Hi Leonid,

Thanks for pointing me to your articles. I am curious as to why you think U-Th enrichment in residual granitic melt would be less common than in fractionated melt from a mafic parent. If a granitic magma is already enriched in incompatible elements shouldn't a derived 'minimum' melt be even more enriched?

Best regards,

Gordon

Well, that seems to be a valid way to look at it. These zircons are actually from a leucosome bearing aplite forming a mantle around a rapakivi granite. Some people have suggested that rapakivi texture might originate from mixing between granitic melts and mantle derived melts. There is certainly evidence for magma mingling along the margin of the main granite body. It may be that the high-U-Th rims are also a consequence of this mixing after the crystallisation of the zircon cores. Thanks for explaining.

Hi Gordon, we published time ago an example of U-Th-rich magmatic overgrowths of zircon in a well-constrained leucogranite. Hope it is useful for you.

hi Gordon; Hydrothermal zircon growth has been described mainly during

late-stage magmatic or post-magmatism crystallization . The main characterization is the formation of high-U zircon margins, which in some cases form simple overgrowths, but more commonly are

embayed into low-U regions in the zircon crystals. These high-U regions may have formed by recrystallization, or dissolution and re-precipitation during an event, which probably was accompanied by regional hydrothermal activity.

There are a common features associated with the overgrowth, changing in colors as a result of U-Th inclusions. Plz check this

(Ra’s Abdah of the north Eastern Desert of Egypt: the role of granitic dykes in the formation of radioactive mineralization, evidenced by zircon morphology and chemistry)

Article Ra’s Abdah of the north Eastern Desert of Egypt: the role of...

hello

every thing related to zircon you can find it in element magazine

February 2007

Volume 3, Number 1

ISSN 1811-5209

Dear Gordon Webb,

The answer to your question "How common are U-Th-rich magmatic overgrowths of zircon in granites and related rock types?" is that it is indeed very commom.

The answer to your questio "How common are U-Th-rich magmatic overgrowths of zircon in granites and related rock types?" is that it is very commom that radiometric dating in zircons (zoned) must take in account every zone individually.

Since every overgrowth means that growth was interrupted and restarted again, it means that magmatic conditions changed (P, T and/or also chemistry). So, U and Th available was maybe different at any restart.

Magma mixing certainly changes magma final composition but magma mingling normally keep both magmas previous compositions and so it may induce new zircon overgrowth but with the same previous zircon composition.

Rapakivi and anti-rapakavi are feldspars textures largely related to magma mixing but some say it can also simply be self magma composition change while the minerals remove one constituent and enrichs in another (Na, K, Ca exchange).

It is very plausible that while parts of two magmas mix while at other parts where P and T conditions might be different they do not mix but only mingle.

I myself mapped a granitic pluton in which there were not only mixing and mingling between the two original members but also the mixed resultant hybrid magma mingling with its both previous magmas.