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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