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GEOCHEMICAL TRENDS OF
PHANEROZOIC GRANITE FORMATION
Grebenschikova V.I., Noskov D.À.
Institute of Geochemistry SB
RAS, Irkutsk, Russia, vgreb@igc.irk.ru
The main bulk
of granitoids in the Earth’s crust consists of large (some
hundred and thousand cubic kilometers) batholith-type plutons.
The process of batholith formation is a principal geochemical feature
of
continental crust. Granite formation causes generation of
associations of sequential phases and granitoid facies, linked
between each other by a regular evolution of chemical composition
(trend). The trend is referred to as the tendency to vary chemical
composition of rocks, as is displayed in different variation
diagrams.
Some components of the main
trend of granite formation have been identified through a comparative
analysis of chemical composition of the main phases granitoids
covering from 60 to 90% of the area of model granitoid batholiths.
They are plagiogranite, tonalite, monzonite-granodiorite,
granodiorite and granite, all of them corresponding to special models
of batholith formation.
The initial
part of the universe trend of granite formation consists of
plagiogranites, which result from melting the crust, both of oceanic
or suboceanic types. These granites belong to the tholeiitic,
essentially sodic series of rocks. They have low Al2O3
(<
15 %), high abundance of alkalis, granitophyle elements (Rb, Li, Cs,
Be, Nb, Ta, W), Cr and Ni. Their Cr/V ratio is lower than 0.1,
whereas K/Rb (700-1200) and Na/K ratios have the highest values
(5-10) as compared to the other palingenic granitoids. The REE
contents are low. The plot exhibits gentle sloping of normalized
compositions and absence or inessential presence of Eu minimum. The
primary ratio of strontium isotopes is close to 0.704. The
Enisey batholith (Western Sayan Mts.)
exemplifies this type of batholith.
The
next part of the main geochemical trend represents
tonalities-trondjemites. These granitoids are formed in the areas of
oceanic-continental crust transition (supra-subduction environment).
Tonalites and trondjemites of the main phase belong to palingenic
granitoids of calc-alkaline series with low alkalinity, which relates
them to «mantle» granitoids. They are distinguished by
increased content of Ba, Sr, B and ferrum-group elements, high Na/K
ratio (1.5-2), lower Ê,
Li, Cs, Sn, W, Mo, Be, Ta, Nb contents, increased K/Rb ratio
(350-600) and low Cr/V ratio (~0.4). The amphibolite low crust was
most likely the protolith. The primary ratio of strontium isotopes in
these rocks was close to 0.704, which indicates the influence of
mantle sources or protolith of mantle origin. The Sumsunur batholith
(Eastern Sayan Mts.) exemplifies this type of batholith.
In the areas
of ocean-continent (island arc, subduction environments) transition
the granitoid batholiths of monzo-granodiorite composition are
formed. The specific feature of such plutons is presence in the
endocontact parts of monzonitoid facies and lowered alkalinity of
granodiorites and granites of the main facies. In granodiorites the
Na/K ratio is in excess of 1. Against palingenic granitoids of normal
alkalinity there are decreased contents of K, Li, Rb, Cs, Sn, W, Mo,
Be, Ta and Nb; higher Sr, Ba, B abundances, as well as ferrum-group
elements. The K/Rb ratio decreases against granitoids of tonalite
model up to 300-400. The Cr/V ratio, on the contrary, increases (~0.8
– 0.9). The primary ratio of strontium isotopes is close to
0.704. The Solgonsky batholith (Kuznetsky Alatau) represents this
type.
The areas
with granodiorite continental crust host
calc-alkaline granitoids of normal alkalinity; they continue the main
trend into the area of silicate composition. They are distinguished
by relatively increased contents of granitephyle rare elements up to
the appearance in some massifs of rare-metal facies in the rocks of
the main facies. The granodiorites of such massifs are characterized
by the following parameters: Na/K ≥ 1; typical contents of Rb, Cs,
Sn, W, Mo, Be, Ta, Nb; relatively increased abundances of Ba, Li, Gd.
In contrast to granitoids of the previous part of the trend, there
is commonly low Eu minimum, but K/Rb and Cr/V ratios decrease
(200-300 and ~0.6, accordingly). The primary ratio of strontium
isotopes reaches 0.705. Formation of granodiorite magma is apparently
associated with regressive granitization of the lowermost part of
upper continental crust. The Kyrinsky (Eastern Trans-Baikal) and
Onon (Mongolia) batholiths represent this type.
The
granitoids that formed in «mature» (granodiorite-granite)
continental crust, have silicate (granite-leucogranite) composition
of rocks; they represent the final part of the main continental crust
trend. They have increased and high contents of granitophyle rare
elements, up to occurrence of rare-metal Li-F facies in the rocks of
the main phase and rare-metal composition of the main phase.
Granites of this type have anchieutectic compositions with a shift to
the potassium field. The following features are the case: increased
contents of K, Li, Rb, Cs, Sn, W, Mo, Cu, Be, Ta, Nb, Zr, Hf, Th and
U; low abundances of Cu, Cr, Ni, Co, Zn; low K/Rb ratio (50-200);
lowered Ba/Sr ratio (1.5-2.5); increased REE contents and profound
Eu minimum; low Cr/V(~ 0.3) ratio; higher and variable primary ratio
of strontium isotopes 0.705-0.715. Formation of similar granites may
be explained by granitization of mature continental crust and the
following fluid magmatic differentiation of the resultant granitoid
magma. They
are associated with rare metal deposits (Sn, Li, Ta, Nb, W, Mo, Be,
etc.) and colored stones. This is exemplified by the
Aduchelon-Sherlova Gora batholith (Eastern Trans-Baikal area).
Therefore,
the main phases of palingenic granitoid batholiths of East Siberia
include «main trend of the crustal granite formation»,
i.e. accumulation of a series of incompatible components of the
mantle:
silica, alkalis, Ba, Sr, REE, Hf, Ta, Nb, F and Sn, with subsequent
depletion of rocks with Ba, Sr, ferrum-group elements and enrichment
in granite – and fluidphyllic elements at final stages. In the
series: granitoids of suboceanic blocks of the crust –
granitoids of structures of transitional type – granitoids of
continental and mature continental crust, the evolutionary trends of
the latter continue and as if «complete» the trends of
indicator elements of granitoids formed in the areas of the least
mature crust. This can be easily shown in different correlation
diagrams. This pattern evidently reflects the geochemical evolution
of significant «palingenic»
granite formation through the continental crust formation processes.
It can be referred to as the «trend of increasing
continentality» of the Earth’s crust.
The work is
supported by RFBR grants ¹ 06-05-65054, ¹ 08-07-98003-ð_Siberia
and Integration project grant SB RAS ¹ 6.17.
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