Postmagmatic zircons in
rapakivi granites of the Salmi batolith and high potassium granite
massives in its North-West surrounding (Karelia).
Rodionov N.V.*, Presniakov S.L.**, Kapitonov I.N.**,
* SPbU, geological
faculty, St.Petersburg, Russia;
** VSEGEI, St.Petersburg,
One of the results of our investigation of pegmatites
connected with the Salmi rapakivi granites in its Nort—West contact with
host metasedimentary rocks (Hopunvaara, Torpussuo) was finding that the
rocks contain as ancient suconcordant zirons (1480-1520 Ma), so some
late generations with U-Pb age from 230 to 500 Ma, for which presence
of excess radiogenic hafnium is characteristic [Sundblad, e.a., 2011].
For this young zircons the value of calculated model hafnium age (T DM
Hf) in lower, than age by U-Pb isotopic system, thus them have
metasomatic origin [Paleoproterozoic…,2011]. In this rocks are present
only some new generation, but no “core-rim” structures were identified,
thus the question are these zircon crystals from the given rocks, or it
is contamination, is still open. Moreover, contamination of the rocks
can be supported by observation, that the rocks have no visible
secondary alteration features.
Investigation of the Torpussuo alaskite topaz bearing
granites (a little massive near contact of the Salmi massive with Ladoga
gneisses), have shown, that in this case there are zircon crystals with
clear bright in CL cores and powerful dark rims with thickness about
30-50 microns. Investigation of isotopic U-Pb system in these zircons
revealed, that the inner cores have subconcordant age 1576±82 Ma, and
the rims have high uranium content (up to 22000 ppm U) and demonstrate a
range of concordant U-Pb ages from 70 to 560 Ma.
To study this fenomena in details we investigated
zircons from the rocks of I intrusive phase of the Salmi batolith itself
and from small Nietjarvi intrusion, believed to be analogous to the II
intrusive phase of the Salmi rapakivi granites. These rocks also do not
reveal any traces of secondary postmagmatic alteration, and we specially
checked zircon grains for outer rims, and have got the follows results:
1). Zircons from Nietjarvi massive have not visible
rims, and concordant U-Pb age 1512±12 Ma was obtained.
2). In the Salmi grannies the zircons as a rule
exhibit structure “low-uranium core (30-50 ppm) – relatively high
uranium crust (150-600 ppm)”. The measurements by SIMS SHRIMP-II have
shown, that all them are subconcordant, contain less than 0.7% of common
206Pb, and have characteristic for magmatic rocks
Th/U=0.4-0.8. The age estimation is 1541±12 Ma (weightered average
207Pb/206Pb), which is in concordance with ID TIMS data
[Amelin, e.a., 1997]; In some zircon crystals were found very thin (<10
microns), dark in CL rims. Their investigation revealed that them are
high uranium (4500-9500 ppm), strongly discordant, contain from 0.7 to
40% of common 206Pb, and show low Th/U=0.09-0.15. Obtained U-Pb
isotopic data are forming a Discordia with intercepts 444±15 and
1540±15 Ma (Fig.1). Lower intercept from our point of view corresponds
to some real geological event, which was not fixed by ID TIMS, probably
due to discrimination of zircons before the analysis. High uranium thin
rims probably were totally recrystalyzed at thermal event of the
High uranium thin rims could originate during
postmagmatic stage of fluid-autometasomatic alteration of the rocks.
This mean migration of high strength field elements zirconium and
hafnium, fro which a water-halogene fluid required. Presence of thik
high uranium rims in the topaz Torpussuo Li-F granites confirm it.
Circulation of such fluids outside the massives of granites must lead to
input of radiogenic hafnium from ancient host rocks, which comes to the
fluid phase due to recrystallization of minerals with high Lu/Hf ratio
(apatite, garnet). Investigation of Lu-Hf isotopic system in zircons do
confirm this model (Fig.2). All outer high-uranium rims of the zircons
in Salmi and Torpussuo granites exhibit some excess radiogenic hafnium,
proofing their metasomatic genesis. The inner “normal magmatic” cores
the Salmi and Torpussuo zircons as well as zircons from Nietjarvi within
the errors are belonging to the correlation field for magmatic rocks
(TA), hence them really correspond to crystallization time of the
Fig.1. U-Pb system in zircons of the Salmi batoloth rapakivi rock. Bold
lines difine error ellipses for outer high uranium rims of the zircons.
Fig.2. Isotopic Hf-Nd systematics of the granites.
Dotted lines define correlation area for magmatic rocks “terrestrial
This work was supported by
SPbU grants 188.8.131.521 and 184.108.40.2061
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Lu-Hf isotopic systems in zircons from pegmatitoids and granite stocks
connected with the Salmi Rapakivi Batholith, Ladoga area, Russia.
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