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| Titel |
Zircon and baddeleyite from the economic ultramafic-mafic Noril'sk-1 intrusion (Russia): Hf-isotope constraints on source composition |
| VerfasserIn |
K. N. Malitch, E. A. Belousova, I. Yu. Badanina, W. L. Griffin |
| 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 |
250066441
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| Zusammenfassung |
| The ultramafic-mafic Noril’sk-1 intrusion in the northwestern part of the Siberian
Craton (Russia) represents one of three known Noril’sk-type, ore-bearing intrusions,
which host one of the world’s major economic sulphide platinum-group-element
(PGE)-Cu-Ni deposits. Zircon and baddeleyite dated previously both by SHRIMP (i.e.
248.0 ± 3.7 Ma, Campbell et al. 1992) and ID-TIMS (251.1 ± 3.6 Ma, Kamo et al.
1996) have been restricted to one lithology (e.g. leucogabbro) of the Noril’sk-1
intrusion.
To better constrain the age of igneous event and sources involved in its generation our
multi-technique study utilized ten rock samples characteristic of unmineralized and
mineralized lithologies. The rocks investigated comprise (from top to bottom) gabbro-diorite
(sample N1-1), leucogabbro (N1-3), olivine-free gabbro (N1-2 and N1-4), olivine-bearing
gabbro (N1-5), olivine gabbro (N1-6), plagiowehrlite and plagiodunite (N1-7),
taxitic-textured rocks comprising melanotroctolite, olivine gabbro with relics of ultramafic
rocks (N1-8, N1-9) and contact fine-grained gabbro (N1-10). Sulphide PGE-Cu-Ni ores occur
in ultramafic (N1-7) and taxitic-textured rocks (N1-8 and N1-9), which have thickness of
about 17 m, whereas the low-sulphide horizon of about 1 m thick occurs in the upper part of
intrusion (N1-3).
In situ U-Pb analyses of zircon from these rocks, combined with detailed study of crystal
morphology and internal structure, identify four zircon populations (Malitch et al. 2012). The
U-Pb ages of baddeleyite and the defined zircon populations cover a significant time span,
from Late Paleozoic to Early Mesozoic (e.g., 290 ± 2.8; 261.3 ± 1.6; 245.7 ± 1.1; 236.5 ±
1.8 and 226.7 ± 0.9 Ma). The established distribution of U-Pb ages implies that
crystallization of baddeleyite and zircon corresponds to several stages of protracted
evolution of ultramafic-mafic magmas at deep-seated staging chambers and/or probably
characterizes interaction of distinct magmas during formation of the Noril’sk-1 intrusion,
which served as the favorable factor for accumulation of ores of unique scales and
concentrations,
To test this hypothesis, in situ Hf-isotope data were collected on the dated spots within
single zircon grains. The analysis used a New Wave LUV213 laser-ablation microprobe
attached to a Nu plasma MC-ICP-MS at GEMOC (Griffin et al. 2002). Hf-isotope
results grouped on the basis of lithology show notable differences. Zircons from the
unmineralized ‘layered rock sequence’ (e.g., olivine-free gabbro, olivine-bearing gabbro and
olivine gabbro) are characterized by the most ‘radiogenic’ initial 176Hf/177Hf and
some of ÉHf values close to those of the Depleted Mantle. Irrespective of zircon
population most radiogenic Hf-isotope compositions are typical for olivine-free gabbro
(mean ÉHf 7.3 ± 1.1 for sample N1-4), olivine-bearing gabbro (9.2 ± 3.8, sample
N1-5) and olivine gabbro (8.3 ± 2.0, sample N1-6). In contrast, zircons from the
leucogabbro that encloses the low-sulphide horizon (N1-3), and plagiowehrlite (N1-7) and
taxitic-textured rocks (N1-8 and N1-9) with disseminated sulphide ores have less radiogenic
Hf-isotope values (e.g., mean ÉHf6.2 ± 1.4, 5.9 ± 2.3, 6.4 ± 1.2 and 4.9±1.4,
respectively). The least radiogenic values (ÉHffrom -2.9 to +2.3, mean ÉHf = 0.1 ± 1.9)
are recorded in gabbro-diorite from the upper part of intrusion. The baddeleyite
from olivine-free gabbro has the narrowest range of ÉHf values (e.g. 6.8-8.4), with
a mean of ÉHfof 7.6 ± 0.8, closely matching that of zircon (mean ÉHf= 7.3 ±
1.1).
Zircons from the leucogabbro that hosts the low-sulphide horizon (N1-3), and ultramafic
and taxitic-textured lithologies with disseminated sulphide ores (N1-7, N1-8 and N1-9) have
less radiogenic ÉHf values than those in barren lithologies. The Hf-isotope data for zircons
from ore-bearing rocks thus suggest that the Noril’sk magmas represent mixing between a
juvenile source equivalent to the Depleted Mantle and a subcontinental lithospheric source
probably at least Neoproterozoic in age. We propose that the SCLM component is especially
prominent in the mineralized portions of the intrusion. This is consistent with the
suggestion of Zhang et al (2008) that ancient cratonic lithospheric mantle may
have contributed significantly to the PGE and Ni budget of the “fertile” Siberian
Large Igneous Province. Small population of zircons from the gabbro-diorite show
the least ‘radiogenic’ Hf-isotope values, indicating the input of a distinctly older
lithospheric, possibly crustal, component, being consistent with a hybrid nature of this
lithology.
Our approach for deciphering the origin of zircon and baddeleyite from mafic and
ultramafic rocks provides a unique set of U-Pb and Hf-isotope constraints on temporal
evolution and petrologic history of the Noril’sk-1 intrusion. The study was supported by
Uralian Branch of Russian Academy of Sciences (12-U-5-1038).
Refereneces:
Campbell I.H., Czamanske G.K., Fedorenko V.A., Hill R.I., Stepanov V. (1992)
Synchronism of the Siberian traps and the Permian-Triassic boundary. Science 255,
1760-1763.
Griffin W.L., Wang X., Jackson S.E., Pearson N.J., O’Reilly S.Y., Xu X., Zhou X. (2002)
Zircon chemistry and magma genesis, SE China: in-situ analysis of Hf isotopes, Pingtan and
Tonglu igneous complexes. Lithos 61, 237-269.
Kamo S.L., Czamanske G.K., Krogh T.E. (1996) A minimum U-Pb age for Siberian
flood-basalt volcanism. Geochim. Cosmochim. Acta 60, 3505-3511.
Malitch K.N., Badanina I.Yu., Belousova E.A., Tuganova E.V. (2012) Results of U-Pb
dating of zircon and baddeleyite from the Noril’sk-1 ultramafic-mafic intrusion (Russia).
Russian Geology and Geophysics 53(2), 123-130.
Zhang M., O’Reilly S.Y., Wang K-L., Hronsky J., Griffin W.L. (2008) Flood basalts and
metallogeny: The lithospheric connection. Earth-Science Reviews 86, 145-174. |
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