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


Biharia Unit (BHA)

Version 1

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

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

Definition

The Biharia Unit represent a predominantly meta-igneous suite, possibly generated in an island-arc tectonic setting, or in a rift-like tectonic setting. Metamorphosed during Variscan Orogeny at greenschist facies degree.

Geographic Position

The Biharia Unit is exposed in the middle part of the Biharia Nappe System, oriented E-W, between eastern termination of the Munţii Apuseni crystalline and the central part of Biharia Massif. The distance between the extreme points is around 70 km.

Maps

Geological Map of Romania 1:1.000.000 (Săndulescu et al., 1978),

Geological Map of Romania 1:200.000, Brad sheet (Lupu et al., 1967),

Geological Map of Romania 1:50.000, sheets: Biharia (Bordea et al., 1988), Avram Iancu (Dimitrescu et al., 1977), Câmpeni (Dimitrescu et al., 1974), Poşaga (Balintoni et al., 1987), Valea Ierii (Hârtopanu et al., 1982),

Aditional maps: Ionescu (1962), Bleahu and Dimitrescu (1963), Balintoni (1985).

Boundaries

Before Alpine thrustings, Biharia unit has been situated between Somes Unit and Baia de Aries Unit. Its actual position is between Baia de Aries and Garda nappes, in the middle part of Biharia nappe system.

Structural Position

The Biharia Unit forms the main part of the Biharia pre-Gosau Nappe where it is transgressively overlain by a fragment of Paiuseni Unit and by the Vulturese-Belioara Unit. Within the Poiana Nappe, Piatra Graitoare Scale and Highis-Muncel Nappe, it is found as thin strips below other transgressive fragments of the Paiuseni Unit. The Biharia Nappe System supports Gosau sediments as a post-tectonic cover.

Subunits

Every fragment of the Biharia Unit located in the Poiana Nappe, Piatra Grăitoare Scale, Biharia and Highiş-Muncel Nappes constitute separate subunits. The complete lithology of the Biharia Unit is only developed in the Biharia Nappe. In the Poiana Nappe, Piatra Grăitoare Scale, Biharia and Highiş-Muncel Nappes, there appear thin strips of greenschists, which can not be fairly correlated with each other.

Correlation

Such litologies are missing in the other Carpathian branches. Some far away correlations have been proposed by Dimitrescu (in Ianovici et al., 1976), with the Vlasina Complex in Serbia and with the Gelnica Series in the Western Carpathians. We can not support such correlations. Concerning the lithology, Gelnica series is alike to the Tulges Unit from East Carpathians and not to the Biharia Unit.

Age of Protolith, Geochemistry

Pană (1998), Pană and Balintoni (2000), reported Latest Cambrian-Earliest Ordovician U/Pb ages on zircons from metatrondhjemites, interlayered with metabasites. They are considered protolith ages for the island arc suite, and represent the oldest isotopic ages obtained from the Munţii Apuseni crystalline till now.

Pană (1998), and Pană and Balintoni (2000), tried to characterize the Biharia meta-igneous suite from a geochemical point of view. Chemical compositions are well grouped in the gabbro-quartz diorite and tonalite (tronhjemites) fields, with a gap between acidic and mafic compositions, tipical for a bimodal magmatism. The granitoids show a predominantly sub-alkaline peraluminous character, and the mafic metaaluminous rocks an alkaline tendency. The trace element discrimination plots for Ti/Cr, V/Ti, Zr/Y, Ti/Zr ratios, and the Zr-Ti-Y ternary diagram, show in the case of mafic rocks a dispersion between the fields of ocean floor basalts, low potassium tholeiites, within plate basalts and island arc basalts. If we consider only the U/Ti diagram (Shervais, 1982), then two thirds of points plot in the island arc basalts field. Because the Biharia Unit separates two contrasting continental fragments (Baia de Arieş and Someş Units), we prefer the island arc suite hypothesis. In the same sense pleads the absence of a basement for the Biharia Unit too.

Lithology, Mineralogy, Metamorphic Grade

Dimitrescu (1958), Ionescu (1962), Mârza (1969), Dimitrescu (in Ianovici et al., 1976) have described the lithology of the Biharia Unit. The meta-igneous rocks are represented by two groups: metagranites and metabasites. Among metabasites, Ionescu (1962), mentioned metagabbros, metadiorites and greenschists with albite porphiroblasts, while Mârza (1969) described metadolerites, metaanamesites, metabasalts and chlorite schists with albite porphiroblasts.

The terrigenous or chemically precipitated rocks appear subordinately as thin lenses of carbonate rocks, white or graphite quartzites, and especially as sericite-chlorite quartz schists.

Very specific and widespread for the Biharia Unit are the chlorite schists with albite porphiroblasts. Generally the green color predominates. As metamorphic rocks, the metabasites are represented by different varieties of amphibolites.

