Dear Hassina and other zircon lover

        It has been observed by many workers that Th/U ratio is generally lower than 0.1 in case of metamorphic overgrowth. But this has to see in close association with the CL images of the zircon over growth which most of the time (that is what our experiences are) is without any oscillatory growth zones (Black et al., 1986; Williams, 1992; Vavra et al., 1996; Rober and Finger, 1997). The cause of the overgrowth is attributed to dissolution, transport and precipitation of zircon from a metamorphic fluid. If these features are present than it is obvious that metamorphic over growth is there.

L. P. Black, I. S. Williams and W. Compston, Four zircon ages from one rock: the history of a 3930 Ma-old granulite from Mount Sones, Enderby Land, Antarctica. Contrib. Mineral. Petrol. 94 (1986), pp. 427 437
 
I. S. Williams (1992). Some observations on the use of zircon U-Pb geochronology in the study of granitic rocks. Trans. R. Soc. Edinb. 83 (1992), pp. 447-458.
 
G. Vavra,  D. Gebauer, R. Schmid and W. Compston, Multiple zircon growth and recrystallisation during polyphase Late Carboniferous to Triassic metamorphism in granulites of the Ivrea Zone (Southern Alps): an ion microprobe (SHRIMP) study. Contrib. Mineral. Petrol. 122 (1996), pp. 337-358.
 
M. P. Roberts and F. Finger, Do U-Pb zircon ages from granulites reflect peak metamorphic conditions? Geology 25 (1997), pp. 319-322.


I hope these observation will be of some help to you.

Cheers

Sandeep


At 02:58 PM 7/27/2005, you wrote:
Dear friends:
I read somewhere (I think in Hoskin & Schaltergger, 2003 - apologies if I
give the wrong reference - I donšt remember the right one) that in general
the Th/U content of igneous zircons is  something like Th/U > 0.5 and in
metamorphic zircons it is < 0.05.
I would like to ask if this is what you see in general (from your experience
with zircon dating) and how much we can rely on this compositional feature
to  discriminate between what is "metamorphic" and what is "magmatic" in the
absence of clear textural evidence (such as zoning in igneous zircons).
I thank you very much in advance for your reply.
Wishing you all the best,
hassina

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Dr. Sandeep Singh
Assistant Professor
Department of Earth Sciences
Indian Institute of Technology Roorkee
Roorkee - 247 667
INDIA

phone: +91-1332-285559 (off.)
         +91-1332-285086 & 275043

mobile: +91-9412073043

fax: +91-1332-273560

web page: http://www.iitr.ac.in/acads/depts/ESC/faculty/facthtml/sandpfes.shtml
                  www.sandeepsingh.20m.com
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