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COMPOSITION AND GENESIS OF
GRANITOIDS FROM BATHOLITHIC BELTS OF THE VERKHOYANSK-KOLYMA
MESOZOIDES
Diamond and Precious Metal
Geology Institute SB RAS, Yakutsk, Russia,
v.a.trunilina@diamond.ysn.ru
The
Late Jurassic-Early Cretaceous granitoids of the Verkhoyansk-Kolyma
Mesozoides form plutons of the Main and Northern belts. The Main
belt extends along the zone of collision of the Verkhoyansk
continental margin and the Kolyma-Omolon microcontinent. The
Northern belt represents a westward-widening “wedge”,
tracing, in the south, the collision zone, and, in the north and
east, the regional fault zone stretching along the boundaries of the
Polousnyy forearc and the Svyatoy Nos-Oloy island arc zones
(Spector, Grinenko, 1995). The granitoid plutons are developed
within the collision zone and consist of five rock associations,
which, as evidenced by geological data, sequentially change one
another in the evolution history of the Mesozoides. The sixth
association is distinguished for only the Northern belt plutons
localized in the regional fault zone.
1)
Gabbro-tonalite-granodiorite-plagiogranite
association (161-154 Ma). Its distinctive minerals are
andesine-labradorite with bytownite cores, Cr-bearing magnesian
diopside-augite, moderately ferruginous orthopyroxene, and edenite.
They occur as relics, while the predominant minerals are
andesine-oligoclase, high-ordered orthoclase, and moderately
ferruginous amphibole and biotite, the latter being compositionally
similar to biotite from the rocks of the gabbro-granite series. The
granitoids are metaluminous, magnesian, and calcareous. The REE
contents are low, characteristic of the rocks formed above the
subduction zone. With regard to the main petrochemical coefficients
and weakly differentiated REE trends, the rocks correspond to M-type
granitoids. The most workable hypothesis for their origin is
crystallization differentiation of low-K basaltic magma.
2)
Diorite-granodiorite-granite
association (140-159 Ma). Typomorphic compositions of the early
magmatic minerals are close to those in the rocks of the previous
association. However, the rocks here display evident signs of
inequilibrium crystallization: in the diorites, pyroxene-plagioclase
and pyroxene-amphibole intergrowths are set in a fine-grained
granite matrix, while in the granodiorites and granites, the
quartz-feldspar matrix includes intergrown idiomorphic grains of
magnesian clinopyroxene, labradorite-bytownite, and edenite. The
rocks are metaluminous, calcareous, and alkali-calcareous, with a
varying Fe-content. In terms of their (Na+K)/Al (avg.
0.6)-Al/(Ca+Na+K) (avg. 0.9) (Maeda, 1990), K/Rb-Rb (212-170),
Sr-Rb/Sr (150-1.1) ratios (Roob et al., 1983), they belong to I-type
granitoids of crustal-mantle origin. The presence in them of an
inequilibrium mineral association and the identity of the
compositions of pyroxenes and early amphibole to those of the
previous association may be interpreted as the result of a limited
syntexis of the mantle melt and a granitoid melt that formed during
the ascent of the basic diapir to the lower crust levels.
3)
Granodiorite-granite association
(137-151 Ma). The rocks are characterized by a medium structural
ordering of feldspars and the presence of moderately ferruginous
dark-colored minerals such as amphobole and biotite. The latter is
similar to biotite from the rocks of the granodiorite-granite
series. The rocks are alkali-calcareous, mostly ferruginous, and
slightly supersaturated in alumina. They have an intermediate
composition between the I- and S-type granitoids (IS-type).
4)
Granite-leucogranite
association (133-148 Ma). It includes biotite and two-mica granites
and leucogranites. Feldspars are high-ordered; biotite is
ferruginous, enriched in F, similar to that from standard granites.