The metamorphic mineral association of the metabasites comprises: amphiboles (green and brown hornblende), plagioclase (andesine, oligoclase, albite), chlorites (clinochlore, proclorite, pennine), epidote, clinozoisite, biotite, ferrostilpnomelane, ilmenite, titanite, leucoxene, zircon, apatite, calcite, magnetite and Fe-oxides.

In this general mineral association, can be separated two mineral parageneses of different metamorphic grades.

Green and brown hornblende, tremolite, antophilite, andesine, oligoclase, biotite, ilmenite and possible a part of prochlorite and epidote belong to the first metamorphic paragenesis.

Actinolite, clinochlore, pennine, albite, majority of the epidote, zoisite, clinozoisite, ferrostilpnomelane, leucoxene, calcite, Fe-oxides, belongs to the second metamorphic paragenesis. Ionescu (1962), cites pyroxene as magmatic relics, replaced by amphiboles.

As metamorphic mineral transformation, have been observed:

hornblende -- actinolite

hornblende -- clinochlore

biotite -- pennine

biotite -- phengite + Fe-oxides

plagioclase -- albite + epidote group minerals

ilmenite -- leucoxene

They are retrogressive transformations from the amphibolite or the epidote-amphibolite metamorphic facies, toward the greenschist facies, chlorite zone. Generally, the second paragenesis predominates and it is associated with a strong foliation. The mineral association of metagranites includes: oligoclase, chlorite, epidote, zoisite, garnet, zircon and Fe-oxides.

The main mineral transformations observable in these rocks are:

oligoclase -- albite + epidote

biotite -- chlorite

garnet -- chlorite

In conclusion, a first metamorphic event, probably in the epidote-albite amphibolite degree has been followed by a second one, in the greenschist facies, chlorite zone.

The first metamorphic event can be temporally correlated with the magmatic event known at the boundary between Biharia Unit and Codru assemblage (Dallmeyer et al., 1999), where these authors obtained 40Ar/39Ar ages on amphiboles from migmatites, between 366 - 405 Ma. In our opinion, this event belongs to the Late Caledonian or Early Variscan terrane collage. During the final part of the Variscan orogeny, the Biharia Unit has been exhumed (the 321 Ma 40Ar/39Ar age, Dallmeyer et al., 1999) in an extensional tectonic regime. During this event has been generated the second metamorphic paragenesis. The extensional regime continued during the Late Carboniferrous-Early Permian, when the Paiuşeni Unit transgressed the Biharia Unit. The Cretaceous metamorphic event has been registered in the Biharia Unit only as a partial reset of K/Ar isotopic system.

Geochronology

The isotopic ages have been performed on the Biharia Unit rocks are shown in Table 1.

Table 1.: Geochronology, Biharia Unit

 

Lithology

Age (Ma)

Source

K/Ar WR

Sericite schist

115

Soroiu et al., 1969

K/Ar WR

Chl-Ms-Ab schist

181

Soroiu et al., 1969

K/Ar WR

Sericite schist

106

Soroiu et al., 1969

40Ar/39Ar Ms

Curly schist

Plateau age

Total gas age

Dallmeyer et al., 1999

251.1

40Ar/39Ar Ms

Sheared granite

 

207.4

Dallmeyer et al., 1999

40Ar/39Ar Ms

Sheared granite

 

321.4

Dallmeyer et al., 1999

40Ar/39Ar Ms

Milonitic quartzite

185.9

217.3

Dallmeyer et al., 1999

Sm/Nd WR

Granite (502 Ma)

1.56 Ga (TDM)

Pană, 1998

Sm/Nd WR

Diorite

(T=502)1.77 Ga (TDM)

Pană, 1998

Sm/Nd WR

Granite

(T=502) 2.26 Ga (TDM)

Pană, 1998

Sm/Nd WR

Granite

(T=502) 3.40 Ga (TDM)

Pană, 1998

U/Pb zircon

Granite

502 ± 4 Ma

Pană and Balintoni, 2000

U/Pb zircon

Granite

489,6 ± 6.9 Ma

Pană and Balintoni, 2000

WR: whole rock, Ms: muscovite, Chl: chlorite, Ab: albite

 

Structural Evolution

The Biharia Unit has been affected by several deformation events. During the Late Caledonian or Early Variscan time, as a lower plate, the Biharia island arc suite was dynamo-thermal metamorphosed in a contractional tectonics. During the Variscan orogeny, the Biharia Unit rocks have undergone an extensional deformation, which accentuated the metamorphic foliation and strongly boudinaged the competent rocks. During the Permian period, The Biharia Unit has been rifted and supported a rift bimodal magmatism type and sedimentation. The Biharia Nappe system formed from the penninic margin of the Preapulian craton during Early Cretaceous time. The suite was split in several tectonic units. At the base of the Biharia Nappe was developed a thick mylonitic zone and formed micro- and meso-scale folds and teconitic structures due to ductile deformation favored by the hot body of the Baia de Arieş Unit, overthrusting the Biharia Unit and its Permian and Triassic covers.

