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THE
VERKHISETSKY MARGINAL-CONTINENTAL TONALITE-GRANODIORITE BATHOLITH
(MIDDLE URALS): ISOTOPE-GEOCHRONOLOGICAL AND PETROLOGICAL DATA
Institute
of Geology and Geochemistry UrB RAS, Ekaterinburg, Russia,
zinkova@yandex.ru
The
Verkhisetsky batholith – the Urals largest plutonic body –
is located near Ekaterinburg within the paleocontinental
margin. The massif extends in nearly meridional direction for about
80 km and reaches 30 km in width covering an area of 1800 km2.
The batholith is hosted by Late Silurian(?) and Early Middle Devonian
(Eifelian) volcanic and comagmatic gabbro-granite series. The
tectonic setting, evolution, and composition is similar to giant
Mesozoic and Cenozoic batholiths from the western North and South
America, where granite magmatism is accompanied by small mafic
intrusions, synplutonic mafic dykes and melanocratic enclaves. Such
batholiths have pronounced mineralogical feature – magmatic
epidote – indicator of high total pressure (4-8 kbar), medium
temperature (800-6000
C)
and high contents of water (from 4 to 12%) in granitoid magma (Dawes,
Evans, 1991; Johnston, Wyllie, 1988; Naney, 1983).
The
Verkhisetsky massif consists of more than six plutons composed of
four intrusive series (from older to younger): gabbro-diorite,
tonalite-trondhjemite (low-K), tonalite-granodiorite (K-Na), and
granite.
Rocks of the
gabbro-diorite series occur in the southern, most eroded portion of
batholith and are represented by gabbro-amphibolite and
biotite-amphibole quartz diorite; the latter is predominant variety.
The diorite are migmatized and exhibit an inhomogeneous spotted
“anatectic” structure (Zinkova, Fershtater, 2000) and
filled by swarm of dykes that vary from gabbros to granodiorites by
composition. Morphology, structural-textural features and similarity
of their chemical composition to host diorites to refer them as
synplutonic dykes (Zinkova, Fershtater, 2007).
The major
portion of batholith is composed by granitoids related to the
tonalite-trondhjemite and tonalite-granodiorite series. They are
represented by tonalite, plagiogranodiorite, granodiorite and veins
of plagiogranite and granite. Rocks are cut by numerous synplutonic
dykes and contain melanocratic inclutions. Synplutonic dykes have
intrusive contacts with the host rocks, while at the same time they
are cut by granitic veinlets and disintegrated into isolated
inclusions. The composition of dykes and inclusions varies from
quartz diorites to granodiorites.
The rocks of the granitic series
cut all the previous rocks and occupy the central part of the
batholith. They are represented by two-mica adamellites and granites
and distinguished from older rocks by their massive structure and
lack of deformation.
U-Pb zircon
age of the diorites of the gabbro-dioritic series indicate that they
were intruded and crystallized in the Late Devonian (Famenian) time
(3696(2)
Ma). Low-K granodiorites have a Rb-Sr age of 3166
Ma, whereas K-Na granodiorites yield 32012
Ma. The massive granites of the central bodies give Rb-Sr age of
2765
Ma (Bea et al, 1997; Smirnov et al, 1998). The 207Pb/206Pb
dating of single zircons from different series of the Verkhisetsky
Massif using the Kober technique (Montero et al, 2000) yielded an
average value of 3184
Ma, which coincided with their Rb-Sr age. Detailed study of zircons
from young granites revealed their isotopic heterogeneity: the
207Pb/206Pb
age of cores varies from 314 to 295 Ma, whereas the age of rims
(275-280 Ma) coincides with the Rb-Sr age, indicating their relation
with older tonalite-granodiorite series of the massif.
The obtained
data of isotopic investigation allowed to reveal long and complicated
history of batholith development (D3-P1)
with two main stages. The first stage (D3-C1)
produced subduction-related low-K and K-Na tonalite-granodiorite
series. Their adakite-like chemistry – high Sr (
400 ppm) and Al (
15 w.%) content; the absence of Eu-anomaly; low Yb (
1,5 ppm), Y (
15 ppm) and HFSE (Nb, Ta) content; high Sr/Y (
40) and La/Yb (
20) ratio; higher Na (4-5,2 w.%) content permit to suggest the
subduction of hot young oceanic lithosphere, which might have been
generated by back-arc spreading during the Late Devonian.
The second
stage (C3-P1)
was responsible for the formation of collision-related massive
granite bodies in the central part of the massif.
This work was supported by the
Russian Foundation for Basic Research (project no. 08-05-00018).
References
Zinkova E.A., Fershtater G.B.
Migmatites of gabbro-diorite series of Verkhisetsky massif – a
key to understanding
the genesis of tonalite-granodiorite series. // Yearbook-1999.
Yekaterinburg: Institute of Geology and Geochemistry. 2000.
P.127-134. (In Russian)
Zinkova E.A., Fershtater G.B. Synplutonic dykes in
Verkhisetsk granitoid massif (Middle Urals). // Lithosphera. 2007. ¹
2. P.141-151. (In Russian)
Smirnov V.N., Bea F., Montero P. et
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