SOA-001b

Balintoni, I., Puşte, A., Balica, C. and Stan, R. (2002): Somes Unit - version 1. In: Dunkl, I., Balintoni, I., Frisch, W., Hoxha, L., Janák, M., Koroknai, B., Milovanovic, D., Pamić, J., Székely, B. and Vrabec, M. (Eds.): Metamorphic Map and Database of Carpatho-Balkan-Dinaride Area. http://www.met-map.uni-goettingen.de

Somes Unit (SOA)

Version 1

compiled: I. Balintoni, A. Puşte, C. Balica and R. Stan (2002)

Age of Protolith, Geochemistry
Lithology, Mineralogy, Metamorphic Grade
Thermobarometry
Geochronology
Structural Evolution
Summary
Bibliography
Links

Definition

The Somes Unit comprises an accretion wedge-like metamorphosed lithology of unknown protolith age intruded by Devonian and Late Carboniferous granitoids and by Late Cretaceous calc-alkaline banatites and Tertiary calc-alkaline magmas. Metamorphism in the almandine amphibolite facies during a pre-Variscan event was followed by a high temperature event during the Devonian time and Carboniferous exhumation.

Geographic Position

The Somes Unit makes up the central and northern part – the Bihor autochthonous – of the Apuseni Mountains and also constitutes the basement for the Garda Nappe from the Biharia Nappe System, and Finis Nappe from the Codru Nappe System (Balintoni, 1997).

Maps

Geological Map of Romania 1:1.000.000 (Sandulescu et al., 1978)
Geological Map of Romania 1:200.000, Turda sheet (Lupu et al., 1967), Brad sheet (Bleahu et al., 1967), Cluj sheet (Saulea et al., 1967), Simleul Silvaniei sheet (Lupu et al., 1967)
Geological Map of Romania 1:100.000, Arieseni sheet (Bleahu and Dimitrescu, 1963), Moneasa sheet (Bleahu, 1963)
Geological Map of Romania 1:50.000, sheets: Avram Iancu (Dimitrescu et al., 1977), Valea Ierii (Hartopanu et al., 1982), Poiana Horea (Bleahu et al, 1980), Campeni (Dimitrescu et al., 1974), Ciucea (Stefan et al., 1974), Mezes (Rusu et al., 1977), Muntele Mare (Hartopanu et al., 1982), Plopis (Papaianopol et al., 1977), Rachitele (Mantea et al., 1987), Remeti (Patrulius et al., 1973), Tusa (Marinescu et al., 1982), Zalau (Rusu et al., 1994), Dumbravita (Bleahu et al., 1984).

Boreholes

According to Visarion and Sandulescu (1970), the Somes and Finis type crystalline (Somes Unit), extends towards south of Oradea till the Crisu Negru River. We assume that the granites intercepted in boreholes between Zerind and Curtici belong to Finis Nappe basement too.

Boundaries

To the north, the Somes Unit is covered by the Upper Cretaceous-Tertiary post-tectogenetic sequences of the Transylvanian Depression and of the eastern prolongations of the Pannonian Basin. To the south, its boundaries are represented by the margins of the Biharia and Codru nappe Systems. The Garda and Finis Nappes basements crop out under the bottom of the Biharia Nappe and Dieva Nappe respectively, in half windows. We appreciate that toward north and north-west, boundaries of the Somes Unit are coincident with margins of Tisia block.

Structural Position

The Somes Unit makes the basement of the autochtonous Bihor Unit and of the Garda and Finis Nappes. The Garda Nappe is the lowermost unit of the Biharia Nappe System, and the Finis Nappe is the second one of the Codru Nappe System and the single one comprising metamorphics in its constitution. In a retro-tectonic restoration, the crystalline of the Garda and Finis nappes is placed at the southern margin of the Somes Unit, coming up in contact with the northern margin of the Biharia Unit. In a general sense, the Somes Units forms the main part of the Tisia block crystalline, outward the front of the Biharia and Codru Nappe Systems.

