Meliata Unit (MEL)
compiled:
S.W. Faryad (2002)
completed:
==== ==== ====
Definition
Age of Protolith, Geochemistry
Lithology, Mineralogy, Metamorphic Grade
Thermobarometry
Geochronology
Structural Evolution
Summary
Bibliography
Links
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Definition
Metasedimentary
and metaophiolitic sequences of Permian – Triassic protolith age, experienced
Jurassic blueschist facies and very low-grade metamorphism (Árkai, 1983,
Faryad, 1995, Mozzolli and Vozárová, 1998, Horváth, 2000). It is defined as accretionary
complex formed by closure of the Triassic Meliata Hallstatt oceanic basin
(Faryad, 1997).
Geographic Position
The
Meliata rocks are exposed mostly beneath Silica and Turna nappes along the
southern border of the Gemericum at boundary between Slovakia and Hungary. The
presence of Meliata unit under Silica nappe was confirmed both in the Slovak
and Hungarian territories. Part of the Meliata rocks form overthrusts on the
Gemericum and occur also in the northern parts of the Gemericum.
Maps
Geological map of the
Slovak Curs 1:50000 (Mello et al., 1996), 1:200,000 (Lexa, et al., 2000).
Boreholes
Several boreholes
penetrated the Meliata unit beneath Silica and Turna nappes. The most important
are:
Drzkovce
(Slovakia) DRZ-1 (very low-grade): Mello et al
(1994);
Bohunovo (Slovakia) V6 (blueschust and marble): Diansika (1984);
Brusnik
(Slovakia) BRU-1 (very low-grade rocks (Vozarova
and Vozar, 1992)
Bodva Valley
(Hungary KO-11 blueschists and very low-grade rocks (Horvath, 2000)
Bodva Valley
(Hungary) RM-135 very low-grade rocks (Horvath, 2000)
Boundaries and Structural Position
The
Meliata Unit is part of southwards dipping nappe stack, which is formed from
bottom to top by the Gemericum unit, Meliata unit, Turna nappe and Silica
nappe.
.
Subunits
Based on metamorphic
conditions, the Meliata rocks are represent by:
-
blueschist facies rocks, forming slices overthrusted on the early Paleozoic of
the Gemericum and. Some authors (Mello et al.,1996; Ivan and Kronome, 1995)
interpret them as Bôrka nappe. However metamorphic conditions and lithologies
and lithology of blueschists indicate that they were exhumed from different level and had different
protholits.
-
very low-grade sedimentary sequences with slices of blueschists and
serpentinized ultramafic rocks.
Correlation
The
typical Meliata rock associations (blueschists, serpentinites and very
low-grade schists with radiolarites) are exposed in the southern parts of the
Gemericum under Silica and Turna nappes and form tectonic slices on the
basement rocks in the western parts. Very low-grade metamorphosed basic and
sedimentary rocks with rediolarite, exposed in NW part of the Gemericum
(Jaklovce village), are assumed to be part of the Meliata unit. Protolith of
the Meliata Unit are correlated with the Mesozoic Hallstatt facies in the
Eastern Alps. They are interpreted as Triassic Meliata-Hallstatt oceanic basin,
which was closed by Jurassic subduction.
Age of Protolith, Geochemistry
Based
on paleogeographic reconstruction (Dercourt et al., 1990; Kozur and Mock, 1995;
Stampli, 1998; Plasienka, 1997; Neugebauer, 2001), the Triassic Meliata oceanic
basin was a westwards prolongation of the Varder Ocean and bounded the Western
Carpathian-Austroalpine block from southeast. The very low-grade rocks which envelope
the blueschists are characterized by deep sea, mostly terrigenous sediments and
by ultramafites. Sedimentary rocks from the Meliata type locality are
radiolarian limestones, cherts and different kinds of shales. Biostratigraphic
results gave a Late Triassic to Early Jurassic age for their sedimentation
(Mock, 1978; Kozur and Mock, 1995; Mello, 1993). In some localities they are
associated with blueschist facies white marbles. Geochemically, the blueschist
facies metabasalts have composition between MORB and arc basalts (Faryad, 1995;
Ivan and Kronome, 1995). Sedimentary textures, mainly high amounts of quartz
pebbles in metaconglomerates, suggest that the protoliths were shallow-water
clastic sediments formed on continental margin. Some blueschist were formed
from earlier amphibolite facies basement rocks (Faryad, 1988). Although there
are no geochronological data supporting a Paleozoic age of the earlier
amphibolite facies metamorphism, a correlation with the Gneiss-Amphibolite
Complex of the Gemericum can be envisaged from lithology and amphibolite facies
mineralogy.
The
ultramafic rocks are represented by lizardite-chryzotile serpentinites that
derived from dunite and harzburgite (Hovorka et al., 1985). Some ultramafic
rocks from the eastern sector are classified as pyroxenite of websterite
composition.
Lithology, Mineralogy, Metamorphic Grade
Regarding
lithology and metamorphic history, the Meliata blueschists are classified into
four groups: a) the most common rocks are marbles intercalating metabasalts and
phyllites. b) metabasites and micaschists with relics of early Paleozoic white
mica (probably derived from basement rocks) are without marbles. c) phyllites
and phengite quartzite and metaconglomerates. d) earlier amphibolite facies
basement rocks, overprinted by blueschist facies metamorphism. Besides of
common phases (glaucophane, epidote, albite and titanite, the blueschists may
contain Na- and Na-Ca pyroxene (aegirine, jadeite (Jd70) and
omphacite (Faryad and Hoinkes, 1999), phengite (Si = 3.3-3.5 a.f.u.) and
garnet. Glaucophane-free phyllites and quartzites contain chloritoid and
phengite (Si = 3.3-3.4 a.f.u.). A retrograde stage of blueschist facies
metamorphism is recorded by the appearance of actinolite rimming blue amphibole
and by formation of Act + Ab symplectites after glaucophane. Mylonitized
blueschist facies phyllites with relic chloritoid indicate ductile deformation
associated with greenschist facies assemblages. Preservation of blueschist
facies assemblages in most metabasites and locally in phyllites suggests a
rapid uplift of the Meliata blueschists.
