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Deep sources and isotope
age of alkali-carbonatite and Pt-bearing ultramafic-mafic associations
of the Urals
Rusin A.I.,*
Krasnobaev A.A.,* Baneva N.N.,* Medvedeva E.V.,** Valizer P.M.**
*
Institute of Geology and Geochemistry UB RAS,
Ekaterinburg, Russia
** Ilmeny
State Reserve UB RAS, Miass, Russia
rusin@igg.uran.ru
The concentric-zonal
ultramafic-mafic massifs of foldbelts and alkali-carbonatite complexes
of platforms are similar in structure expressed in dunite or olivinite
core and rimming its wehrlite, clinopyroxenite and other zones distinct
in the level of alkalinity and basicity of the rock associations
(Efimov, 2010; Dubrovsky, 2011). The differences in chemical composition
of the concentric-zonal massifs are considered to be related to the
various geodynamic settings. The Ural Pt-bearing
belt is an etalon of
dunite-clinopyroxenite-gabbro massifs throughout the foldbelts.
Its association with complexes of the Tagil
paleoisland arc is considered to be the evidence of its formation at
supra-subduction setting. The absolutely another
geological-tectonic setting of alkali-carbonatite massifs and
peculiarities of material composition indicate their relation to the
riftogenic activization of stable areas of the continental crust.
For the long-term period,
the Ilmeno-Vishnevogorsk alkali province was regarded as the genotype of
alkali-carbonatite complexes of orogenic belts. The absence of relation
to the deep mantle magmatism was considered as distinctive peculiarity
of these complexes. We established that numerous mafic-ultramafic rocks
of this province are characterized by anomalous high concentrations of
rare and rare earth elements and include the fragments of
alkali-ultramafic intrusion of the platform type disintegrated in the
zone of the regional postcollision shift (Rusin et al., 2006). The
ultramafic-mafic, alkali-ultramafic (metafoidolite), and
miaskite-carbonatite typical rock associations may be distinguished in
the structure of this intrusion. The geological interrelations of the
first two associations observing in the separate blocks (Nyashevo and
others) allow the supposition on the concentric-zonal structure of the
intrusion. However, the results of the thorough study of metafoidolites
are quite significant for understanding of its genesis (Rusin et al.,
2012). The finding of the relict Cros+Px+Ky assemblage in these rocks
indicates the grospidite level of origination of the primary melts. The
Nd and Sr isotopic data of metafoidolites and related carbonatites
testify to their generation from the EM1 and EM2 types. The U-Pb age of
zircons is evidenced by the Riphean-Vendian (662 and 543 Ma) age of
alkali-ultramafic association. The Lu-Hf system of zircons from
miaskite-carbonatite association showed similarity of the primary Hf
isotopic ratios in miaskites, nepheline pegmatites and carbonatites of
the Vishnevogorsk massif (εHf
= 3.5–5.7) corresponding to moderately depleted mantle (Nedosekova et
al., 2010). The age of generation of the primary melts of
alkali-carbonatite association is 790–880 Ma in accordance to the model
Lu-Hf age of the early zircons. Together with age of zircons from
metafoidolites, these data allow us to suggest that formation of
alkali-ultramafic intrusion of the Ilmenogorsk zone occurred before the
opening of the Ural paleoocean and was related to the Riphean-Vendian
riftogenic activization caused by the deep-seated mantle plumes.
The material evidences of
the Precambrian plume processes in the Urals are mostly expressed in the
paleocontinental sector. There are dike swarms tracing along the entire
Ural foldbelt, layered intrusions (Kusa-Kopan and Sarany belts), and
manifestations of alkali-ultramafic magmatism (Suroyam massif, Chetlass
carbonatite complex, etc.). At the same time, no significant eruptions
of plateau basalts or their denudation products in the Riphean
stratotypical sections are known in the Urals and this gives a ground to
state that the products of deep partial melting accumulated in the
basement of the Late Precambrian crust as a result of the dry plume
underplating. This is confirmed by the transitional zone at the
crust/mantle boundary registered by the URSEIS-95 seismic profile. The
huge volumes of Late Precambrian gabbro in massifs of Pt-bearing
belt, the universal
development of high-temperature plastic (brittle-plastic) deformations
in them related, as suggested, to the riftogenic lithospheric extension,
and also petrochemical and metallogenic (Pt, Cr, and Ti-magnetite ores)
similarity of the main rock associations with platform intrusions of the
central type served as a base for the conclusions on the plume
(underplating) nature of Pt-bearing belts (Rusin et al., 2009). The
absence of harzburgites in the Pt-bearing
belt (the obligatory
element of the oceanic crust), isotopic-geochronological data, and
similarity of REE distribution trends with rock associations
accompanying the dunite core of the Kondyor massif (Efimov, 2010, Figs.
4 and 5) exclude the formation of the Pt-bearing belt massifs at
supra-subduction setting.
