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| Titel |
Timing of metasomatism in a subcontinental mantle: evidence from zircon at Finero (Italy) |
| VerfasserIn |
I. Yu. Badanina, K. N. Malitch |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250065116
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| Zusammenfassung |
| The Finero phlogopite-peridotite represents a metasomatized residual mantle harzburgite,
exposed at the base of the lower-crust section in the Ivrea Zone, Western Alps (Hartmann and
Wedepohl 1993). It forms the core of a concentrically zoned sequence of internal layered
gabbro, amphibole-rich peridotite and external gabbro. The phlogopite peridotite contains
small-size chromitite bodies, with a suite of accessory minerals such as phlogopite, apatite,
Ca-Mg carbonates, zirconolite, zircon, thorianite and uraninite, proposed to form during
alkaline-carbonatitic metasomatism process within the mantle (Zaccarini et al. 2004). In this
study, the combined application of a non-destructive technique to separate zircon from their
host rocks (see details at http://www.natires.com) and in-situ analytical technique for
compositional and isotopic analysis (SHRIMP-II at Russian Geological Research
Institute, St. Petersburg) has provided new more detailed age constraints on the
formation of chromitite and related metasomatic events within a mantle tectonite at
Finero.
Chromitite samples derived from the dump in the prospecting trenches of Rio Creves. In
thin sections, zircon occurs as relatively large (up to 200 μm) grains characterized by
subhedral to euhedral shapes. Separated grains of zircon form two distinct populations.
Dominant zircon population is pale pink and characterized by different shapes (subhedral,
subrounded or elongated). In cathodoluminescense, the main set of population is represented
by complex grains, which show development of core-rim relationship (most likely
recrystallized rim on a preserved core). Subordinate zircon grains are colourless.
They are characterized by a smoky cathodoluminescense, with almost no internal
pattern.
Three main U-Pb age clusters have been recognized. The youngest age cluster,
typical for subordinate colourless zircon population and rims in complex grains
of dominant pale pink population, show two concordant 206Pb/238U ages (e.g.,
208.6 ± 4.0 Ma, MSWD=2.0; P=0.16, n=8 and 194.9 ± 3.4 Ma, MSWD=0.45;
P=0.50, n=3, respectively). Other age clusters are characterized by the cores and rims
observed in composite grains. They yielded concordant 206Pb/238U ages of 288.3
± 7.3 Ma (MSWD=3.3, n=6) and 248.6 ± 3.3 Ma (MSWD=0.13, P=0.72, n=8),
respectively.
Since the pioneering work of Exley et al. (1982), the complex metasomatic history at
Finero has received much attention. New U-Pb results are consistent with the age range
obtained for mantle rocks, the phlogopite peridotite (293 ± 13 Ma, Voshage et al. 1987) and
chromitite (208 ± 2 Ma, Grieco et al. 2001). The former age estimate, based on a Rb-Sr
whole-rock isochron for six phlogopite-bearing peridotites and one phlogopite pyroxenite,
has been interpreted as time of K metasomatic enrichment of the harzburgite. This event has
been coeval with the intrusion of alkaline ultramafic magmas into the deep crust of the Ivrea
Zone during the late Carboniferous (287 ± 3 Ma, Garuti et al. 2001). The U-Pb age of 208±2
Ma for zircon at Alpe Polunia, attributed by Grieco et al. (2001) to one of the major
metasomatic episodes, is corroborated by a subordinate subset of zircon grains at Rio
Creves. The U-Pb zircon ages identified in this study thus show notable differences.
Our U-Pb data do not concur with the assumption of a single metasomatic event
during chromitite formation. In contrast, we suggest a prolonged formation and
multistage evolution of zircon growth, as mirrored by multiple U-Pb ages. U-Pb results
for zircons from two chromitite localities (Alpe Polunia and Rio Creves) place
tight constraints on their different temporal evolution. We presume that Hf-isotope
data of zircon and Os-isotope data of laurite, to be investigated in the future, will
shed new light on the sources of materials involved in a subcontinental mantle at
Finero.
This investigation was supported by Uralian Branch of Russian Academy of Sciences
(grant 12-P-5-1020).
References:
Exley, R.A., Sills, J.D., Smith, J.V. (1982) Geochemistry of micas from the Finero
spinel-lherzolite, Italian Alps. Contrib. Mineral. Petrol. 81, 59-63.
Garuti, G., Bea, F., Zaccarini, F, Montero, P. (2001) Age, geochemistry and petrogenesis
of the ultramafic pipes in the Ivrea Zone, NW Italy. J. Petrol. 42, 433-457.
Grieco, G., Ferrario, A., Von Quadt, A., Koepprel, V., Mathez, E.A. (2001) The
zircon-bearing chromitites of the phlogopite peridotite of Finero (Ivrea Zone, Southern
Alps): evidence and geochronology of a metasomatized mantle slab. J. Petrol. 42,
89-101.
Hartmann, G., Wedepohl, K.H. (1993) The composition of peridotite tectonites from the
Ivrea Complex, northern Italy: resudues from melt extraction. Geoch. Cosmoch. Acta 57,
1761-1782.
Voshage, H., Hunziker, J.C., Hofmann, A.W., Zingg, A. (1987) A Nd and Sr
isotopic study of the Ivrea Zone, Southern Alps. N-Italy. Contrib. Mineral. Petrol. 97,
31-42.
Zaccarini, F., Stumpfl, E., Garuti, G. (2004) Zirconolite and Zr-Th-U minerals in
chromitites of the Finero complex, Western Alps, Italy: evidence for carbonatite-type
metasomatism in a subcontinental mantle plume. Canad. Mineral. 42, 1825-1845. |
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