Structure and formation of the crystalline crust in Lithuania

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Structure and formation of the crystalline crust in Lithuania Powered By Docstoc
					   POLSKIE TOWARZYSTWO MINERALOGICZNE – PRACE SPECJALNE
      MINERALOGICAL SOCIETY OF POLAND – SPECIAL PAPERS
                 Zeszyt 26, 2005; Volume 26, 2005

Gediminas MOTUZA1

    STRUCTURE AND FORMATION OF THE CRYSTALLINE CRUST
                      IN LITHUANIA

Abstract: The recent integrated petrological, radiological and geophysical investigatons of the
Precambrian crystalline basement in the south-west part of the East European Craton provide more
detailed characteristic of supracrustal and intrusive complexes in principal domains: West Lithuanian
Granulite Domain (WLGD); East Lithuanian Domain (ELD); Podlasie-Belarus Granulite Belt
(PBGD). Presented new evidences and implications for their tectonic environment and age of
formation of the crust in each domain and particularly suture zone between WLGD and ELD, defined
as Middle Lithuanian Suture zone (MLSZ) and regarded as relic volcanic island arc.

Key words: tectonic evolution, East European Craton, Precambrian, Lithuania

                   GEOLOGICAL SETTING AND PREVIOUS STUDIES
    The Precambrian crystalline basement in the SW part of the East European Craton
(former Baltica palaeocontinent) is entirely covered by Phanerozoic and, in places, also by
Neoproterozoic unmetamorphosed sediments. Thickness of this cover is variable, from few
tenths of meters on the Belarus-Mazurian high to more than 3000 meters in the central part
of the Baltic sedimentary basin. The structure of the upper lithosphere in Lithuania and
adjacent areas is, thus, merely revealed by geohysical and drill core data.
    First direct data on the geology of the Precambrian crystalline basement have been
provided by wells drilled in the thirties of the 20th century. A systematic drilling was
commenced during the sixties in order to help geological mapping, prospection for oil and
ore deposits, and the development of gas storages. In general, c. 520 wells were drilled into
the crystalline basement in the territory of Lithuania. A number of wells have been also
drilled in adjacent countries of Belarus, Latvia and Poland. Basic potential field data have
been acquired in the course of systematic mapping carried out at different scales (1:200 000
– 1:10 000) across the whole territory of Lithuania and the Baltic Sea offshore. These data
were digitised and reprocessed to a format compatible with analogous data from the
adjacent countries. Deep seismic soundings were performed along numerous profiles
during international experiments Eurobridge, Polonaise, Kochtla-Jarve–Sovietsk, Baltic Sea
and others (Ankundinov et al. 1994). These data were supplemented by local
magnetotelluric investigations (Burakhovich et al. 1990).

                           STRUCTURE OF THE LITHOSPHERE
   The seismic structure of the upper lithosphere is revealed by DSS data acquired along
profiles Eurobridge, Polonaise P4, P5 and Kohtla-Jarve-Sovietsk and other experiments on
the Baltic Sea offshore. P- and S- wave velocity models were compiled by Eurobridge

––––––––––––––––––––––––––
1
  Vilnius University, Faculty of Natural Sciences, Department of Geology and Mineralogy, Ciurlionio
21/27, 2009 Vilnius Lithuania, e-mail: Gediminas.Motuza@gf.vu.lt




