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U-Pb SHRIMP Geochronology
and Geochemical Peculiarities of Zircons from Igneous Rocks of Lomonosov
Submarine Massif: an Evidence for Proterozoic Basement Presence in the
Northern Margin of the Black Sea Depression?
Shniukova K.
Institute of Geochemistry,
Mineralogy and Ore Formation NAS of Ukraine, Kiev, Ukraine
shniukova@igmof.gov.ua
Lomonosov submarine massif
(LSM) located in the Black Sea 24 miles to the south-west from
Sevastopol is studying since its discovery in 1989. LSM is composed of
volcanic and plutonic rocks which outcrops extend for 40 km along the
continental slope at the depth of 500-1800 m. As a result of study LSM
has been considered to be a fragment of paleoisland arc stretching from
south-west to north-east, that is subparallel to the Mountainous Crimea
(Shnyukov et al, 1997). Three island-arc series have been divided among
the LSM volcanic rocks, namely high-magnesian (HM), moderate-magnesian
(MM) and shoshonite. First two series are spread out in the central and
eastern parts of LSM and represented by full number of normal series
rocks: basalt – andesibasalt – andesite – dacite – plagiorhyodacite –
plagiorhyolite. The third subalkaline series occupies the western part
of LSM and is out of this review. Plutonic rocks embrace full number of
rocks, too: gabbro – diorite – quartz diorite – tonalite – plagiogranite
(trondhjemite) and are comagmatic to the MM volcanic series.
HM and MM volcanites are
petrographically identical; they have both porphyric and aphyric
varieties. Basic and partly intermediate rocks are spilitized.
Phenocrysts are represented by plagioclase, augite (endiopside), rarely
orthopyroxene. An absence of olivine and magnetite and the presence of
quartz in the groundmass of even basics are characteristic. Intermediate
and acidic plutonic rocks have the hypabyssal signs and contain
hornblende and high-An plagioclase. Petrochemically both normal volcanic
series and plutonites are calc-alkaline; tholeiites are almost absent.
All acidic rocks follow trondhjemitic trend. HM and MM series are
clearly separated in all petrochemical and geochemichal diagrams; their
common features are low TiO2, high FeO*, low K2O
contents. HM series whose basic members contain up to 19% MgO while
acidic ones up to 4% has been identified with low-Ca boninite series (CaO/Al2O3<0,75),
although TiO2 content in it corresponds rather to komatiites
than boninites (<0,8%) and there are many basic rocks with too low SiO2
content (<53%). Their REE patterns being slightly LREE-enriched
unlike more flat MM
basics ones aren’t typically boninitic U-shaped and are characterized by
negative Eu anomaly due to plagioclase fractionation.
Both volcanic and plutonic
rocks of the LSM were dated by K-Ar method in the nineties (37
determinations) (Shnyukov et al, 1997; Shcherbakov, Shnyukova, 2000).
Datings refer mainly to Cretaceous and Palaeogene (from 147 to 41 Ma,
isolated datings 170 and 26-31 Ma). On their basis two stages of
magmatic activity in LSM were established , namely Early Cretaceous
(with maximum of about 125 Ma) and Late Cretaceous – Early Paleogene
(with maximum of about 65 Ma). A petrochemical zonality together with
rejuvenation was outlined in LSM approximately across arc’s strike from
south-east towards north-west that is from supposed arc front (boninites)
to its back (shoshonites).
In order to obtain more
reliable age determination of the LSM magmatism, zircons have been
picked out in the various normal series rocks from the central part of
LSM. Zircons are rare in the MM series rocks being singular in the HM
series ones. U-Pb zircon dating has been performed under the direction
of S.Sergeev using secondary ion microprobe SIMS SHRIMP-II at CIR FGUP
“VSEGEI” in 6 samples: one from HM andesibasalt (5 grains, 15 spots);
one from tonalite (3 grains, 6 spots); one from MM andesite (10 grains,
10 spots); one from MM plagiorhyodacite (1 grain, 5 spots); two samples
from MM plagiorhyolite (15 grains, 15 spots; 1 grain, 3 spots). All
dated zircons are igneous and homogeneous, practically all age values
are concordant, but the values themselves appeared to be unexpected.
The most young dating
(Bajocian) has been obtained for the zircon from MM andesite (168,5±1,7
Ma) that was dredged at the same station within LSM where similar
andesite yielded the most ancient K-Ar age among effusives, exactly 147
Ma. K-Ar datings of sedimentary rocks (argillites) from another
close-located station gave age determinations in the range of 155-166 Ma
while the diorite from the same station yielded single estimation of 170
Ma. So, in the central part of LSM Jurassic block is surely present,
which was confirmed by U-Pb zircon dating.
