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GRANITE
FORMATION IN NORTHERN BAIKAL-VITIM FOLD AREA
Buldygerov V.V.
Irkutsk
State University, Irkutsk, Russia, buldygerov@irk.ru
Granitoids
compose a larger part of the Baikal-Vitim Fold Area (BVFA) in the
present erosional section. The most anicient granitoids dated are of
the Early Proterozoic age. They have been revealed in significant
volumes along the peripheries of BVFA and fragmentarily in its
central part. The Baikal-Patom arc formed at that time under effect
of the ellipsoidal shape plume elongated in north-eastern direction
(Buldygerov, 2006). The bodies of granitoids have elongated shape due
to the arc structure. Granitoids of plagio-series were the first to
form (Kocherikovsky, Ugol’kansky complexes). They were preceded
by volcanites of main and acid compositions and intrusions of basic
magma. Subsequently, potassium-sodium granitoids (Primorsky, Chuisky,
Ileirsky and other complexes) became widely spread in conditions of
amphibolite facies metamorphism. Metasomatic transformation of
substrate occurred with gradual leveling the composition
(granitization) under effect of input and redistribution of
incoherent elements. Melting the substrate and magma transition to
the surface up to origin of volcanic facies started on achievement of
the composition being eutectoidal for the present P-T conditions. In
this case, they were included in volcano-plutonic belts like the
North Baikal one that became spread in southern Siberian Platform and
completed formation of its basement (Buldygerov, Sobachenko, 2005).
The next
stage of intensive granite formation within BVFA fell onto the Late
Riphean-Vendian as a result of plume new activization. It was
preceded by volcanites of completely differentiated
basalt-plagiorhyolite series and intrusions of main composition.
Subsequently, granitoids of plagio-series (Tallainsky, Gulinsky
complexes) formed. They were mostly represented in the present
erosional section by the shifted bodies that also subordinated to the
arched structures at introduction, but localized nearer to the centre
of the Baikal-Patom arc. They are characterized by widely observed
processes of hybridization with formation of middle composition
varieties. Then formation of potassium-sodium composition granitoids
followed (Lower Angara, Lesnoy, Bambukoisky complexes). It was
preceded by intense metasomatic transformation of substrate that
widely occurred even in conditions of green schist facies
metamorphism of substrate with formation of big quartz-feldspar
metasomatite fields in the Sredninsky complex. In addition, initial
formation of significantly plagioclase metasomatites was found, and
then intensive development of potassium feldspar occurred. The
melting and magmatic replacement of substrate with formation of
hardly shifted bodies took place in conditions amphibolite facies
metamorphism.
The most
intense process of granite formation occurred in the Early Paleozoic
when the granitoid bulk mass of the Angara-Vitim batholith
(Svetlinsky complex) extended in north-eastern direction formed in
the central Baikal-Patom arc. The formation of the batholith
granitoid bulk mass was preceded by the volcanic processes with
formation of the Padrinskaya volcano-plutonic association and its
analogues. It belongs to the contrast basalt-leucorhyolite formation
and is characterized by simultaneous eruptions and introductions of
subvolcanic bodies of tholeite-basalt and leucorhyolite, mostly
potassium compositions. In that period, the granitoid formation of
composition from diorites to plagiogranites and bifeldspar granites
occurred by magmatic replacement with preliminary metasomatic
transformation. In addition, diorite-granodiorite varieties more
often originated at the beginning of substrate granitization and were
then replaced by granites. Numerous not shifted relics are
established in variously granitized substrate. In some areas,
conditions for mechanical introduction of granitoids into the
enclosing rocks appeared.
A final stage
of intense granite formation is at the boundary of the Carboniferous
and Permian (Konkudera-Mamakan complex). It was preceded by formation
of the Synnyrsky complex ultra-alkaline and potassium bodies in zones
of deep faults. This stage is characterized by intense
silica-potassium metasomatosis, mostly overlapped the pre-existing
granitoids. As a result, porphyroblastic varieties became widespread.
Shifted bodies in insignificant volumes originated. The location and
shape of Paleozoic granitoid bodies also subordinated to arched
structures.
In Late Paleozoic and Mesozoic,
the processes of granite formation mainly shifted to the south of
BVFA. Only not large separated bodies of subalkaline granites
(Deminsky, Batrakansky, Oporogsky complexes) and dykes of
diorite-plagiogranite formation (Aglan-Yansky complex) are known in
the northern part. They were preceded by dykes of main and
lamprophyre compositions.
Thus, minimally 5 stages of
granite formation are distinguished in the northern BVFA. It occurred
as a result of periodically activated plume representing the flow of
mostly incoherent elements and energy. That flow had ellipsoid-like
shape with formation of the Baikal-Patom arc and initially effected
even the whole south of Siberian Platform and then gradually
concentrated in central part of this arc. Magmatic hearths of main or
alkaline compositions that intruded into the crust and (or) appeared
on the surface, originated as a rule under effect of plume, initially
as a result of mantle substrate metasomatic transformation.
Subsequently, the flow of matter and energy intruded into the crust
that resulted in metamorphism and granitization of substrate with
gradual dilution of composition to eutectoidal one for the present
P-T conditions. These processes resulted in volume increase and
formation of positive structures, respectively. In most cases,
intense granite formation was preceded by volcanic processes in
conditions of still poorly heated brittle crust. Silica and alkalis
were mainly input, with sodium and then potassium dominating among
the alkalis. Magmatic hearths of various depths could simultaneously
exist.
The above material testifies to
polychronic formation of the Angara-Vitim batholith. Each subsequent
stage of granite formation resulted in transformation of previous
granitoids that explained the presence of mostly “young”
isotope age datings. More ancient datings only preserved in the
peripheries of the area and roof pendants of batholith.
References
Buldygerov V.V. (2006) Formation and
evolution of Baikal-Patom arc // Izvestia vuzov Sibiri. Series of
Earth’s sciences.
Irkutsk. Izd-vo of Irkutsk STU. Issue 9-10. P.28-31.
Buldygerov V.V., Sobachenko V.N.
(2005) Problems of North Baikal volcano-plutonic belt geology.
Irkutsk. Irkutsk State University. 184p.
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