Fig. 1 Structural map of Apuseni Mountains (from Balintoni, ____)

Subunits

We distinguish three subunits within the Somes Unit: the metamorphic sequences of the Bihor autochthonous and of the Garda and Finis Nappes respectively. A characteristic of the Garda Nappe basement is the presence of migmatized amphibolites. In the basement of the Finis Nappe, there are several granite bodies, probably of different ages.

Correlation

Kräutner (1988) tried a lithostratigraphic correlations of the Somes Unit with the Sebes-Lotru and Dragsan Units from South Carpathians, with the Bretila Unit from East Carpathians, with Jaraba Group from Western Carpathians, with the Muride Crystalline from Austro-Alpine and with the Rhodopian Super-group (its lower part) from Rhodopes. Such correlations can not be accepted as relevant because of great distances and because of the absence of protolith’s isotopic ages. Another reason is the paucity of data in order to support reliable ages for metamorphic events. Also, the geochemical data necessary for tectonic setting models are scarce.

Age of Protolith, Geochemistry

The protolith ages of the Somes Unit are unknown. Several logic reasonings, valuable till acquisition of some new isotopic ages, afford us to suppose a Precambrian age for its protoliths. Thus we mention: first, the island arc Biharia Unit which separates the Somes and Baia de Aries continental crust fragments, yelded 502 Ma and 489 Ma protolith ages (Pana, 1998; Pana and Balintoni, 2000). Second, the subduction of the Biharia island arc suite beneath the Garda amphibolites, generated the 400 Ma Codru migmatites (Dallmeyer et al., 1999), named by Balintoni (1997), Aries granitoids. Because the Garda amphibolites were already metamorphosed during the Aries granitoids formation, the initial metamorphism of the Somes Unit has probably been pre-Devonian.

Balintoni (1985, 1997), included in the Somes Unit, the Arada Formation and the Codru Complex (Dimitrescu, 1988) the last one constituting the basement of Finis Nappe. This is important for understanding the Somes Unit tectonic setting, because the Arada and Codru sequences comprises significant metabasite bodies. A systematic geochemistry did not conduct for the Somes Unit rocks, but Dimitrescu (1988) notes "Leptyno-Amphibolite" associations in the Bihor "Group", and Arada "Formation" and orthoamphibolites (meta-gabbros and meta-hornblendites) in the Codru "Complex". The association of metabasites with acid metavolcanics paragneisses and micaschists, and the scarce appearance of carbonate and graphitic rocks suggested to Balintoni (1997) to propose an accretion wedge-like tectonic setting for the Somes Unit. It can also be supposed an island arc meta-suite or both, remnants from an accretion wedge complex and from an island arc conjoint suite.

Lithology, Mineralogy, Metamorphic Grade

The lithology of Somes Unit has been synthesized by Balintoni (1997) according to Dimitrescu (1988), and the sheets of Geological Map of Romania, 1:50.000. The lower part (Valea Cosuri Micaschist Formation, Dimitrescu, 1988), consists of micaschists and paragneisses, and a few amphibolites, mica-quartzite and K-feldspar gneiss intercalations. Over these follows the "Giurcuta Leptyno-Amphibolite Formation" (Dimitrescu, 1988), formed from micaschists, K-feldspar gneisses (leptynites), feldspar-quartzites and amphibolites. The third term is constitutes by the "Upper Micaschist Formation" (Dimitrescu, 1988), exhibiting a micaschist suite with some leptynite, white or graphite quartzitic intercalations.

The fourth term, the "Arada formation" (Dimitrescu, 1988), consists from retrogressive micaschists (Balintoni, 1985), amphibolites, wite gneisses, metaporphiroids and a few graphite quartzites. It is also known a carbonate lens.

The position of the fifth term "Codru Complex" (Dimitrescu, 1988), is not clear because it makes up the Garda Nappe basement. It is formed from migmatized amphibolites.

In the Bihor Unit are known pegmatitic mobilisates and it is also pierced by granitoids. The Muntele Mare granite is an intrusive body with a hornfels aureole containing recristallized biotite and muscovite along with sillimanite, cordierite, or andalusite (Dimitrescu, 1988). The thermal contact zone is about 1km large. The Muntele Mare granite intruded the Bihor autochthonous unit, and Pana (1998), obtained an isotopic age on zircon of 295 Ma (Late Carboniferous).

