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EVOLUTION
OF PERMIAN-TRIASSIC MAGMATISM IN WEST ALTAI-SAYAN FOLD REGION AS A
RESULT OF MANTLE PLUME – HETEROGENIOUS LITHOSPHERE INTERACTION
Vladimirov A.G., Kruk N.N.
Institute of Geology and
Mineralogy SB RAS, Novosibirsk, Russia, vladimir@uiggm.nsc.ru
Most famous
manifestations of magmatism concerned with mantle plumes activity are
the large igneous provinces (LIP) on Precambrian cratons including
wide trapp fields, layering basic-ultrabasic massifs, dike belts of
K-basites, carbonatite intrusions etc. However as now became
more obvious the LIP formed within fold belts have peculiar features.
The main of them is anomalously wide granitoid appearance that dozens
of times exceed a basic-ultrabasic rocks by volume. Their composition
reflects, on the one hand, the peaculiaritis of the coexisting mantle
melts and, on the other hand, – lithospheric structure and
composition.
Western part
of Altai-Sayan Fold Region (ASFR) is very indicative of this.
Collision of Siberian and Kazakhstan continents together with
activity of Tarim and Siberian plumes (Dobretsov et. al., 2005;
Borisenko et. al., 2006) resulted in Hot Shear System (HSS) formation
here in Late Paleozoic – Early Mesozoic (Vladimirov et. al.,
2008).
The described
region has heterogeneous basement, in which fragments of
paleoseamounts, pre–, inter– and back–arc basins,
island arcs, turbidite basins, fillded in thick stratas of Si
oversaturated sedimentary rooks.
Permian-Triassic
mantle magmatism is presented by three geochemical types. First one,
prevalent by volume involves trapps of Kuznetsk basin and basic dike
belts of Terekhtinsky complex. Rocks of this type are mainly
tholeitic seldom subalkaline ones that moderately enriched in TiO2
(1.5-2.3 wt.%), P2O5 (0,3-0,8 wt. %) and REE (total 150-300 ppm) and
similar to initial basalts of Siberian trapp province (Kruk et. al.,
1999). Age of this basites synchronous to main volume Siberian trapps
– 253-249 Ma (Fedoseev et al., 2001; Dobretsov et. al., 2005).
Isotopic composition (E(Nd)t=
+2… +4) shows on their forming from lithospheric mantle. The
second type presented by monzogabbro, monzodiorites and syenites in
gabbro-granite associations composing single massifs in large faults
zones. These rock are subalkaline, enriched in K (up to 6 wt.%), Zr
(330 ppm), anomalous Sr (up to 9000 ppm) and Ba (up to 3000 ppm).
Forming of these rocks (251-248 Ma (Vladimirov et. al., 1997;
Shokalsky et. al., 2000) are subsynchronous with trapps. Third type
is alkaline basalts and lamprophyres of Chuisky complex compose dike
belts system in Southern part of Altai Mountains (Obolenskaya, 1971
etc.). These rocks strongly enriched in K2O (up to 9 wt.%), Rb (up to
300 ppm), Ba (up to 3000 ppm), REE (up to 400 ppm), Zr (up to 850
ppm), Th and U (up to 57 and 9 ppm respectively). They intruded in
range of 248-238 Ma. Isotopic composition (E(Nd)t=
-2… -4) shows on their forming from mantle source of plume
nature.
Permian-Triassic
granitoid magmatism in Western part of ASFR characterized of “motley”
geochemical types and long period of forming. Earliest and least
common are the granitoids of aforenamed gabbro-granite associations
with age 251-248 Ma. These rocks are subalkaline and succeed
rare-element composition from earlier basites, their geochemical and
isotopic composition shows mixed mantle-crust genesis. The main
volume of Permian-Triassic granitoids presented by
granodiorite-granite and granite-leucogranite associations with age
250-240 Ma and has not a basic “predecessors”. These
granitoids has clear correlation of its composition with composition
of holding terrains. In zones with mafic substratum (seamounts and
island arcs) granitoids has calc-alkaline and subalkaline composition
whereas within turbidite paleobasins granitoids has plumasite
composition with geochemical features of A-granites. Sm-Nd
composition of granitoids also analogous to rocks of holding terrains
(Kruk et. al., 2001; Plotnikov et. al., 2002) that indicate the
“autonomous” crust genesis of granitoids. Latest
granitoids have rare metal composition and make some large mainly
leucogranitic massifs in South of Altai Mountains. Formation of these
intrusions take a period from 240 to 215 Ma (Vladimirov et. al.,
1997; Annikova et.al, 2006). Granitoids enriched in lithophilous
elements and in some cases accompanied by Mo-W mineralization.
Petrological study of these granitoids shows that their generation
was caused by long-time differentiation of crust granite magmas with
recurring interaction with mantle alkali-basaltic melts.
Thus the
evolution of Permian-Triassic magmatism in Western part of ASFR
reflect the interaction of Siberian plume with heterogeneous
lithosphere. Character of the evolution depended on the one hand on
variety of mantle magmas and on the other hand on consistent change
of granite forming mechanisms – from mixing mantle and crustal
magmas on earliest stage through sizeable anatexis, forming and
intrusion weakly differentiate granitoids even to long time
differentiation of crustal magmas with thermal and fluid influence
from mantle alkaline basaltic melts.
Investigations
was carried out with Financial Support of Presidium SB RAS
(Integration project No 7.10.2 and 6.11) and Russian Fund of Basic
Researches (project N 07-05-00853, 08-05-00974).
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