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Phanerozoic time
alteration of the Precambrian rocks from the Onega structure (Karelia).
Lokhov K.I.*,
Kapitonov I.N.**, Antonov A.V.**, Sergeev S.A.**
* SPbU,geological
faculty, St.Petersburg, Russia;
** VSEGEI, St.Petersburg,
Russia.
Kirill_Lokhov@vsegei.ru
At investigation of volcanics and high carbon (shungite bearing)
metasedimants from Ludikovi sequence of the Onega structure it was
found, that them contain numerous zircons with Phanerozoic ages (Goltsin,
e.a., 2010). Investigation of Precambrian rocks from the Onega
parametric drill hole (OPD) have shown, that as Paleoproterozoic
volcanics, so Neoarchean granitoids of the basement, suffered polystage
endogenic alterations. Such late processes are known for some parts of
the Baltic shield
[Larson, Tullborg, 1998, Sergeev, e.a., 2008 and others].
Investigation of zircons from plagioclase-microcline granites of the
basement in OPD revealed that even in the less altered rocks the zircons
exhibit as some partial loss of radiogenic lead, so crystallization of
the new generations (structures “core-rim”) at, at least two stages of
the rock alteration, and the last one corresponds to Phanerozoic event(s).
The Ludikovi volcanics contain: a). some discordant zircons, by which
lead loss at numerous events of Phanerozoic time van be estimated; b).
new generations of zircon crystals, and the given precision of the SIMS
SHRIMP-II do not allow estimate if there are existing some groups of
concordant zircons with ages 148, 165, 200, 300 and 650 Ma, or them
belong to unique discordia with parameters 154±6 and 619±54 Ma.
Investigation of Lu-Hf isotopic system in these zircons show, that there
exist several age groups of zircons, which are containing excess and
sequently more radiogenic hafnium. Such zircons could be originated at
low-temperature fluid-metasomatic processes, during which, some minerals
from sedimentary rocks with high Lu/Hf ratio, about 0.15, were
recrystallized. These minerals could be phosphates from terrigenic rocks
of Ludicovi, formed near 2000 Ma ago. Presence of powerful sequence of
salts (halite) in the basement of Paleoproterozoic rocks from OPD [Paleoproterozoic…,2011]
defined circulation in the rocks of water-halogene fluids, which
transported high strength field elements (Zr, Hf) effectively [Korzhinskaya,
Ivanov, 1988, Aja, e.a., 1995 and others], thus one can expect elevated
concentration of hafnium in the metasomatic zircons. Zircons with excess
radiogenic hafnium, for which calculated model age by hafnium (T DM Hf)
is less, than calculated age by U-Pb isotopic system, are marking
Phanerozoic time events of rock alteration in the Onega structure.
Notably in these zircons really are extremely high hafnium contents:
from 1.5 to 7%.
Presence of salt sequence in the basement of Paleoproterozoic
sedimentary-volcanic rocks in the Onega structure, which are responsible
for water-halogene fluid circulation, define, that in this structure
some young processes are fixed more contrast as in comparison with
Archean granitoids and volcanics, in which these events are fixed in
rear cases and in from of some partial loss of radiogenic lead from
zircons, leading to their discordance. One can expect, that in the rock
reach by halogens, for example in lithum-fluorine granites, analogous
effects also are widely represented.
This work was supported by SPbU grants
3.37.81.2011 and 3.37.86.2011.
Literature:
Goltsin N.A., Lokhov K.I., Kapitonov
I.N., e.a. Polystage alterations of high carbon sedimentary rocks of
Ludikovi of the Onega structure. // Regional Geology and Metallogeny,
2010, 41, p.66-79.
Korzhinskaya V.S., Ivanov I.P.
Experimental study incoengruent dissolution of zircon in the system ZrO2-SiO2-H2O-HCl
at T=400-600oC and P=1 kbar. // DAN, 1988, v.299, N4, p.
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Paleoproterozoic Onega structure Ed.
By L.V.Glushanin, N.V.Sharov, V.V.Shiptsov, Petrozavodsk, Karelian sci.
center RAS, 2011 432p.
Sergeev S.A., Lobach-Zhuchenko S.B.,
Arestova N.A., e.a. Age and geochemistry of zircons from oldiest
granitoids of the Vyg river, South-East Karelia. // Geokhimiya, 2008,
№6, p.647-659.
Aja S.U., Wood S.A., Williams-Jones
A.E. The aqueous geochemistry of Zr and the solubility of some Zr-bearing
minerals. Applied geochemistry, 1995, v.10, Iss 6, p. 603-620.
Larson S.A., Tullborg E-L. Why Baltic
shield zircons yield late Paleozoic lower-intercept ages on U-Pb
Concordia. Geology, 1998, v.26, p. 919-922. |