Summary

The Biharia Unit probably represents an island arc suite, metamorphosed during the Devonian time. It is situated between two contrasting continental fragments and forms a Late Caledonian or Early Variscan suture. During the Late Variscan orogeny, it has been exhumed and undergone an extensional deformation. In this event the first metamorphic paragenesis generated in the PT field of albite-epidote metamorphic facies has been retrogressed in the chlorite zone of the greenschist facies. An Early Permian rifting favored the formation of a volcano-sedimentary suite pierced by bimodal intrusions. An Early Triassic carbonate platform cover is preserved in the eastern part of the Biharia Nappe. During the Early Cretaceous, the Biharia Nappe System was sheared from the penninic margin of the Preapulian Craton, as an antithetical nappe pile. During this event, the Early Permian and Early triassic covers of the Biharia Unit have been dynamo-thermal metamorphosed. During the Late Cretaceous, the Apuseni Mountains have been pierced by the banatitic calc-alkaline magmatism. Finally, the Tertiary extensional calc-alkaline magmas invaded the graben structures SE-NW oriented.

Bibliography

Balintoni, I. (1985): Corélation des unités lithostratigraphiques et tectoniques longeant le ruisseau d’Aries entre la valée de Iara et le Mont Gaina (Monts Apuseni). D. S. Inst. Geol. Geofiz., LXIX/5 (1982), 5-15, Bucuresti.

Balintoni, I. (1997): Geotectonica terenurilor metamorfice din Romania. Ed. Carpatica, Cluj Napoca, 176pp.

Balintoni, I., Lupu, M., Iancu, V. and Lazar C. (1987): Geological Map of Romania sc. 1:50.000, Posaga sheet, Inst. Geol. Geofiz. Bucuresti.

Bleahu, M. and Dimitrescu, R. (1963): ): Geological Map of Romania sc. 1:100.000, Arieseni sheet, Inst. Geol. Bucuresti.

Bordea, S., Dimitrescu, R., Mantea, Gh., Stefan, A., Bordea, J., Bleahu, M. and Costea, C. (1988): Geological Map of Romania sc. 1:50.000, Biharia sheet, Inst. Geol. Geofiz. Bucuresti.

Dallmeyer, R. D., Pana, D., Neubauer, F.and Erdmer, P. (1999): Tectonothermal evolution of the Apuseni Mountains, Romania: Resolution of Variscan versus Alpine events with 40Ar/39Ar ages. Journal of Geology, 107, 329-352.

Dimitrescu, R. (1958): Studiul geologic si petrografic al regiunii dintre Garda si Lupsa. Anu. Com. Geol., XXXI, 51-119, Bucuresti

Dimitrescu, R., Bleahu, M. and Lupu, M. (1977): Geological Map of Romania sc. 1:50.000, Avram Iancu sheet, Inst. Geol. Geofiz. Bucuresti.

Dimitrescu, R., Bordea, J. and Bordea, S. (1974): Geological Map of Romania sc. 1:50.000, Campeni sheet, Inst. Geol. Geofiz. Bucuresti.

Hartopanu, I., Hartopanu, P., Balintoni, I., Borcos, M., Rusu, A. and Lupu, M. (1982): Geological Map of Romania sc. 1:50.000, Valea Ierii sheet, Inst. Geol. Geofiz. Bucuresti.

Ianovici, V., Borcos, M., Bleahu, M., Patrulius, D., Dimitrescu, R. and Savu, H. (1976): Geologia Muntilor Apuseni. Ed. Acad. Rom., Bucuresti, 667pp.

Ionescu, C. (1962): Cercetari geologice si petrografice in cristalinul Muntilor Bihor (Biharia). Anu. Com. Geol., XXXII, 167-198, Bucuresti

Lupu, M., Borcos, M., Dimian, M., Lupu, D. and Dimitrescu, R. (1967): Geological Map of Romania sc. 1:200.000, Turda sheet, Inst. Geol. Bucuresti.

Marza, I. (1969): Evolutia unitatilor cristaline dni sud-estul Muntelui Mare. Ed. Acad. Rom., Bucuresti, 166p.

Pana, D. (1998): Petrogenesis and tectonics of the basement rocks of the Apuseni Mountains: significance for the Alpine tectonics of the Carpatho-Pannonian region. PhD Thesis, University of Alberta, Canada, 356pp..

Pana, D. and Balintoni, I. (2000): Igneous protoliths of the Biharia lithotectonic assemblage: timing of intrusion, geochemical considerations, tectonic setting. Studia Univ. Babes-Bolyai, Geologia, XLV, 1, 3-22, Cluj Napoca.

Soroiu, M., popescu, G., Kasper, V. and Dimitrescu, R. (1969): Contributions preliminaires a la géochronologie des massifs cristallins des Monts Apuseni. An. St. Univ. "Al. I. Cuza", Sect. II, b (Geologie), XV, 25-33, Iasi.

Links

Descriptions of the neighbouring Units:

Baia de Arieş Unit (BAA)
Păiuşeni Unit (PAA)
Somes Unit (SOA)
Vulturese - Belioara Unit (VBA)

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