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Seismic Working Group (2001), Polonaise Seismic Working group (Grad et al. 2003) and
individual researchers (Giese 1998). Additionally, gravity field along the Eurobridge profile
was modelled using GM SYS software in co-operation with L. Korabliova (Motuza et al.
2000). Summarising the available data, a number of blocks of different structure and
composition can be distinguished in the upper lithosphere along the Eurobridge DSS
profile, which generaly coincide with tectonic domains revealed based on petrological,
potential field, and mapping data (Fig. 1):
I. Vestervik-Gotland block (VG);
II. West Lithuanian Granulite Domain (WLGD);
III. East Lithuanian Domain (ELD);
IV. Podlasie-Belarus Granulite Belt (PBGB).
     The Moho depth and thickness of the crust (Fig. 2), as well as its internal structure and
composition, varies significantly along the profile. The thickness of the crust is 47–51 km
in the westernmost VG block, 42-44 km in the WLGD block while in the EL and PBGB
blocks it ranges between 50 and 57 km, being the most variable. Deepening of the Moho
towards the boundary between the WLGD and ELD takes place on a distance of 35–40 km.
     The seismic velocities in the uppermost part of the mantle vary between 8.65 and 8.9
km/s and the corresponding density is 3374-3406 kg/m3, increasing from the west to east
along the profile. In the central part of WLGD, the seismic velocities (8,1-8,5 km/s) and the
density of the mantle (3358 kg/m3) are essentially lower. This might have been produced by
metasomatism or by partial melting. The latter interpretation is supported by
megnetotelluric investigations, indicating the low resistivity zone at the same depth,
interpreted as partially molten rocks (Burakhovich et al. 1990). In the marginal parts of the
WLGD local reflectors appear at a depth of 82–73 km in the mantle. The origin of these
reflectors is not clear and various explanations are possible. R.Giese interpreted them as
bodies of higher density, where seismic velocity increases up to 9.2 km/s (Giese, 1998).
The density was modelled L.Korabliova using GM SYS software as 3622 kg/m3 (Motuza et
al. 2000). The high density of the bodies points to the presence of high density minerals,
possibly garnet and pyroxene. These bodies can potentially represent delaminated slices of
the crust, which sank in the mantle (cf. Defant & Kepezhinskas 2002).
     The thickness of the lower crust in the VG and in ELD blocks is up to 22–25 km,
whereas in the WLGD block it is only 10 km. The density of the lower crust along the
Eurobridge profile varies between 2900–3000 kg/m3, with the exception of the WLGD
block. The latter reveals density of c. 2850 kg/m3 i.e. below the average density of the
lower continental crust (2900 kg/m3), which is presumably due to the presence of felsic or
intermediate granulites along with mafic rocks or/and migmatites. The low density also
implies the scarcity of rocks rich in garnet and pyroxene.
     The seismic middle crust is characterised by Vp, varying between 6.5 and 6.75 km/s and
the corresponding densities between 2820 and 2845 kg/m3. In WLGD block the density is
lower: 2750–2800 kg/m3. Taking into account the increase of density of same lithology
with depth, such an average density corresponds to predominantly felsic lithology, simmilar
as to the lithology in the upper crust. This presumption implies that the geological middle
crust is absent in the WLGD, whereas the felsic upper crust is much thicker. Such a
structure of the crust is indicated there by a regional negative gravity anomaly located
roughly in the area of the WLGD block and is supported by results of modelling (Fig. 1). In
this way the thickness of the felsic upper crust in WLGD is up to 35 km, while in the
adjacent blocks it varies between 13 and 20 km. The density varies between 2650 kg/m3 in
the uppermost part up to 2750–2760 kg/m3 in the lower part. Based on the lithology on the
surface of the crystalline crust, as known from drilling, and on the interpretation of




70
Fig.1 The structure of the upper lithosphere along Eurobridge Deep Seismic Sounding (DSS)
profile (Motuza et al. 2000).




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potential field data, granitoid and charnockitoid intrusives as well as migmatites with relics
of felsic and intermediate supracrustals are prevailing in the upper crust of WLGD. In other
blocks, particularly the EL, migmatites are predominant with abundant relics of
supracrustals, mainly of maphic metavolcanics. The seismic velocity model shows a low
velocity layer in the upper crust, about 7.5 km thick at the depth of c. 10 km. The low
velocity layer is situated mainly in the area of the WLGD and in the eastern part of the VG
block. The interpreted density of this layer (2650–2700 kg/m3) is slightly lower when
compared to the surrounding rocks. It might be interpreted as granite formed in a cratonic
crust.
    The Middle Lithuanian Suture zone is considered a boundary or transitional zone
between the WLGD and ELD blocks. In this zone takes place thickening of the crust,
verging out the low velocity layer, and an abrupt change in the other physical parameters of
the crust and mantle. The width of transitional zone is about 35-40 km. The contact
between ELD and PBGB is not so prominent, taking into account the general variability of
the crust and the particular layers in both blocks. Provisionally, the contact is defined along
the rims of granulite outcrops on the recent basement surface. Such a structural variability
of the crust, as well as sharp changes in the lateral and vertical trends of rock properties,
can indicate delamination of the crust into slices and blocks and their lateral and vertical
displacements. Subhorizontal reflectors in the lower and middle crust, observed along the
Eurobridge DSS profile, can be explained as shear zones along the detachment surfaces.
Tectonic movements might have resulted in the formation of a system of thrusts in the
upper crust, uplift of the granulite blocks inserted into lower-grade rocks and intense
mylonitisation along their rims fixed by drilling.