However, the Cretaceous-Paleogene
age of the main stages of LSM magmatism was not confirmed. Moreover,
both volcanic LSM series appeared to be not continuously differentiated
but consisting of multiple-aged products. Zircons from the most acidic
rocks of MM series (plagiorhyolites) yielded the most ancient
Paleoproterozoic mean ages, exactly 2030±12 and 2022±10 Ma. These
zircons haven’t any geochemical peculiarities and correspond to ones
from appropriate rocks, like zircon from Bajocian andesite. Whereas a
little younger zircon (Late Paleoproterozoic) from three more basic
rocks displays very specific composition. Mean ages are the next: for MM
plagiorhyodacite 1761±44, for tonalite 1759±25, for HM andesibasalt
1794±42 Ma. These zircons are notable for low U concentrations (average
7,6; 25,2; 4,4 ppm, respectively) by usual Th concentrations resulting
in extremely high Th/U ratios (in first two cases average 16; in the
last 111, range from 10 to 262!). At that these zircons contain high
percentage of non-radiogenic Pb by common not high Pb content. It should
be noted that one zircon grain from tonalite with ordinary Th/U ratio
gave age of 636±20 Ma.
Zircons with such unusual
Th/U ratios are few and occur only in some alkaline rocks, such as
syenite and nepheline syenite pegmatites (Belousova et al, 2002).
Zircons there may contain numerous inclusions like LSM ones. It is
difficult to explain an occurrence of such zircons in the island-arc
rocks and all the more in boninites.
Certainly, it might be supposed that Proterozoic zircons are xenogenic;
their source is a metamorphic basement where they occured in turn due to
Gondwanian provenance. Such explanation of Proterozoic detrital zircons
is given for the neighbouring Black Sea structures of Greater Caucasus
and North Dobrogea (Balica et al, 2011; Somin, 2009). But zircon
from LSM first, has fresh unrounded magmatic habit and second, differs
from metamorphic rocks zircons by geochemical peculiarities.
It seems too daring
to concede that zircon age is contemporaneous with true age of LSM
rocks; on the other hand, all zircon sampling points are indeed located
within the Pre-Cretaceous area according to the LSM map built on the
basis of new seismic profile interpretation. Possibly, K-Ar datings
reflect the rejuvenation of K-Ar system caused by secondary processes
during the Cretaceous-Paleogene.
No doubt that the
crystalline basement of the northern periphery of the Black Sea
depression encloses or enclosed Pre-Riphean blocks, one of which may
have situated in the LSM base. The only structure of the Black Sea
depression known to have Precambrian basement is the Andrusov Ridge
served as a provenance for Upper Jurassic conglomerates of the
Mountainous Crimea. These conglomerates often contain the pebbles of
microcline granites whose sourceland was considered by many researchers
to locate to the south of present-day Crimea and whose K-Ar dating
yielded ages in the range of 850-1500 Ma (Boyko et al, 1989). Remarkable
that these granites are more or less affected by dynamic metamorphism
that in fact was dated for microcline. Initial “magmatic” age for
zircons from these granites was estimated by total lead method as no
less than 1000-1500 Ma (isolated datings up to 2100 Ma) (Andreev et al,
1993). Apropos, magmatic zircons from these granites display rather high
usual U content. It is this stage of magmatism that zircons from most
acidic LSM rocks probably refer to. In the issue, a Mid-Black Sea High
with granitized Proterozoic basement whose existance was persisting by
many native geologists and geophysicists since the seventies (The
Earth’s Crust..., 1975) might occupy not only base of Andrusov and
Shatsky Ridges as it is considered now but go on further to the
west-north-west till LSM; alkaline rocks are assumed to be its component
here.
The data obtained give rise
to doubt about an island-arc origin of some calc-alkaline LSM rocks,
espesially of HM series suspected to be boninitic. With Proterozoic
datings beeing taken as a truth all the ancient history of the Black Sea
depression needs a revision, so far as generation mechanisms for high-Mg
melts in the
Phanerozoic and Paleoproterozoic are distinctive in kind. In the
Phanerozoic their petrogenesis is related to the subduction processes in
the active continental margins and island arcs, while in the
Paleoproterozoic their production might be caused by assimilation of
low-crustal material by mantle plume melts within intraplate
environment, as it was shown for high-magnesian volcanites of Vetreny
Belt in Karelia (Sharkov et al, 1997). To clear up the situation in the
near future more isotope analysis should be done.
SHRIMP analyses were partly financially supported by OC”Rosneft”.
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