Formation conditions of
ultrabasic and basic dike rocks from Yllymakh and Ryabinovy Massifs,
Rokosova E. Yu.
University, Novosibirsk, Russia
V.S. Sobolev Institute
of Geology and Mineralogy SB RAS, Novosibirsk, Russia
Yllymakh and Ryabinovy
alkaline massifs (Central Aldan) of Mesozoic age belong to an intricate
volcano-plutonic structures. These massifs consist of intrusive,
volcanic and dike rocks of potassic series. They are about 40-50 km2
in area. In these massifs, almost the entire range of rocks was
found, from the potassic alkaline ultrabasic rocks through basic and
intermediate ones to alkaline granosyenites and granites. The formation
related to the activity of
the Mesozoic rift structure of the Aldanian shield. The location of the
ultrabasic and basic rocks of Ryabinovy massif is limited, mainly,
within the alkali-picrite neck at the north-eastern side of the massif.
The ultrabasic rocks of the Yllymakh massif are not outcropped and are
located at a depth. This is confirmed by the presence of nodules and
xenoliths of the pyroxenite and wehrlite in basic rocks (Kostyuk
et al, 1990).
We have studied dikes of
the shonkinite-picrites which are located within the alkali-picrite neck
of the Ryabinovy massif and alkaline basic dike rock of the Yllymakh
massif. Shonkinite-picrites consist of (vol.%) 56-51 clinopyroxene,
25-30 Fe-Mg mica, 10-12 potassium feldspar, 4-3 apatite, 2 magnetite,
2-3 sphene and rutile. Alkaline basic dike rock consist of (vol.%) 60
clinopyroxene, 15 olivine, 10 pseudoleucite, 2 K-Na feldspar, 3
magnetite and sulfides, 10 glass.
The alkaline basic dike rock
differ in bulk chemical
composition from the shonkinite-picrites in higher contents of
(50,2 vs 43,4), Al2O3 (12,5 vs 5,5), Na2O
(2,7 vs 1,6), K2O (5,1 vs 3,6) and lower Fe2O3
(10,4 vs 15,5), MgO (7,7 vs 10,6), CaO (9,3 vs 14,2).
Clinopyroxenes of the considered rocks
are evenly colored
(yellowish-green, dark-green) and zoned. In zoned clinopyroxenes, cores
are yellowish-green and rims - dark-green. The content of the FeO and Na2O
increases from cores to rims in the
zoned clinopyroxenes. The
clinopyroxenes of the alkaline basic dike rock differ in
composition from the clinopyroxenes
of the shonkinite-picrite in higher contents of (wt%) Al2O3
(2-4,7 vs 0,65-1,3), MgO (12,9-15,8
vs 9,1-13,6) and lower Na2O (0,6-1,1 vs 1,8-5,3), FeO (
5,5-9,6 vs 7,9-14,7). The clinopyroxenes of the alkaline basic
dike rock, according to international classification, are ascribed to
augite and the clinopyroxenes of
the shonkinite-picrite are ascribed to diopside and aegirine.
Clinopyroxenes of the considered
rocks, except for Ba and Nb, are significantly enriched in trace and
rare elements relative to primitive mantle. It should be noted that the
rims of zoned phenocrysts have higher contents of trace elements than
their cores. Clinopyroxenes are more enriched in LREE and less enriched
in HREE. It is generally accepted that clinopyroxene accumulates LREE
during melting of garnet-spinel facies mantle.
studying the primary melt inclusions in the clinopyroxenes of the
shonkinite-picrite of the
RYABINOVY massif, it was found that at the crystallization stage of
clinopyroxenes the melt was heterogeneous and consisted of immiscible
silicate, silicate-carbonate, carbonate-salt, and carbonate fractions.
The carbonate-salt and carbonate melts separated from silicate magma
were enriched in Ca, alkalies, CO2, S, Cl and were an
immiscible fraction of carbonatite melt. The temperature of the
heterogenization of melts is above 1120-1170 °C. The composition of the
silicate melts in the homogenized inclusions in diopside of the
shonkinite-picrite corresponds to tephrite. The contents of Si, Al,
alkalis increases, and the contents of Mg, Fe, Ca decreases in the melts
of the homogenized inclusions from the later mineral – aegirine, i.e.
the composition of melts has evolved towards of the alkali
studying the glassy and partly crystallized melt inclusions in minerals
of the alkaline
basic dike rock of the YLLYMAKH massif, it was found that compositions
of the unheated silicate glasses from primary inclusions in clinopyroxenes
and secondary inclusions in olivine are enough highly siliceous
(56.4 - 62 wt.% SiO2) and are close to the composition of the
alkali pulaskite developed on the massif (Kostyuk
et al, 1990). The average composition of unheated primary glassy
inclusions in Ti-magnetite contains up to 53 wt. % SiO2, 23.6
wt. % Al2O3, 10 wt. % alkalis and is close to the
composition of nephelinic syenite (tinguaite), which are also developed
on the massif. The unheated glass of the primary inclusions in apatite
have highly siliceous, monzonitic composition.