                                       LITHOLOGY
    The major crustal domains characterised above differ in their lithostratigraphy and
tectonic evolution. Specific lithological complexes are formed in the zone of junction
between the WLGD and ELD, defined as Middle Lithuanian Suture zone (MLSZ). The
principal lithotectonic and genetic units are presented on the chronostratigraphic chart on
Fig. 2.

Supracrustal rocks:
     Precambrian crystalline basement of Lithuania and adjacent areas was formed in high
grade metamorphic conditions corresponding to amphibolite and granulite facies. The
supracrustal sequence is heavily migmatised and original metasediments are preserved
mainly in the form of palaeosome of migmatites and relic bodies. The lower possible age
limit of supracrustal rocks in the ELD is estimated by the youngest age of detrital zircon at
ca 1,91 Ga, whereas the upper possible limit is indicated by the age of the oldest migmatite
neosome – 1,884 Ga (Mansfeld 2001; Claesson et al. 2001). Metasedimentary rocks of the
ELD are predominantly felsic biotite-plagioclase-quartz gneisses, in places with garnet and
sillimanite - mainly grywackes and arcoses, in places with admixture of volcanic mainly
pyroclastic material. They are associated with accompanied by mafic gneisses, marbles and
metapelites. The latter rocks are characterized by fine and even grained lepidogranoblastic
texture, banded structure regarded as relics of primary bedding, caused mainly by variation
in biotite content. Zircon is present in detrital well-rounded grains, regarded as, whose age
varies from 2,16 to 2,71 Gy in the same well (Marfin et al. 1987; Mansfeld 2001). The
geochemical data implies two main sources of material in the sedimentary rocks: one with
mafic rocks, probably mainly local, and the other with felsic rocks, which might be both
local volcanics and granitoids from a distant block of an older, Proterozoic and/or Archaean




72
   Fig. 2 The chronology of the rock complexes of the crystalline basement of Lithuania

continental crust. The εNd ratio (Table 1) in biotite-plagioclase-quartz gneisses from the
well Šalčia 403 is intermediate between that typical of Archaean (εNd= –9) and of
Palaeoproterozoic (εNd= +4) rocks and might be interpreted as blending of the material of
different age source rocks. Higher values (well Lazdijai-32) might be caused by addition of
volcanic material. The position of points corresponding to the ELD rocks on a CIA diagram
demonstrate that plagioclases and primary clastic material of the metasediments was not
weathered. This is confirmed also by a positive correlation between Al2O3 and Na2O+CaO
characteristics for a greywacke with different content of plagioclase.
    Marbles were found just in few places in southern Lithuania. In the central part of an
area in the Varėna iron ore zone, bodies of dolomite marble are present in few wells.
Presumably, they are tectonically deformed beds, which occupy a specific stratigraphic
position between the sequences of predominantly felsic gneisses and amphibolites. The
marbles are largely substituted by skarns, including rich magnetite ores.
    Mafic metavolcanics are the predominant type of supracrustal rocks in the ELD. They
are represented by amphibolites and pyroxen plagioclase gneisses, containing both




