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Mafic rocks of
West Transbaikalia:
geochemical
and
geochronological data
Antsiferova T.
Geological Institute of SB
RAS, Ulan-Ude, Russia,
antsifer@gin.bscnet.ru
At present, there are
number of different models of large granitoid batholith formation.
Mantle magmas as an additional source of heat and fluids play a key role
there. Direct evidence of mantle magma participation are combined
dykes, melanocratic inclusions in granitoids and synplutonic mafic
rocks (Litvinovsky et
al, 1993; Yarmolyuk et
al, 1997; Gordienko et
al, 2003).
The Late Paleozoic
intrusive rocks (general trend from high-K calc-alkaline to alkali and
peralkaline granitoids) totally occupy no less than 80 % of the
territory of Transbaikalia (200,000 km2)
(Litvinovsky et
al, 1993; Kovalenko et al., 2004). Five intrusive formations were
emplaced throughout the region. The largest from them is Barguzin
complex of high-K calc-alkaline granites making up the
Angara-Vitim batholiths (AVB)
150,000 km2 in area.
U-Pb and Rb-Sr isotopic
zircon dating shows that the Late Paleozoic magmatic cycle lasted almost
continuously from ~330 to ~275 Ma and included two stages of granitoid
magmatism: early – 330-310 Ma (Barguzin complex is
typomorphic for AVB) and
at late stage formation of geochemically distinct intrusive suites
occurred with total or partial overlap in time - 310-275
Ma (monzonite, quartz monzonite,
quartz syenite)
(Tsygankov et
al, 2010, Litvinovsky et.al., 2011).
Mafic rocks are found in
all intrusive complexes except for the earliest Barguzin suite.
Gabbros, according to some
estimates, come
up to 10%, but their temporal relationship with
associating granitoids in
most cases is
unknown. We assume that some part of gabbros could relate to
granitoid magmatism. The evidence
of such relationship is geochronological data and geochemical similarities of
gabbros with synplutonic mafic rocks
of dated granitoid plutons.
Our studies were
concentrated in the
central part of the
West Transbaikalia,
in the lower course of Kurba River and neighboring areas of the
Ulan-Burgasy Range, at 150-160 km
north-east of Ulan-Ude. We
have studied three mafic bodies confined to tree different granitoid
suites that were
formed in
the interval 305-278 Ma (Tsygankov
et al, 2010): Orefiev pluton
(Chivyrkui intrusive complex), Khasurta pluton
(Lower Selenga suite)
and Unegetey
pluton (Zaza suite).
In the TAS
(total alkalis vs. SiO2)
classification of Middlemost
(1997) almost
all the samples plot in the monzogabbro, monzodiorite,
alkaline and normal gabbro fields. The Magnesium Mg#=100x (MgO / (MgO +
FeO*) in rocks vary from 50 to 39%, TiO2 (0.8 – 1.5 wt. %, in
mafic rocks of Khasurta pluton
till 2.4 %). All gabbros are highly-aluminous (15 – 20 wt.
%
Al2O3).
Mafic rocks from
Orefiev and Khasurta pluton are potassium enrichment (0.5 – 2.7 and more
wt % К2О)
and can be referred to as high-K calc-alkaline to shoshonitic. Unegetey
rocks are differed by minimal potassium alkalinity that does not exceed
0.5 wt. % К2О.
High-K calc-alkaline mafic rocks have higher contents of Ba and Sr
(about 1000 ppm, in average), Rb (to 100 ppm) at comparatively not high
Zr (no more than 200 ppm).
The content of Ba
and Sr
in the rocks of
Unegetey
pluton is on average 600-800 ppm
respectively, and Rb
and Zr
- 13-32 ppm respectively.
The REE
patterns of the mafic
rocks from all three plutons are quite uniform. They are LREE-enriched
(La/Yb)N
=8–23, and
the Eu anomaly is generally weak to absent
(Eu/Eu* 0.67 – 1.11). These
characteristics indicate that
these rocks
are similar to continental basalts commonly found in interiors of
cratons. Overall, all the mafic
rocks under consideration are distinguished from OIB by clear Nb (less
Ti, Zr, Hf)
troughs, and Pb, Sr peaks.
The negative Nb anomaly reflects
the geochemical
characteristics of mantle
source, and
Pb,
Sr peaks
- the contribution of crustal
contamination. More
latest mafic rocks of West Transbaikalia
have same geochemical features.
In a few samples
δ18O
was measured in plagioclase from Orefiev gabbro which showed mantle-like
values (8.1–8.2‰). However, all mafic rocks are characterized by
negative εNd(Т)
values ranging from -7 to -4.
The data
set also indicates that mafic
rocks and associated granitoids show roughly similar isotopic
compositions (Litvinovsky et
al, 2011).
It
should be noted, that all Late Paleozoic mafic rocks of
Transbaikalia
have negative εNd(Т)
values. It is likely that lithospheric mantle was enriched in the
ancient acid crust material subducted during accretion and collision of
the two continental plates. This inference is supported by mildly
enhanced I(Sr) ratios, 0,7050 –
0,7061.
U-Pb isotopic zircon (from
Ol-gabbro of Orefiev pluton) dating shows age of gabbros 290 ± 5 Ma (MSWD=0.44).
In view of the fact that gabbros of Orefiev and Khasurta plutons have
geochemical similarities,
this age (290 Ma) can be extrapolated to
the latter.
It agrees with geological and geochronological data
according to which Q-syenite-monzonite of Lower Selenga suite (U-Pb
and Rb-Sr age 283.7
± 5.3) intrude mafic
rocks of Khasurta pluton.
Thus,
the geochemical similarity
of mafic
rocks indicates that the formation of mafic
magmas was related to the
same region of the
mantle magma
generation. U-Pb isotopic
data confirm the synchronism of high-alkaline mafic
and granitoid magmatism of
the West Transbaikalia in the
Late Paleozoic. According
to isotopic age of
Orefiev gabbros (290
Ma), these mafic rocks relate to high-K
quartz-syenite and monzonite. The
problem of mafic
rocks associated
with granites of Barguzin
suite still open.
References:
Gordienko I.V., Kiselev A.I.,
Lashkevich V.V. Delamination of lithosphere and related magmatism in
fold areas (on example of southern Siberian platform fold rim) //
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OGGGGN RAS. 1999-2001, edited by D.V.Rundquist. M:. 2003. OGGGGN.
P. 185-199.
Kovalenko V.I., Yarmolyuk
V.V., Kovach V.P., Kotov A.B., Kozlovsky A.M., Salnikova E.B., Larin
A.M. Isotope provinces, mechanisms of generation and sources of the
continental crust in the Central Asian Mobile Belt: geological and
isotopic evidence // Journal of Asian Earth Sciences. 2004. Vol.
23. Р.
605-627.
Litvinovsky B.A., Tsygankov
A.A., Jahn B.M., Be’eri-ShlevinY. Origin and evolution of overlapping
calc-alkaline and alksline magmas: The Late Paleozoic post-collisionaligneos
province Transbaikalia //Lithos. 2011. Vol. 125. P. 845-874.
Litvinovsky B.A.,
Zanvilevich A.N., Alakshin A.M. et al. The Angara-Vitim batholith is the
largest granitoid pluton. Novosibirsk. UIGGM SB RAS. 1993. P.
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Tsygankov A.A., Litvinivsky
B.A., Jahn B.M., Reichow M., Liu D.Y., Larionov A.N., Presnyakov S.L.,
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