The
very low-grade rocks which envelope the blueschists are characterized by deep
sea, mostly terrigenous sediments and by ultramafites. Metamorphic minerals in
metasediments are white mica, chlorite, pyrophyllite and albite. Besides
lizardite and chrysotile, the ultramafic rocks contain relic olivine,
orthopyroxene and spinel (Hovorka et
al., 1985).
.
Thermobarometry
P-T conditions for
blueschists facies metamorphism were estimated using petrogentic grids,
thermodynamic datasets and software programs Ge0-Calc, Tweeq (Brown et al.,
1989, Berman, 1996) and Thermo-Calc (Holland and Powell, 1990; Powell and
Holland, 1996). Blueschist facies metamorphism
with maximum P-T conditions of 13 kbar at 450 oC (Faryad, 1995) was
followed by greenschist facies metamorphism, which resulted in formation of
biotite and actinolite for that 9-10
kbar and 400 oC were calculated (Faryad, 1995b). The sedimentary
series and related basalts of the Meliata unit suffered high-T anchizonal
prograde regional metamorphism, the temperature and pressure of which might
vary between c. 280 and 340 oC and c. 2.5 and 5 kbar (Árkai et al,
submitted)
Geochronology
Blueschists
|
|
rock type |
locality |
Mean |
Range (n) |
Source |
|
Ar/Ar white mica |
metabasite |
Hačava |
155 |
(1) |
1 |
|
Ar/Ar white mica |
metabasite |
Hačava |
155 |
(1) |
2 |
|
Ar/Ar white mica |
metabasalt |
|
151.9 ± 0.3 |
(1) |
3 |
|
Ar/Ar white mica |
metabasalt |
|
154.8 ± 0.9 |
(1) |
3 |
|
Ar/Ar white mica |
phyllite |
|
218-222 |
(2) |
3 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
1-Malusky
et al,.l (1993), 2-Dalmayer et al,. (1993), 3- Faryad and Henjes-Kunst (1997)
Very
low-grade rocks
|
|
rock type |
locality |
Mean |
Range (n) |
Source |
|
K/Ar white mica |
phyllite |
Meliata |
128.3±5.0 |
(1) |
1 |
|
K/Ar white mica |
phyllite |
Meliata |
134.0±5.2 |
(1) |
1 |
|
K/Ar white mica |
phyllite |
Držkovce |
115.8±4.5 |
(1) |
1 |
|
K/Ar white mica |
phyllite |
Držkovce |
126.2±4.9 |
(1) |
1 |
|
K/Ar white mica |
phyllite |
Držkovce |
142.7±5.7 |
(1) |
1 |
|
K/Ar white mica |
phyllite |
Držkovce |
149.2±5.7 |
(1) |
1 |
|
K/Ar white mica |
phyllite |
Rož.
Bystre |
127.0±5.1 |
(1) |
1 |
|
K/Ar white mica |
phyllite |
Rož.
Bystre |
142.7±5.6 |
(1) |
1 |
|
K/Ar white mica |
phyllite |
Rož.
Bystre |
122.8±4.8 |
(1) |
1 |
|
K/Ar white mica |
phyllite |
Hačava |
144.9±5.7 |
(1) |
1 |
|
K/Ar white mica |
phyllite |
Hacava |
137.8±5.4 |
(1) |
1 |
|
K/Ar white mica |
phyllite |
Hacava |
137.2±5.4 |
(1) |
1 |
|
K/Ar white mica |
phyllite |
Hacava |
145.3±5.5 |
(1) |
1 |
1-Arkai
et al. (submited)
Structural Evolution
With
exception of calcite marbles, which mostly indicate no foliation, other
blueschist facies rocks are rarely outcropped. Primary structure discernible in
phyllites is bedding (So). Foliation (S1) is mostly
parallel to bedding and generally strikes approximately EW and dips with an
angle of 20-50o towards the S. Stretching lineation is developed on
S1 planes and indicates N-NNE direction of shear sense. Some
phyllites are characterized by isoclinal folds that fold S1 fabric
(Faryad et al., 1997).
Deformation
during blueschist facies metamorphism is only locally preserved. It is
characterized by parallel orientation of glaucophane in phyllite and by the
presence of inclusion-free glaucophane in the strain shadows of coarse-grained
titanite-rich glaucophane in metabasites. Lath-shaped porphyroblasts or
rosettes of chloritoid, which partly overgrow the S1 foliation and
some Na-pyroxene in glaucophane-bearing phyllites probably, formed during the
final stage of D1 deformation.
Microfabrics
and mineral assemblages indicate that deformation mostly reflects exhumation
history that was accompanied by retrogression of blueschist to greenschist
facies. Shear sense criteria, such as asymmetric sheared clasts and pressure
shadows around porphyroblasts, are consistent with a top to the ENE sense of
shear. Up to 10 cm large s- or d-type clasts of basaltic composition
mantled in carbonate matrix indicate northverging sense of thrusting. In some coarse-grained
metabasites, the foliation flows around glaucophane with strain shadows filled
by quartz and phengite. Mylonitized black phyllites, which usually occur along
thrust faults, contain s-type mantled porphyroblasts
of glaucophane pseudomorphs. The
strain shadows adjacent to porphyroclasts consist of quartz and mica.
Summary
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