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COMPARATIVE
GEOLOGICAL-GEOCHEMICAL ANALYSIS OF GRANITOIDS IN PEGMATITE FIELDS OF
DIFFERENT FORMATIONS
Zagorsky V.Ye., Makagon V.M.
Institute
of Geochemistry SB RAS, Irkutsk, Russia, victzag@igc.irk.ru
In
systematics of granitic pegmatites (Zagorsky et al., 2003) three
groups of pegmatitic formations are distinguished on the basis of
initial pressure at early magmatic stage of pegmatite
crystallization: formations of low, moderate and high pressures. The
first group includes crystal-bearing (with crystals of minerals in
miarolitic
cavities) and rare-metal – rare-earth formations of
low-pressure pegmatites. The second group is represented by only
rare-metal formation of pegmatites. The third high-pressure group
joints rare-metal – muscovite and proper muscovite pegmatites
of the mica-bearing formation, and non-specialized (ceramic) and
uranium – rare-earth pegmatites of the feldspar formation.
Pegmatites of all formations associate spatially with granitic
massifs or series of other morphological bodies but their genetic
links are not obvious in many cases.
In a sequence from low-pressure
to high-pressure pegmatitic formations, follow tendencies in changing
of sizes, morphology, inner structure and composition of granitic
bodies, as well as role of different processes at formation of
granitoids, are recognized (Makrygina et al., 1990; Zagorsky et al.,
1997; 1999; Shmakin et al., 2007):
♦ decrease of granitoid
bodie sizes;
♦ changing of their
morphology: from intricately constructed polychronic plutons which
are composed by granitoids of several polyphase seria, quite often
with features of ring structure, through interformational layer-like
batholiths, sometimes with cupolas, toward net-, echelon-,
stockwork-like bodies, and staircase vein seria.
♦ decrease
of phase granitoid diversity, and increasing of vein facies
varieties role – aplites, orthotectites, granite-pegmatites;
♦ increase
of normative K-feldspar (and K) role relative to albite (and Na) in
extreme members of formation sequence, that is to say in the mostly
low- and high-pressure pegmatitic formations;
♦ increase of Ca role;
♦ decrease of alkalinity,
and increasing of alumina enrichment (with greatest alkalinity of
granitoids in pegmatite fields of rare-metal – rare-earth
formation);
♦ decrease
of F role, and increasing of CO2
role;
♦ decrease of enrichment
degree of granitoids in granitophil elements (Li, Rb, Cs, Sn, Ta, Nb,
Zr, Hf) simultaneously with increase of Ba and Sr contents;
♦ decrease of geochemical
changeability within granitoid bodies;
♦ anomalous enrichment in
REE, Nb and Zr of granitoids with increased alkalinity in fields of
rare-metal – rare-earth pegmatitic formation;
♦ availability of
granitoids with positive Eu-anomalies in fields of high-pressure
pegmatites;
♦ decrease of magmatic
differentiation role, and increase of granitization,
ultrametamorphism and anatexis processes influence;
♦ replacement of A- and
I-type granites by S-type granites;
♦ decrease of mantle
influence and role of allochthonous granitoids, and increasing of
autochthonous granites role.
In pegmatite
fields of all formations, a variability of structure-texture
peculiarities and composition increases in sequence from early to
late phases of granitoids. Time gap between pegmatites and spatially
associated granitoids is established for a number of low- and
moderate-pressure pegmatite fields.
Analysis of
linkage between pegmatites of different formations and their
geodynamic tectonic settings shows that as the intensity of granite-
and pegmatite-forming processes increases, geotectonic enviroments
may be ranged as following: a) island arks; b) continental margins
above seismofocal zones; c) collision zones, continental rifting
zones, and domains of within-plate magmatism over mantle plumes.
There are no strict links between pegmatitic formations and types of
geotectonic settings, but it is possible to speak about preferential
coincidence of pegmatites of different formations with
one
or another geodynamic enviroments. As a whole, in the sequence from
low- to high-pressure pegmatitic formations, a list of geotectonic
settings, which are capable to result corresponding pegmatites,
reduces. So, miarolitic pegmatites of the crystal-bearing formation
are known in context of all above-mentioned geodynamic regimes, while
most high-pressure uranium – rare-earth and non-specialized
(ceramic) pegmatites of the feldspar formation are typical for
collision zones and areas of high degree of metamorphism and
granitization within Precambrian shields. Large fields and belts of
rare-metal pegmatites are characteristic of collision zones and
intracontinental riftogenic trough structures, and scales of
pegmatite-producing processes correlate with scales of these
geotectonic structures. To form fields of rare-metal pegmatites, the
most important factor is not a vector of geodynamic efforts
(compression or extention), but presence of thick “mature”
crust which is cut by deeply penetrating (possibly up to mantle)
tectonic structures facilitating influence of deep sources of energy
and substance on crust chambers of granite and pegmatite formation
(Zagorsky, Makagon, 2005). Largest provinces of mica-bearing
pegmatites are situated within folded belts, on place of former
paleorifts or their framing complexes (Tkachyov, 1995). In spite of
granite- and pegmatite-producing processes have proceeded here during
postinversial stage, under conditions of compression, spatial
gravitation of pegmatite fields to negative tectonic structures
indicates that latter exerted long-time influence on
pegmatite-forming processes.
The study is supported by RFBR,
project 08-05-00471.
References
Makrygina V.A., Makagon
V.M., Zagorsky V.Ye., Shmakin B.M. Mica-bearing pegmatites. V.1.
Granitic pegmatites. Novosibirsk: Nauka, 1990. 233p. (In Russian)
Tkachyov A.V. Geotectonic nature of the Mama
muscovite-bearing region // Otechestvennaya geologiya, 1995, ¹ 7.
P.17-24. (In Russian)
Zagorsky V.Ye., Makagon V.M.
Petrology problems of pegmatitic rare metals deposits // Precious and
rare metals of Siberia and Far East: ore-forming systems of deposits
of complex and nontraditional ore types. Proceedings of Conference.
V.1. Irkutsk: Institute of Geography, SB RAS, 2005. P.59-60. (In
Russian)
Zagorsky V.Ye., Makagon V.M., Shmakin
B.M. Systhematics of granitic pegmatites // Russian geology and
geophysics, 2003, V.44, ¹ 5. P.403-416.
Zagorsky V.Ye., Makagon V.M., Shmakin B.M., Makrygina
V.A., Kuznetsova L.G. Rare-metal pegmatites. V.2. Granitic
pegmatites. Novosibirsk: Nauka, 1997. 285p. (In Russian)
Zagorsky V.Ye., Peretyazhko I.S.,
Shmakin B.M. Miarolitic pegmatites. V.3. Granitic pegmatites.
Novosibirsk: Nauka, 1999. 485p. (In Russian)
Shmakin B.M., Zagorsky V.Ye., Makagon V.M. Rare-metal – rare-earth pegmatites. Pegmatites of unusual
composition. V.4. Granitic pegmatites. Novosibirsk: Nauka, 2007.
432p. (In Russian)
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