The rocks are peraluminous, ferruginous, alkali-calcareous to
calc-alkaline, with a distinct Eu minimum in the REE trends. In
terms of their compositional characteristics, particularly (Na+K)/Al
(0.7) – Al/(Ca+Na+K) (1.25), K/Rb – Rb (170-250) ratios
and I0
values (>0.710) they correspond to S-type crustal granites.
5)
Alkali-feldspar
granite and granosyenites
(86-114 Ma).The rock exposures are confined to the intersections of
large post-fold faults. Characteristic are mesoperthitic
K-Na-feldspars with sanidine inclusions and labradorite relics;
moderately and highly ferruginous amphibole and V-bearing aegirine
(in granosyenites); rather highly ferruginous Cl-rich biotite;
orthite, pyrope-almandine (up to 23.6% py), titanomagnetite,
chromite, and native iron. The rocks are peraluminous, ferruginous,
calc-alkaline to alkali-calcareous. Their high Sr/Rb ratios (4-44)
match those of the derivatives of the melts generated in the
continental crust and of alkali-basaltic magmas (Tischendorf,
Palchen, 1985). The apparent magma generation depth is 39-42 km
(lower crust or the crust-mantle boundary). Compositionally they
correspond to A-type granites. It is supposed that the parent melts
were derived from the lower crustal substrata under the influence of
deep heat and fluid fluxes.
Thus,
in the course of formation of the Main belt and the central and
southwestern parts of the Northern belt, the derivatives of the
mantle melts changed to crustal-mantle and then to lower and upper
crustal ones. Subsequent
to stabilization of the Mesozoides, granitoids of crustal-mantle
genesis are seen again.
6)
The
granitoid plutons of the Northern belt,
located in the regional fault zones (120-134 Ma) are characterized
by a widely ranging petrographic composition – from quartz
diorites and monzodiorites to biotite granites. Their emplacement
was predated and postdated by volcanic outflows, which suggests the
permeability of the crust was kept high throughout the Early
Cretaceous epoch. As with I-type granitoids, two inequilibrium
mineral associations are found here. They are distinguished by high
Cr and Na in clinopyroxenes, low SiO2
and high Al2O3,
Na2O,
and Cl in amphiboles, and high F and Fe contents in biotites.
Restites are composed of fine-grained jadeite aggregates. The rocks
arc peraluminous, moderately ferruginous, alkali-calcareous to
calc-alkaline. In terms of their high K contents, the distribution
pattern of trace elements, La/Yb ratios (30 and higher), the
presence of the Eu-maximum in the REE trends and low Io values
(7054-7072) the rocks belong to the
granitoids of the latitic series
of the continent extension zones (L-type). Typomorphic compositions
of the early minerals suggest they crystallized from a basic melt of
elevated alkalinity, which probably trapped, in the course of
intrusion, small xenoliths of the granulite or eclogite substratum.
However, the apparent depths of magma generation are lower crustal
(33-28 km). The granitoid parent melts were derived from the crustal
amphibolites under the influence of heat and fluid fluxes of highly
alkaline deep magmas. It is supposed that the latter partly mixed
with the produced granitoid melts.
This work was supported by
grant 06-05-96008 from the Russian Foundation for Basic Research.
References
Maeda J. 1990. Opening
of the Kuril Basin deduced from the magmatic history of Central
Hokkaido, northern Japan
// Tectonophysics, N 174, P.235-255.
Roob, M.G., Gladkov,
N.G., Pavlov, V.A., Roob, A.K. and Troneva, N.V., 1983. Alkalies and
strontium in ore-bearing (Sn, W, Ta) differentiated magmatic
associations // Dokl. AN SSSR, V 268, N 6, P.1463-1466
(in Russian).
Spector, V.B. and
Grinenko, V.S., 1995. Geological Map of Yakutia, scale 1:500 000.
Lower Yana block. St. Petersburg.
Tischendorf G.and
Palchen W., 1985. Zur klassification von Granitoides //Z. Geol.
Wiss. Berlin,
Bd.13.
Hf.5.
S.615-627.
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