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BASITE
PRECURSORS OF LATE PALEOZOIC GRANITOIDS IN WEST TRANSBAIKALIA:
MINERALOGICAL
AND GEOCHEMICAL CHARACTERISTICS, GEOLOGICAL EFFECTS
Antsiferova T.N.,
Tsygankov A.A.,
Vrublevskaya T.T.
Geological
Institute SB RAS, Ulan-Ude, Russia, antsifer@gin.bscnet.ru
Scales of
granitoid magmatism in West Transbaikalia rather do not have
analogues in the world. By present, the reliabile
isotope-geochronological data that the overwhelming part of
granitoids in the region, including the Angara-Vitim batholith (AVB)
formed in the Late Paleozoic have been obtained. The world practice
shows that large granitoid magmatism cannot occur without
participation of mantle basite magmas. In West Transbaikalia, the
occurrences of synplutonic basites were mainly revealed in
association with the later Late Paleozoic and Mesozoic granitoids
related to AVB. Evidence of mantle magma participation (combined
dykes, synplutonic basite intrusions, melanocratic inclusions in
granitoids) in formation of Barguzin granites being dominant in the
region and typomorphic for AVB has not obtained, and the known
examples from literature (Nesterikha, Romanovsky quartz-monzonite
plutons Litvinovsky et al., 1992) seem obscure in view of the present
geochronological data, as they likely fix the later magmatic events.
At the same
time, not large (from several hundreds m2
to first tens km2),
but rather numerous bodies (relics) of pre-granite gabbroids, at
least part of them being probably quite associated with granitoid
magmatism, occur in the areas of the Late Paleozoic granitoid
development.
We have
studied several such bodies within the Late Paleozoic autochthonous
gneiss-granites of the Barguzin complex (Orefjevsky massif), Zaza
granites (Unegetei relics), and gabbroids within the Khasurta
monzonite-quartz syenite massif in the basin of the Lower Kurba
river. The time interval of host granitoid formation equals 325-284
Ma (Tsygankov et al., 2007).
Mineralogical
and geochemical typization of the above basite magmatism occurrences
was the aim of the carried out studies. In addition, we considered
the following hypotheses: a) basites that are genetically or
paragenetically associated with the Late Paleozoic granitoids should
have the interplate geochemical specific features; b) mineralogical
and geochemical similarity of rocks from separated outcrops, and
mainly, their similarity to synplutonic basites from contemporaneous
Romanovsky (278 Ma, Tsygankov et al. 2007) and Nesterikha (298 Ma,
Kozubova et al., 1980) massifs will testify to their genetic
affinity, indirectly indicating an association with granitoid
magmatism as well. In contrast, differences will indicate the lack of
such association.
The
Orefjevsky and Unegetei massifs are composed by middle-grained
amphibolized gabbros, where unaltered varieties, including olivine
gabbros and gabbro-pyroxenites, sometimes occur. Gabbroids are
replaced by gabbro-diorites, monzodiorites and monzonites nearer the
contact with granitoids. The Khasurta gabbroids are quite saturated
by veins and dykes of granites and quartz syenites. As a result, the
initial unhybridized varieties only fragmentarily preserved.
The studied
gabbroids are more often composed by magnesial hornfels and
plagioclase, with the composition ranging from labradorite (65% an)
to andesine (42% an). Monocline pyroxene varies from diopside to
augite by composition. Hypersthene is present in gabbroids of the
Unegetei relics. Primary magmatic biotite belongs to the
phlogopite-annite isomorphic suite. Olivine was only revealed in the
Orefjevsky massif, where it contains fayalite component from 32 to
52%. In hybridized varieties, potassium feldspar that contains albite
component up to 10% originates. The accessory minerals are magnetite,
ilmenite, apatite and sphene.
The chemical
composition of the studied gabbroids varies widely enough. In
unaltered gabbroids, silica the content of silica varies from 41 to
50-52 wt.%, and that of magnesium (Mg#) from 58 to 39%. In Harker’s
plots, the trend of the Unegetei relic gabbroids is sharply differed
from all the rest, though middle composition of rocks is rather
similar. The analogous regularities are specific of microelement
composition as well. This data by itself hardly contributes to
solution of the problem. Therefore, we used petrogeochemical data on
synplutonic basites studied by B.A.Litvinovsky with co-authors
(Litvinovsky et al., 1992) within the Romanovsky (Vitim plateau) and
Nesterikha (Barguzin ridge) quartz-monzonite massifs. Macro- and
microelement compositions of these basites are practically similar to
those of the Kurba river basin gabbroids. The composition of rock
forming minerals does not discover significant deviations from the
described above either. The studied gabbroids, including synplutonic
basites of the Romanovsky and Nesterikha massifs are characterized by
moderate contents of titanium (averagely, 1.2 wt.% Ti) and silica
(17.3 wt.%, in average) and higher contents of alkalis (1.7-5 wt.%
Na2O
and 0.4-4 wt.% K2O
without hybridized varieties). In addition, the Unegetei gabbroids
are differed by minimal potassium alkalinity that does not exceed 0.5
wt.%. The higher contents of Ba and Sr (averagely, ~1000 ppm), Rb (to
100 ppm at comparatively not high Zr (no more than 200 ppm) are
specific.
REE
distribution in the studied gabbroids is characterized by sharp
enrichment in LREE that makes them similar to basites of interpolate
type. The presence of Nb negative anomaly and Pb sharp positive
anomaly is mostly significant. However, these geochemical
peculiarities can be associated with intense crust contamination of
mantle magmas.
It should be
noted that the above mentioned isotope (U-Pb) age of the Romanovsky
(279 Ma) and Nesterikha massif (Tsygankov et al., 2007; Kozubova et
al., 1980) monzonitoids corresponds to the time interval of the
Barguzin granitoid formation in the Lower Kurba river and adjacent
areas of the Ulan Burgasy ridge (325-284 Ma) (Tsygankov et al.,
1974). Such coincidence is considered not accidental and permits to
suggest that numerous gabbroid relics of various size belong to one
and the same stage of mantle magmatism. In one case (Kurba river
basin), introduction of basite magmas just likely preceded the
formation of granitoids, and it was synchronous with it (synplutonic
dykes) – in other cases (Romanovsky, Nesterikha massifs).
Thus, the
obtained geological and mineralogical-geochemical data allow to
conclude that two types are distinguished among the studied gabbroid
relics of the Kurba river basin and adjacent areas of the Ulan
Burgasy ridge. One of them (with higher potassium alkalinity) is
similar by composition to synplutonic basites that occur within
granitoids of the Angara-Vitim batholith. These gabbroids, likely
excluding the Unegetei type, fix the single stage of mantle magmatism
preceding the development of granitoid intrusions in the studied
region.
References
Kozubova L.A., Mirkina S.L., Rublev A.G. et al. (1980)
Radiological age and peculiarities of the Chivyrkui pluton
composition (Baikal mountain area) // Doklady of AN. V.251. N4.
P.948-951.
Litvinovsky B.A., Zanvilevich A.N.,
Alakshin A.M. et al. (1992) The Angara-Vitim batholith is the largest
granitoid pluton. Novosibirsk. Izd-vo of UIGGM SB RAS. 141p.
Tsygankov A.A., Matukov D.I., Berezhnaya N.G. et al.
(2007) Sources of magmas and stages of the Late Paleozoic granitoid
development in West Transbaikalia // Geology and Geophysics. V.48.
N1. P.156-180.
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