The Aries intrusions affected the Garda nappe basement. For the Madrigesti granite (Finis Nappe basement), Pana (1998), recorded a U/Pb isotopic age on zircon of 392 Ma, and for the Aries (Codru) granodiorite of 372 Ma (Middle and Late Devonian). From the basement of Finis Nappe are also known the Siria and the Codru granites, wich according to Stan (1989), have a Permian age (Permian rock xenolith engulfed).

The mineral association of the Somes Unit rocks includes several successive metamorphic parageneses. According to Hartopanu (in Hartopanu et al., 1982) for the first paragenesis, the cyanite-staurolite are the characteristic mineral, for the second one the sillimanite, and the third one has a retrogressive character, containing biotite and chlorite. Balintoni, (1985), advocated the retrogressive evolution of the "Arada Formation", as sustained by Giusca et al. (1967).

Based on successive parageneses we can imagine the following evolution: regional metamorphism of an accretion wedge in a contractional tectonic setting, until the staurolite-kyanite zone of the amphibolite facies; the age of this event is unknown. A supplementary heating during the Aries type Upper Devonian migmatization, when collided the Biharia Island Arc and the Bihor Unit continental fragment. Then it was generated the sillimanite. Regional extension and exhumation to the end of Variscan Orogeny, accompanied by a generalized retrogression and intrusive granite emplacement. This event has been followed by the Permian volcanism and continental sedimentation. During the Mesozoic, great sedimentary suites accumulated and during the Early Cretaceous the marginal part of the Somes Unit has been sheared and participated to the formation of the Codru and Biharia Nappe Systems. A very strong and widespread Later Cretaceous calc-alkaline "Banatitic" suite invaded the Bihor crystalline.

Thermobarometry

Table 1. Thermometric data measured on the formations of Somes Unit

Thermometer	Lithology	Mineral	Range ⁰C			Method authors	Source
Thermometer	Lithology	Mineral	Min	Av.	Max	Method authors	Source
Bifeldspars	Pegmatite	K-feldspar	260 – 571			Barth, 1962	Stumbea, 2001
	Pegmatite	Albite	385			Barth, 1962	Stumbea, 2001
	Pegmatite	K-feldspar	250	466	600	Barth, 1962	Stumbea, 2001
	Pegmatite	Plagioclase	320	363	440	Barth, 1962	Stumbea, 2001
	Pegmatite	Microcline	475 - 540			Barth, 1962	Murariu, 2001
	Pegmatite	Microcline	400 – 475			Seck, 1971	Murariu, 2001
	Pegmatite	Plagioclase	405 – 652			Barth and Orville	Murariu, 2001
Muscovite - Paragonite	Pegmatite	Muscovite	250	353	460	Eugster and Yoder, 1955	Stumbea, 2001
	Pegmatite	Muscovite	250 – 485			Eugster and Yoder, 1955	Murariu, 2001
	Pegmatite	Muscovite	250 – 520			Guidotti et al., 1994	Murariu, 2001
Scandium biotite content	Pegmatite	Biotite	510 – 640			Oftedahl, 1943	Murariu, 2001
	Pegmatite	Biotite	600 – 650			Dagelaisky and Krilova,1973	Murariu, 2001
	Micaschist	Biotite	595			Oftedahl, 1943	Murariu, 2001
	Micaschist	Biotite	600			Dagelaisky and Krilova,1973	Murariu, 2001
	Pegmatite	Biotite	630 – 740			Oftedahl, 1943	Stumbea, 2001
	Micaschist	Biotite	590			Oftedahl, 1943	Stumbea, 2001
	Pegmatite	Biotite	645 – 760			Dagelaisky, Krilova,1973	Stumbea, 2001
	Micaschist	Biotite	610			Dagelaisky, Krilova,1973	Stumbea, 2001
Mg/(Mg+Fe+Mn) in the garnet-biotite system	Micaschist	Garnet+ Biotite	542 - 579				Kalmar, 1994
Fe/(Fe+Mg) in biotite δ_O2: T⁰C	Pegmatite	Biotite	555 – 595			Wones, Eugster, 1965	Murariu, 2001
Litium Biotite content	Pegmatite	Biotite	540 – 610			Pomarleanu and Movileanu,1977	Murariu, 2001
Litium Biotite content	Micaschist	Biotite	650			Pomarleanu and Movileanu,1977	Murariu, 2001
Mackenzie diagram	Pegmatite	Plagioclase	490 – 600			Mackenzie, 1957	Stumbea, 2001
Muscovite polytypes	Pegmatite	Muscovite	>400				Stumbea, 2001
Q-Ab-Or system	Pegmatite	Quartz+Albite +Orthoclase	700 - 800			Luth and Tuttle, 1969	Stumbea, 2001