The
mineralogical-geochemical and isotopic-geochronological studies of
zircons from all rock associations of the Pt-bearing
belt allow us to
establish the extreme duration of mantle zircon formation. Three age
groups (2852–2656, 1608–564, and 495–463 Ma) were determined in zircons
from dunites, clinopyroxenites, and gabbro. They are characterized by
specific isotopic-geochemical peculiarities indicating the probable
endogenic source (Anikina et al., 2012). The Archean and Early
Proterozoic zircons are characterized by the wide variations in U
(34–1891 ppm) and Th (5–560 ppm) contents and Th/U ratio (0.2–1.47) and
high REE concentrations (377–1723 ppm). Their Hf isotopic composition
indicates mostly juvenile origin from the sources significantly distinct
in Lu/Hf ratio. The Late Proterozoic igneous zircons in anorthite gabbro
have moderate U (68–306 ppm) and Th (46–638 ppm) contents and Th/U ratio
(0.4–1.6) and relatively low REE contents (220–600 ppm). Their
composition reflects the dry crystallization from the mafic melt and the
primary Hf isotopic ratios are similar to the chondrite values (εHf
= –3.6 to +3.0). The Late Proterozoic zircon from dunites is a result of
transformation of older crystals from the depleted mantle (εHf
= 14.6). Zircon is enriched in LREE that is similar to the Archean
zircon from olivine-anorthite gabbro. The Early Paleozoic zircons from
dunites (495–463 Ma), olivine-anorthite gabbro (450 Ma), and labradorite
gabbro (428 Ma) are identical in morphology and composition. They have
similar primary Hf isotopic ratio and
εHf
from +9 up to +15. This assumes the common source (TDMHf =
0.5–0.6 Ga) with higher Lu/Hf ratio in comparison with the source of
ancient zircon. Probably, these zircons record the exhumation of the
mantle block into the continental crust.
Presently, the conjugated
study of U-Pb and Lu-Hf isotopic systems of zircons is broadly used for
the solution of problems of the origination and evolution of the deep
mantle rocks. The finding of the Late Precambrian zircons with similar
primary Hf isotopic ratio in alkali-ultramafic association of the
Ilmenogorsk zone and concentric-zonal massifs of the Pt-bearing
belt, together with petro-
and geochemical characteristic of the rock associations, allows
conclusion on formation of these complexes during the riftogenic stage
of the development of the Urals foldbelt. The endogenic sources of these
complexes could be a result of the influence of mantle plumes. The
obviously expressed variations in Hf isotopic ratios in zircons
different in age could be related not only to the change of endogenic
sources but to the total effect of metasomatic, magmatic, and
metamorphic processes in the mantle reservoirs. The principal
possibility of the enrichment of zircons in radiogenic and
non-radiogenic Hf limits the interpretation of the nature of endogenic
source and requires the obligatory account of data on Sm-Nd ages of the
rocks (Lokhov et al., 2009). First of all, this concerns the Late
Paleozoic and Mesozoic ages of zircons reflecting the time of collision
and postcollision events in the Urals. Their Hf isotopic ratios could be
inherited and related to the early metamorphic evolution of the
transformation of the primary substrate.
This study
was supported by the Interdisciplinary project of the Urals Branch of
RAS no. 12-С-5-1011
and the project no. 12-И
5-2035 jointly conducted with Siberian and Far East Branches of RAS.
References
Anikina, E.V., Krasnobaev,
A.A., Rusin, A.I. et al. Isotopic-geochemical
characteristics of zircons from dunite, pyroxenite, and gabbro of the
Urals Pt-bearing belt // Dokl. Earth Sci. 2012. Vol. 443. Pt 1.
P. 513-516.
Dubrovsky M.I. Systematics
and petrogenesis of the rocks from concentric-zonal ultramafic massifs
// Lithosphere. 2010. N 1. P. 34-45.
Efimov A.A. The results of
the centenary study of the Ural Pt-bearing Belt // Lithosphere. 2010. N
5. P. 134-153.
Lokhov K.I., Kapitonov
I.N., Bogomolov E.S. et al. Geochemistry of Hf isotopes from zircons and
Nd isotopes from the rocks as a tool for correct interpretation of U-Pb
geochronogical information and evaluation of the ore potential of mafic
intrusions // In: Archean Granite-Greenstone Systems and Their Younger
Analogues. Petrozavodsk. 2009. P. 109-114.
Nedosekova I.L., Belousova
E.A., Sharygin V.V. Lu-Hf isotope composition of zircons from the
Ilmeny-Vishnevogorsk complex (results of LA-ICP-MS study) //
Yearbook-2009, IGG UBr RAS. Yekaterinburg, 2010. P. 283-288.
Rusin A.I., Valizer P.M.,
Krasnobaev A.A. et al. Origin of the
garnet-anorthite-clinopyroxene-amphibole rocks from the Ilmenogorsk
Complex (Southern Urals) // Lithosphere. 2011. N 1. P. 91-109.
Rusin A.I., Krasnobaev
A.A., Rusin I.A. et al. Alkali-ultramafic association of the
Ilmeny-Vishnevy Mountains // In: Geochemistry, Petrology, Mineralogy,
and Genesis of Alkaline Rocks. Miass: UB RAS, 2006. P. 222-227.
Rusin A.I., Rusin I.A.,
Krasnobaev A.A. New interpretation of nature of the Ural Pt-bearing belt
// In: Mafic-Ultramafic Complexes of Fold Regions and Related Deposits.
Vol. 2. Yekaterinburg, 2009. P. 154-157. |