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hypersthene and clinopyroxene. Fine- to medium-grained, nematogranoblastic texture is
characteristic of these rocks, in places with relics of primary porphyry and ophitic texture.
The geochemical characteristics of the supracrustal mafic rocks mostly correspond to
tholeiitic basalts, although some of them demonstrate features of calc-alkaline series. Based
on trace element content, might be distinguished relatively primitive, little differentiated,
metabasalts with low content of REE, characteristic of tholeiites and more differentiated
metabasites enriched in LREE, which is characteristic of calk-alkaline basalts.
    On a tectonic setting diagrams the majority of analyses of amphibolites fall in the field
of subduction-related volcanic island arc .
    In the WLGD, the predominant supracrustal rocks are metapelitic and felsic gneisses
(metapsammites), whereas marbles and mafic rocks are practically absent. Felsic gneisses
are composed of feldspars, quartz, biotite, garnet in different proportions, in places
containing also clinopyroxene, silimanite, cordierite, with magnetite and herzinite as typical
accessories. Plagioclase is usually predominant, quartz content being variable. The texture
is typically very fine-grained and banded, probably reflecting primary bedding. Metapelites
are distinguished by high content of alumina-rich minerals and of general Al2O3 (up to
25%). In comparison with ELD metasediments, the WLGD rocks contain less Na, K more
Fe, Mg, Ti. This might indicate that source rocks of WLGD metasediments where more
mafic. The CIA diagram implies that the plagioclase in primary clastic material of
metasediments was strongly weathered in contrast to metasedimentary rocks of the ELD.
The negative correlation between the Al2O3 and Na2O+CaO is also characteristic of pelites.
The εNd ratio from the WLGD (wells Pociai-3 ir Lauksargiai-5) varies between -2,2 and
-3,1, which may indicate a higher input of Archaean material. The possible time interval of
formation of WLGD supracrustral rocks is between 2,145 and 1,849 Ga. The first figure is
an age of the youngest detrital zircon (Claesson et al. 2001) and the second – the age of the
oldest plutonic rocks (charnockitoids). As most probable time is regareded 1.87-1.86 Ga –
the time of deposition in the Västervik Basin (South central Sweden) which is nearest part
of Fennoscandian Shield (Sultan et al. 2004)
    The MLSZ includes adjacent margins of the WLGD and ELD occupied by a specific
succession of supracrustal rocks, regarded as a particular lithotectonic unit. In the western
part of the MLSZ (corresponding to the easternmost section of the WLGD), there occurs a
dismembered belt of pyroxene-biotite-hornblende-feldspar-quartz gneisses with well
developed porphyry texture, interpreted as metavolcanics of andesitic-dacitic composition.
The porphyry texture is formed by idiomorphic and clearly zoned plagioclase phenocrists
dispersed in a fine-grained matrix. In the eastern part of the MLSZ (western margin of the
ELD), the amphibolites predominate, showing geochemical features of subduction-related
magmatic arc volcanics manifested by well diffferentiated REE patterns, Nb negative
anomaly or other indices characteristic to calk-alkaline series. The age of the MLSZ rocks
is constrained by single datings of mafic metavolcanics, andesitic-dacitic metavolcanics and
granitoids at 1.867 Ga (Rimsa et al. 2001), 1.842 and 1.836 Ga, respectively.

Migmatites:
    The migmatites are the most waidspread rock type in the basement of Lithuania. They
comprise different textural varieties, including nebulite (diatexite), and are classified
according to the composition of palaeosome and leucosome. The palaeosome consists of
supracrustal rocks typical of particular crustal domains and its composition mirrors the
distribution of supracrustal sequences before the migmatisation. The neosome of the
migmatites is mostly of granitic and, in places, also of tonalitic and charnockitic
composition, dependent on the type of a protolith and the metamorphic grade. The




74
migmatites with granitic neosome are the most widespread, especially in the ELD. A
tonalitic neosome is more common in migmatised andesitic-dacitic metavolcanics in the
MLSZ. Charnockitic migmatites are known from the granulite bodies in the ELD and
WLGD. Developed on expense of metapelites, they often contain mesosome or
melanosome enriched in garnet, cordierite, biotite. The formation of the migmatites took
place in few stages in a wide time span. The oldest age of a neosome in NW Belarus is
1.884 Ga (Claesson et al. 2001). In Lithuania, the radiometric age of migmatites (U-Pb on
monazite) is constrained only in the WLGD to 1800 Ma (Skridlaite et al. 2004). This age
indicates that certain stage of migmatisatisation in the WLGD might have been induced by
an emplacement of large charnockitoid intrusions and following regional contact
metamorphism. Simmilar model is presumed for soth-central Sweden (Väisänen et al.
2004).