Regarding these data there are some observation that can be made.

1) The least temperatures were recorded by muscovite. It probably closed during the Variscan thermo-tectonic event.

2) The highest temperatures have been registered by the Q-Ab-Or system and scandium content in biotite.

3) Some biotite and biotite-garnet systems probably conserved mixed temperatures including ones of the first thermo-tectonic event.

Geochronology

There are some important radiometric ages, performed especially on granitoids. They constrain a few metamorphic and magmatic events but the protolith ages are missing.

Table 2. Geochronological data obtained on Somes Unit

Method	Lithology	Age (Ma)			Source	Locality
K/Ar WR	Micaschist	227±8			Pavelescu et al., 1975	Cosului Valley
K/Ar Hbl	Amphibolite	209±6			Pavelescu et al., 1975	Iara Valley
K/Ar Bt	Quartz-Feldspar schists	79±5			Pavelescu et al., 1975	Iara Valley
K/Ar WR	Garnet micaschist	268			Soroiu et al., 1969	Cosului Valley
K/Ar WR	Garnet micaschist	381			Soroiu et al., 1969	Bulzului Valley
K/Ar Ms	Muntele Mare pegmatite	237±6			Pavelescu et al., 1975	Valea Mare, Bistra
K/Ar Ms	Muntele Mare pegmatite	199±6			Pavelescu et al., 1975	Valea Mare, Bistra
K/Ar Bt	Muntele Mare pegmatite	119±3			Pavelescu et al., 1975	Valea Mare, Bistra
K/Ar Bt	Muntele Mare pegmatite	85±3			Pavelescu et al., 1975	Calatele
K/Ar Bt	Muntele Mare pegmatite	90			Soroiu et al., 1969	Somesul Rece Valley
K/Ar KF	Muntele Mare pegmatite	89			Soroiu et al., 1969	Somesul Rece Valley
K/Ar Bt	Muntele Mare pegmatite	115			Soroiu et al., 1969	Valea Mare, Bistra
K/Ar Bt		160			Soroiu et al., 1969	Valea Mare, Bistra
K/Ar Ms	Muntele Mare pegmatite	156			Soroiu et al., 1969	Somesul Rece Valley
K/Ar Ms	Muntele Mare pegmatite	194			Soroiu et al., 1969	Valea Mare, Bistra
K/Ar Ms	Muntele Mare pegmatite	232			Soroiu et al., 1969	Valea Mare, Bistra
		Plateau Age	ICA	Total Gas Age
⁴⁰Ar/³⁹Ar Ms	Muntele Mare granite	191.3		189.8	Dallmeyer et al., 1999	Marisele Village
⁴⁰Ar/³⁹Ar Ms	Arada retrogr. plagiogneiss			253.0	Dallmeyer et al., 1999	Bistra Valley
⁴⁰Ar/³⁹Ar Ms	Sericite quartzite	302.5		295.1	Dallmeyer et al., 1999	Rusesti Village
⁴⁰Ar/³⁹Ar Hbl	Amphibolite	306.1	300	266.8	Dallmeyer et al., 1999	Ciurturi Village
⁴⁰Ar/³⁹Ar Ms	Retrogressed plagiogneiss	313.8		311.4	Dallmeyer et al., 1999	Ciurturi Village
⁴⁰Ar/³⁹Ar Hbl	Amphibolite	316.7	313.0	319.6	Dallmeyer et al., 1999	Belis Village
⁴⁰Ar/³⁹Ar Ms	Plagiogneiss	122.9		116.0	Dallmeyer et al., 1999	Iara Valley
⁴⁰Ar/³⁹Ar Ms	Mylonitic orthogneiss	100.6		99.6	Dallmeyer et al., 1999	Tarnita Lake
