THE EAST SAYAN ADAKITE
GRANITOID MAGMATISM, GENETIC PECULIARITIES AND SOURCES OF SUBSTANCE
of Geochemistry SB RAS, Irkutsk, Russia,,
and moderately acid varieties of calc-alkaline volcanic and plutonic
magmatic rocks that have original geochemical characteristics are
called adakites (Defant, 1990; Martin, 2005). Genetic ideas about
this type rock formation are rather reserved. There are three most
likely mechanisms, i.e. 1) melting the subdued oceanic lithosphere
(Defant, 1990); 2) melting the metabasalts in the base of thick
continental crust (Petford, 1996); melting the rocks of mantle cline
that are metasomatized by adakite melt (Brandon, 2002).
combination of conditions that are needed for adakite formation
occurs rather rarely that results in their insignificant distribution
compared to other calc-alkaline granitoids, and origin of the various
age adakite magmatism is considered unique. The East Sayan is one of
such unique regions, where adakites occur within the Vendian
(Kuzmichev, 2004) and Early Paleozoic (Efremov, 2007) granitoids.
conceptual approaches can be used for explanation of geochemical
characteristics inheritance in time, i.e. the concept of single
geochemical reservoir that generated adakite magmas in periods of
large tectonic reconstructions; concept of adakite magmatism
association with geodynamic events of various age.
concept seems most attractive, as spatial association of various age
granitoids hardly appears accidental. And it seems quite
substantiated, taking into consideration their reference to the
Archean sulphur-gneiss stratas, geochemical analogues of adakites.
However, this concept requires the distribution of “single
source” through the whole area of northern Tuva-Mongol
microcontinent (distribution of granitoid massifs with adakite
geochemical characteristics) that contradicts to the modern
geodynamic reconstructions. Their authors consider the region as
collage of varios age terranes that are of various genetic origin
(Kuzmichev, 2004 and references herein).
of adakite magmatism association is also in bad agreement with the
present geodynamic reconstruction. If the formation of the Vendian
adakites can be associated with formation of new subduction zone that
appeared after collision of the Gargan block and Dunzhugur island arc
(Kuzmichev, 2004), the formation of the the Early Paleozoic adakites
cannot be logically elucidated. Taking into account the present day
geodynamic reconstructions, we should relate the formation of these
granitoids to the process of the Tuva-Mongol micricontinent collision
with Siberian craton. But “collisional” adakites are
unknown in the world. The only example described in literature is
associated with melting the detached slab immediately after the
collisional event (Sajona,
Theoretically, adakite formation in collisional zones is possible by
all three (see above) genetic models. However, this problem requires
special studies. The most likely are slab melting in the Early
Paleozoic zone of subduction; remobilization of sulphur-gneiss
basement during the collisional event; remobilization of lithospheric
source formed in one of the previous stages of the regional
geological development (buried slab, lithospheric mantle
metasomatized by adakite melts).
with the above material, the data on sources of substance for the
Vendian and Early Paleozoic adakites acquire the key value. We can
solve the most above problems, with the information on composition,
location and age of these sources being available. The data can be
obtained based on the present genetic models with use of isotope and
geochemical characteristics of various age adakites.
definitely suggest extractustal source of the rock substance.
Geochemical characteristics of the Early Paleozoic adakites allows to
refer them to the LSA-type (Martin, 2005), associating their
formation with melting the lithosphere mantle metasomatized by
adakite melts. The Vendian adakites more correspond to the HAS-type
by their matter composition (Martin, 2005) and could have “slab”
isotope data (Sr,Nd) give the possibility to calculate protolith age
for granitoids of both types. By the obtained calculations, they are
rather similar and range within 2500-2550 Ma that permits to relate
the formation of these granitoids with the single source of
results show that the ancient specialized source existed within the
East Sayan. Its remobilization resulted in the various age adakite
magmatism. In addition, according to the present models of high
niobium basite and adakite formation, it can be in lithospheric
mantle of the region. It excludes the possibility of adakite
formation at account of sulphur-gneiss strata remobilization, but
does not exclude the possibility that all granitoids having the
adakite specificity (including grey gneisses) formed at account of
single source of substance.
is rather difficult and requires more thorough study. It should be
considered that the conclusions about lithospheric position of source
are only based on the hypothetical models of forming this or that
type of rocks that can be altered in time. However, existence of the
source itself is quite evident. Its position is fixed by plutons of
adakites and adakite granites. Theoretically, it can be spread
through the whole northern Tuva-Mongol microcontinent.
Both the age
of the source and its distribution indicate existence of single
continental block, at least, from the boundary of Archean and
Proterozoic (2500 Ma) that contradicts to the present geodynamic
reconstructions. Thus, the obtained results indicate the necessity of
correcting the available ideas about geological structure of the
region and making the schemes of geodynamic development more exact.
Cotten J., Hall
M.L. Adakite-like lavas from Antisana Volcano (Ecuador): Evidence for
slab melt metasomatism beneath Andean Northern Volcanic Zone //
Journal of Petrology. 2002. V. 43. № 2. P.199–217
Defant,M.J.,Drummond,M.S. Derivation of some modern arc
magmas by melting of young subducted lithosphere //Nature, 1990,
(2007) Early Paleozoic adakites of East Sayan. Geochemical
peculiarities and sources of substance // Problems of endogenic
process geochemistry and environment. Materials of conference.
Irkutsk. V.2. p.91-97.
(2004) Tectonic history of Tuva-Mongolian massif: Early-, Late Baikal
and Early Caledonian stages. M.
Smithies R.H., Rapp R., Moyen J.-F., Champion D. An overview of
adakite, tonalite–trondhjemite–granodiorite (TTG), and
sanukitoid: relationships and some implications for crustal evolution
// Lithos, 2005, V.9,
Atherton M. Na-rich partial melts from newly underplated basaltic
crust: the Cordillera Blanca Batholith, Peru // J.Petrol., 1996, V.
37, № 6, P.1491-1521
Sajona F.G., Mauri R.C., Pubellier M., Leterrier J.,
Bellon H., Cotten J. Magmatic source enrichment by slab-derived melts
in young post-collision setting, central Mindango (Philippines) //
Lithos, 2000, V. 54, P.173-206.