Meliata Unit (MEL)

 

 

compiled: S.W. Faryad (2002)

completed:

 

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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.

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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).

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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

The Meliata unit, situated in the SE part of the Western Carpathians, represents an accretionary complex formed by closure of the Triassic-Jurassic Meliata oceanic basin.  Blueschists exhumed from different depths are assembled with very low-grade sedimentary series and ultramafic rocks in the accretionary wedge. All these rocks are imbricated in a tectonic zone between the Gemericum and the Silica/Turna nappe. Petrological and microstructural analyses indicate a single progressive deformation coincident with prograde metamorphism at blueschist facies conditions. The foliation is defined by preferred orientation of mica, blue amphibole and rarely also by Na-pyroxene. The exhumation path is documented by ductite deformation, formed at blueschist-greenschist facies boundary and at greenschist facies conditions. The E-W directed thrust faults, which are parallel to the foliation, seem to be responsible for the exhumation of the blueschists. Later stages of deformation in phyllites are documented by shear bands that crosscut the blueschist facies foliation. Low-temperature Cretaceous nappe tectonics resulted in brittle deformation and used mostly the older tectonic systems that formed at blueschist-greenschist facies conditions.

 

 

Bibliography

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Árkai P., Faryad SW., Vidal, O. and Kadosa, B. Very low-grade metamorphism of sedimentary rocks of the Meliata unit, Western Carpathians, Slovakia: implications of phyllosilicate characteristics. (J. Erath Sciences)

 Brown, T.H., Berman, R.G. & Perkins, E.H.,1989. PTA-system: A Ge0-Calc software package   for the calculation and display of activity-temperature-pressure phase diagram. American   Mineralogist, 74, 485-487.

Dallmeyer RD, Neubauer H, Fritz H, Putiš M (1993) Variscan vs. Alpine tectonothermal evolution within the Eastern Alps and Western Carpathians, Austria-Slovakia. PAEWCR Conference, September 1993, Stará Lesná, Slovakia). Geol Carpathica 44:255-256

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Dianiška I., 1984. Bohúňovo-Sadrovce (unpublished manuscript). Archív Geol. Survey, Spišská N. Ves, s.123.

Faryad SW (1995) Phase petrology of mafic blueschists of the Meliata Unit (West Carpathians) - Slovakia.  J metamorphic Geol 13:432-448.

Faryad, S.W. (1995b): Phase petrology of mafic blueschists of the Meliata Unit (West Carpathians)-Slovakia.  J. metamorphic Geol, 13, .432-448 .

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Faryad SW, Henjes-Kunst F (1997) K-Ar and Ar-Ar  age constraints of the Meliata  blueschist facies rocks, the Western Carpathians (Slovakia). Tectonophysics 280:141-156

Faryad, S.W. and Hoinkes, G. (1999): Two contrasting mineral assemblages in the Meliata Blueschists, Western Carpathians, Slovakia. Mineralogical magazin, 63, 489-501

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