⁴⁰Ar/³⁹Ar Ms	Chl-Ms schist, Garda	339.9		336.3	Dallmeyer et al., 1999	Huzii Valley
⁴⁰Ar/³⁹Ar Hbl	Chl-Ms schist, Garda	334.6		326.4	Dallmeyer et al., 1999	Huzii Valley
⁴⁰Ar/³⁹Ar Hbl	Amphibolite, Garda			371.1	Dallmeyer et al., 1999	Salasele Valley
⁴⁰Ar/³⁹Ar Hbl	Amphibolite, Garda	336.4	364.0	367.1	Dallmeyer et al., 1999	Iara Valley
⁴⁰Ar/³⁹Ar Hbl	Amphibolite, Garda	337.7		386.8	Dallmeyer et al., 1999	Iara Valley
⁴⁰Ar/³⁹Ar Hbl	Amphibolite, Garda	404.9	400.8	405.0	Dallmeyer et al., 1999	Iara Valley
K/Ar Ms	Pegmatoid granite,Garda	296			Soroiu et al., 1969	Dosul Neagului
K/Ar Ms	Pegmatoid granite,Garda	305			Soroiu et al., 1969	Dosul Neagului
K/Ar Ms	Pegmatoid granite,Garda	326			Soroiu et al., 1969	Valea Mare, Bistra
K/Ar Ms	Pegmatoid granite,Garda	343			Soroiu et al., 1969	Bistra Valley
K/Ar Bt	Trondhjemite, Garda	303			Soroiu et al., 1969	Ariesul Mare, Pojorata
K/Ar Hbl	Pegmatoid granite,Garda	288			Soroiu et al., 1969	Bistra Valley
K/Ar Hbl	Pegmatoid granite,Garda	300			Soroiu et al., 1969	Valea Mare, Bistra
K/Ar Hbl	Hornblendite, Garda	357±10			Pavelescu et al., 1975	Valea Mare, Bistra
K/Ar Ms	Pegmatite, Garda	356±9			Pavelescu et al., 1975	Valea Mare, Bistra
K/Ar Bt	Biotitite, Garda	340±9			Pavelescu et al., 1975	Ariesul Mare, Pojorata
K/Ar Bt	Pegmatoid granite,Garda	281±10			Pavelescu et al., 1975	Ariesul Mare, Pojorata
K/Ar Bt	Trondhjemite, Garda	230±9			Pavelescu et al., 1975	Ariesul Mare, Pojorata
K/Ar WR	Arada Sericite feld schist	49			Soroiu et al., 1969	Valea Mare, Bistra
K/Ar WR	Arada Sericite schist	111			Soroiu et al., 1969	Albacului Valley
K/Ar WR	Arada Sericite feld schist	164			Soroiu et al., 1969	Bistra Valley
K/Ar WR	Arada sheared hornfels	204			Soroiu et al., 1969	Bistra Valley
U/Pb zircon	Madrigesti granite	392			Pana, 1998	Drocea Massif
U/Pb zircon	Codru granodiorite	372.2±1.0			Pana, 1998	Neagu Valley
U/Pb zircon	Muntele Mare Granite	295±1			Pana, 1998	Somesul Rece Valley
Sm/Nd WR	Muntele Mare Granite	t=295 1.62 Ga (T_DM)			Pana, 1998	Somesul Rece Valley
Sm/Nd WR	Gneiss Q-Fp	1.90 Ga (T_DM)			Pana, 1998	Giurcuta
Sm/Nd WR	Gneiss Grt	1.87 Ga (T_DM)			Pana, 1998	Baisoara
Sm/Nd WR	Micaschist-Ky	1.76 Ga (T_DM)			Pana, 1998	Iara
Sm/Nd WR	Gneiss	1.73 Ga (T_DM)			Pana, 1998	Madrigesti
Sm/Nd WR	Granite	t=392 1.75 Ga (T_DM)			Pana, 1998	Madrigesti
Sm/Nd WR	Gneiss Grt-And	1.61 Ga (T_DM)			Pana, 1998	Codru Moma
Sm/Nd WR	Granite	1.24 Ga (T_DM)			Pana, 1998	Codru Moma
Sm/Nd WR	Fibrolite schist	1.63 Ga (T_DM)			Pana, 1998	Huzii Valley
Sm/Nd WR	Granite	1.38 Ga (T_DM)			Pana, 1998	Iara Valley
Sm/Nd WR	Granodiorite (372 Ma)	t=372 0.96 Ga (T_DM)			Pana, 1998	Neagu Valley