Plutonic rocks:
    Major plutonic suites distinguished in Lithuania, in terms of their tectonic setting are
classified as pre- to syn-orogenic and intra-cratonic.
    In the ELD, ultramafics found in several wells as serpentinised peridotite, serpentinite
or serpentine-actinolite-phlogopite-talck rock are considered pre-orogenic intrusives. The
peridotite is characterised by high content of MgO=31.03%, low TiO2=0.35% and
Na2O+K2O=0.45%. Synorogenic inrusions are represented by gabbroic, dioritic to
granodioritic plutons associated in the Randamonys complex, the largest component of
which is the composite Randamonys pluton (25x15 km in map view). Furthermore, a
number of large plutonic bodies of the same complex are supposed, according to potential
field data, to occur in the MLSZ and the eastern part of the WLGD, but their presence is so
far confirmed only in one well (Gėluva-99). The gabbros of the Randamonys complex are
geochemically similar to tholeitic series, while diorite and granodiorite are of calck-alkaline
character. The age of the Randamonys complex is constrained by one dating of granodiorite
(well Zheimiai-347; U-Pb method on zircon) to 1837 Ma (Rimsa et. al. 2001). The age of
gabbroids from well Geluva-99 is 1850 Ma (Skridlaite et al. 2004). These ages overlap with
a period of calck-alkaline magmatism in the MLSZ and in the adjacent part of Belarus
which were estimated at 1850-1802 Ma (Claesson et al. 2001).
    In the WLGD, pre-orogenic rocks comprise rare small plutons of gabbro (well
Krazhante). Large plutons of syn-orogenic charnockitoids and granitoids are recognized in
northern and western parts of the WLGD, as Kurshiai pluton, occupying area of 140x80
km. The latter are mainly of opdalite and mangerite composition with SiO2 content between
56 and 66%. These rocks are medium- to coarse-grained, often mylonitized with common
K-feldspar or plagioclase phenocrysts up to 2-3 cm. Charnockitoids are subalkaline potassic
S-type peraluminous rocks usually garnet-bearing. Detrital kernels in zircons from the
charnockitoids yielded the age of 2.45-2.16 Ga (Claesson et al. 2001). Granitoids are S-
type, often garnet-bearing with fairly common cordierite. Their geochemical features are
similar to those revealed by chranockitoids. Two datings of charnockitoids and two of
granitoids fall in a time span between 1.815 (Claesson et al. 2001) and 1.846 Ga.
    Intra-cratonic intrusions occur in all domains of the Precambrian basement. The largest
is the Ryga batholite of rapakivi granite-gabbro-anorthosite (Bogatikov, Birkis 1972), dated
at 1.58 Ga (Rämö et al. 1996). Other intrusions of this type are present in western
Lithuania, NE Poland, southern Lithuania, NW Belarus and probably in the Kaliningrad
district. The predominant rocks are granitoids, monzodiorites, monzonites, whereas in NE
Poland (Suwałki-Kętrzyn) the AMCG suite is documented as well (Bagiński et al. 2001;
Sundblad et al. 1994; Wiszniewska et al. 2004). The granitoids are mostly ferroan,




                                                                                           75
metaluminous, I-type, enriched in REE and other incompatible elements. They show a
characteristic porphyry texture. Geochronological data from southern Lithuania and Poland
(U-Pb on zircons; Re-Os) indicate time of their emplacement between1.56 and 1.478 Ga
(Sundblad 1994; Dörr et al. 2002; Wiszniewska et al. 2002; Stein et al. 1998). In western
Lithuania similar rocks are dated at 1.46 Ga (Motuza et al. 2004). Rare mafic dykes of
unmetamorphosed diabase or basalt up to a few meters thick are found in few wells both in
the WLGD and ELD; their age is presumably Mezo- or Neoproterozoic.

Metasomatic rocks:
   Metasomatic rocks are widespread in the Varena area in southern Lithuania. The
magnesium and calcium skarn association developed in a few phases from the sedimentary
supracrustal dolomite marble. Skarns are composed of olivine (mainly serpentinised),
spinel, clino- and orthopyroxene, magnetite, hornblende, diopside-plagioclase, phlogopite
and microcline. Magnetite-bearing rocks form iron ore deposits with reserves estimated at
hundreds of million tones.

