon: why not?

Some zircons from the Hercynian Soultz granite, NE France, exhibit complex structures, with double cores and oscillatory-banded overgrowths. These glo meroporphyritic crystals are uncommon for zircons, in which synneusis has n ot previously been described. The cores of the zircons are considered to co rrespond to very quickly formed {100} or {110} prisms. These could slowly s ediment in the magma chamber and cluster together while sinking. The overgr owth rims correspond to a slower growth rate. U/Pb ion-probe dating of the cores and the overgrowth rims provided the sam e age of 327 +/- 6 Ma. Chemical electron microprobe analyses indicated that cores and rims have the same composition for major and trace elements. The sinking of zircons accounts for the abnormally high percentages of zirc on in the lower parts of the granite. The formation of clusters in the earl y stages of crystallization needs stable P/T conditions, a hot, liquid sili cate melt with a high fluid content, and no other minerals crystallizing (I iquidus phase).