The composition of secondary heated
inclusions in olivine showed only insignificant changes with respect to
unheated inclusions, in particular,
insignificant decrease in Si and alkali contents, more significant
decrease in Al, and significant increase in Mg, Fe, and Ca. The
composition of heated inclusions in olivine is close to
the composition of the
pseudoleucite phonolite. The
composition of primary and
secondary homogenized inclusions in clinopyroxenes as compared to
unheated inclusions are significantly enriched in Mg and Ca at very
insignificant decrease of Si, alkalis, Fe. The composition of primary
homogenized inclusions is close to the composition of basic
feldspathoid rocks – malignites, and
the composition of secondary
homogenized inclusions –
pseudoleucite phonolite-monzonite. The secondary melts in olivine
and primary melts in clinopyroxene contained extremely low water content
of 0,01-1,13 wt.% and were
enriched in BaO (0,8-0,1 wt.%), Cl (0,1-0,3 wt.%) and trace elements
(especially LREE and MREE), whose abundances are several times higher
than mantle values.
Fig.1 The evolution trend
of the melts involved in the formation of rocks of the Yllymakh and
Yllymakh massif. Rocks: 1 -
pulaskite, 2 - pseudoleucite phonolite, 3 - malignite, 4 - studied
alkaline basic dike rock with subtracted olivine; 5 - studied alkaline
basic dike rock. Melt inclusions in the minerals from alkaline basic
dike: 6 - unheated inclusions in olivine, 7 - heated inclusions in
olivine, 8- unheated inclusions in pyroxene, 9 - heated inclusions in
pyroxene, 10- unheated inclusions in Ti-magnetite, 11 - unheated
inclusions in apatite.
Ryabinovy massif. Rocks: 12
- minette, 13 – shonkinite-picrite and shonkinite. Melt inclusions from
shonkinite-picrite: 14 - heated and unheated inclusions in pyroxenes.
composition of heated and unheated inclusions in minerals of the studied
rocks of the
Yllymakh and Ryabinovy massifs
define a common evolution
trend of the melts (fig. 1).
It is seen that
crystallization of the melts was accompanied by the decrease in the
contents of calcium, iron, magnesium, and increase in aluminum, silica,
alkalis, which is typical of the evolution of alkaline-basaltoid melts.
The studied alkaline
basic dike rock, malignites, monzonites, pseudoleucite phonolites,
pulaskites of the Yllymakh massif and shonkinite-picrites, shonkinites,
minettes of the Ryabinovy massif are also plotted in the trend.
Such a trend may
indicate that studied rocks of the Yllymakh and Ryabinovy massifs
crystallized from parental magmas with similar alkali-basalt
composition. Undoubtedly, the
entire rock series developed at the Yllymakh massif and dikes of
shonkinite-picrites, shonkinites, minettes of the Ryabinovy massif
was derived via fractional crystallization of parental alkaline
basaltoid magmas. The shonkinite-picrites of the Ryabinovy massif
crystallized at an
earlier high-temperature stage of evolution of the
parental magma, and alkaline
basic dike rock of the Yllymakh
massif - at a later stage.
should be noted that during the crystallization of the studied rocks,
probably, the crustal material/crustal fluids was involved in the
magmatic process. This is evidenced by
the fact that spider-diagramm of the shonkinite-picrites, shonkinites,
minettes of the Ryabinovy massif shows negative Ta, Nb anomalies, and
these rocks have high La/Nb
ratio (La/Nb: 14,1 – for
shonkinite-picrites, 5,5 – for
shonkinites and 8,9 – for
the Nb/Ta–La/Nb diagram with shown fields of mantle and crustal sources
(Barth et al., 2000), glassy melts conserved in the olivine and
clinopyroxene rims are plotted in the influence zone of continental
crust (Panina et al., 2011).
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