Impact rocks:
   The Misarai impact crater, 5 km wide in a diameter is known from southern Lithuania.
Various impact rocks are identified in the crater, including shocked rocks, impact breccia
and suevite (Puura et al. 1994). The age of the impact was roughly estimated as Vendian-
Cambrian, based on acritarch fossils find in the rocks filling the crater.

                                TECTONIC EVOLUTION
    The recent investigations of the western portion of the East European Craton (EEC)
including Lithuania and surrounding areas did not reveal the presence of Archaean crust
(Claesson et al. 2001). Results of radiological dating imply, that the formation of the crust
in this region took place in the Palaeoproterozoic Orosirian period and might be regarded as
part of the Svekofennian orogeny senso lato.
    According top regional models accretion of the WLGD took place towards the NNE
(Nironen, 1996). Nevertheless, there are not revealed direct evidences of island arcs of the
NW-SE strike.
    The formation of a continental crust in the eastern part of Lithuania, NE Poland and
NW Belarus (ELD, PBGB) was complicated by distinct movement of a separate ELD plate
in the WNW direction. The subduction of the ELD plate beneath WLGD plate towards the
WNW took place along the MLSZ, corresponding to an active plate margin and associated
volcanic island arc.
    The supracrustal sequences of the WLGD, ELD and MLSZ form distinct lithotectonic
units differing in their lithological composition, provenance and tectonic setting (Fig.3).
The ELD unit, composed of metapsammites with non-weathered plagioclases, testifies to
rapid sedimentation in an active tectonic environment. The geochemical data clearly
support subduction-related volcanic island arc as its tectonic setting. Calck-alkaline plutons
of mafic-intermediate composition are typical of such an environment. The ELD unit
comprised of intermingling slices of contrasting composition match to the structure of an
accretionary prism. Contrastingly, the WLGD unit, composed by predominantly
metasedimentary felsic and metapelitic gneisses, reveals an occurrence of intensely
weathered plagioclases and the absence of mafic metavolcanics. These features indicate
tectonic quiescence, while the geochemical indices of volcanic island arc are not obvious.
These data rather correspond to a back-arc tectonic setting. The MLSZ comprises mostly
metavolcanics of mafic and felsic to intermediate composition with geochemical signature




76
Fig. 3 The sketch of tectonic structure of crystalline crust of Lithuania

of a volcanic island arc setting. The metavolcanic rocks form belts in the contact zone
between the WLGD and ELD where essential changes in composition and structure of
theentire crust take place. Thus, the MLSZ is considered a volcanic island arc with mafic
forearc unit on its eastern side and felsic-intermediate arc unit on its western side.
    The time span of protolith ages of supracrustal rocks are indirectly estimated at 1.91-
1.88 Ga and 1.945-1.85 Ga in the ELD and WLGD, respectively. The age of the MLSZ
volcanics is more precisely estimated at 1.87-1.84 Ga. Intense plutonic magmatism and
emplacement of syn-orogenic S-type charnockitoids and granitoids in the WLGD took
place in a time span of 1.85-1.836 Ga and may have been related to the same orogenic
event, which led to cratonisation of the WLGD crust. The intense deformation of the
cratonised ELD crust took place due to collision with older Sarmatian palaeocontinent
(Bogdanova et al. 2001). This collisional event provoked large-scale deformation of the
lithosphere, including the delamination of the crust and upper mantle, formation of thrust
systems in the upper crust, uplift of granulite blocks and development of shear zones along




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their margins as well as resultant exhumation of a granulitic lower crust cropping out on a
recent surface of the basement. The strike of thrust traces and of uplifted granulite bodies is
mainly NE-SW. The timing of this tectonic event is constrained by palaeomagnetic (Elming
et al. 1998) and Ar-Ar data to a period between 1,7 and 1,65 Ga (Bogdanova et al. 2001).
Intracratonic plutons of predominantly granitoid, monzodiorite and in places AMCG,
composition intruded the western margin of the EEC from southern Sweden up to NW
Belarus between 1,58-1,42 Ga. A tectonic context of this magmatism is still unknown.
Čečys, Bogdanova (2004) presumed its relation to a hypothetical „Dano-Polonian“orogeny
at the SW margin of the EEC at ca. 1.5-1.42 Ga.

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