WR: whole rock, Hbl: hornblende, Ms: muscovite, Bt: biotite, K-Fp: K-Feldspar

The Cretaceous ages can be connected with the "Banatite" intrusions and with the Cretaceous thrusting. They do not represent a regional metamorphic event.

Structural Evolution

1. Protolith tectonic setting. The Somes Unit has been individualized as an accretion wedge-island arc suite. Its initial metamorphism realized in an unknown context and the protolith ages are missing.

2. Plate tectonics and metamorphism. The metamorphic grade of the first event is suggested by the staurolite-kyanite bearing paragenesis. During this event, the Somes Unit became a continental crust fragment. Later, during Devonian time, the Somes Unit arrived in a collisional relationship with the Biharia Island Arc suite. In this event, the Somes Unit was probably in an upper plate position. It has been supplementary heated until the sillimanite crystallization. In this time, an inceptive anatexis occurred, facilitating the migmatite and pegmatite formation. The Aries intrusions affected the margin of the Somes Unit ("Codru Complex" according to Dimitrescu, 1988), and probably the collision created the thrust structures and favored an Late Devonian partial exhumation of this margin.

At the end of Variscan Orogeny, a generalized extention has been accompanied by intrusive granite emplacement (Muntele Mare, Siria and Codru Moma intrusions), and retrogressive metamorphism toward the chlorite zone of the greenschists facies and Permian deposits have been accommodated. The Permian deposits have been succeeded by Triassic continental platform sediments. The Mesozoic rifting created the Meliata and South Penninic oceanic environments around the Preapulian craton. During the Early Cretaceous convergence, the shearing of its Meliatic and Penninic margins generated the Codru and Biharia nappe systems, respectively. After the nappe emplacement, the calc-alkaline "Banatites" invaded the Muntii Apuseni Structures. This intrusive and extrusive magmatism indicates an Late Cretaceous relaxation tectonics.

In the northern part of Somes Unit, some very large Alpine folds can be mentioned. The hinges of these folds are parallel with the Plopis Summit in the NW area, and with the Meses Summit in the NE area. They can be correlated with the directions of tectonic transport of Codru and Biharia Nappe Systems, being perpendicular to these transport directions.

Summary

The Somes Unit of accretion wedge-island arc affinity, became a continental crust fragment probably before 500 Ma. Around 400 Ma it has been involved in a collisional relationship with the Biharia Island Arc suite. This event generated the Aries migmatites and induced a supplementary heating of the metamorphic sequence from the staurolite-kyanite zone to the sillimanite zone. Before the beginning of Permian period, a widespread extension and exhumation took place, accompanied by granite intrusion and metamorphic retrogression. This event could be conceived as associated with the late Variscan Orogeny instability. The extensional tectonics continued during the Permian and Early Mesozoic as the process of Tethys opening. As a part of Preapulian craton, the Somes Unit was included in the Early Cretaceous convergence. The Codru and Biharia Nappe Systems resulted in this time. The Late Cretaceous relaxation promoted the widespread calc-alkaline "Banatitic" magmatism.

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Descriptions of the neighbouring Units:

Baia de Arieş Unit (BAA)
Biharia Unit (BHA)
Păiuşeni Unit (PAA)
Vulturese - Belioara Unit (VBA)

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