Synneusis of zircon; why not?
Synneusis of zircon; why not?
Mineralogical Magazine (February 2001) 65 (1(428)): 71-79
- absolute age
- Bas-Rhin France
- Carboniferous
- dates
- Europe
- France
- granites
- igneous rocks
- isotope ratios
- isotopes
- lead
- magmas
- major elements
- metals
- nesosilicates
- orogeny
- orthosilicates
- Paleozoic
- Pb-206/Pb-204
- plutonic rocks
- radioactive isotopes
- SEM data
- silicates
- Soultz-sous-Forets France
- stable isotopes
- trace elements
- U/Pb
- Variscan Orogeny
- Western Europe
- zircon
- zircon group
- synneusis
Some zircons from the Hercynian Soultz granite, NE France, exhibit complex structures, with double cores and oscillatory-banded overgrowths. These glomeroporphyritic crystals are uncommon for zircons, in which synneusis has not previously been described. The cores of the zircons are considered to correspond to very quickly formed {100} or {110} prisms. These could slowly sediment in the magma chamber and cluster together while sinking. The overgrowth rims correspond to a slower growth rate. U/Pb ion-probe dating of the cores and the overgrowth rims provided the same age of 327+ or -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 zircon in the lower parts of the granite. The formation of clusters in the early stages of crystallization needs stable P/T conditions, a hot, liquid silicate melt with a high fluid content, and no other minerals crystallizing (liquidus phase).