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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