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

The Baia de Arieş Unit consists of passive continental margin lithologies, of unknown protolith age, intruded by the Vinţa granitoid, the Late Cretaceous calc-alkaline "Banatites" and by the Tertiary extensional calc-alkaline magmas.

Isotopic protolith age were not obtained till present, and Dimitrescu (1988) cited contradictory palynologic data (Early Proterozoic or Devonian). Pană (1998) tried to characterize the marbles from Baia de Arieş Unit using the ¹³C and ¹⁸O isotopes. His data (+1 to +3 for carbon and 19.5 to 23.7 for oxygen), suggest metamorphosed marine limestones of Precambrian – Cambrian age. Mârza (1969) mentions a few orthoamphibolites and porphyroids as metamagmatites. Such rocks are also represented in the Preluca Massif. But the Baia de Arieş Unit lithology is dominated by marbles and micaschists, specific for a passive continental margin sedimentary environment.

The Baia de Arieş Unit is composed of great lithons of carbonate rocks, mica-plagioclase gneisses (named also paragneisses), micaschists, graphite quartzites and schists, white quartzites, and subordinate appear amphibolites and metaporphyroids as metamorphosed acidic vulcanics. Iancu and Balintoni, (1986), discussed the metamorphic parageneses connected to the metamorphic evolution. These authors cited the following paragenetic successions:

These minerals have been only microscopically studied and the chemical variations from a generation to another are difficult to record. For the second mineral generation the sillimanite formation on biotite is quite clear (Mârza, 1969). A part from biotite I transforms in muscovite II. With the second mineral generation is associated an incipient anatexis too, and pegmatite formation. Finally, the third mineral generation indicates a retrogressive event toward the chlorite zone of the greenschist facies.
As a summary the following metamorphic model can be considered.
The Baia de Arieş Unit has been metamorphosed for the first time in the staurolite – kyanite zone of the almandine amphibolite facies. The age of this event is not known. Later, the system temperature arisen and probably its pressure decreased. This event should be correlated with the Devonian collision between the Biharia and Someş Units. We do not have indications for another time interval of Apuseni Mountains crystalline terranes collage. Thus, the Late Variscan Orogeny was not recorded by the Baia de Arieş Unit. The Vinţa granitoid is Permian or Late Triassic (Pană, 1998), and the ⁴⁰Ar/³⁹Ar ages are Alpine (Dallmeyer et al., 1999). The retrogressive mineral generation can be linked with the Mesozoic evolution of the Preapulian craton penninic margin, and the Early Cretaceous overthrusting. The bottom of the Baia de Arieş Nappe arose in the crust above the 500ºC isotherm during the Late Jurassic - Early Cretaceous (Dallmeyer et al., 1999).

U/Pb, Sm/Nd and ⁴⁰Ar/³⁹Ar data were performed by Pană (1998) and Dallmeyer et al. (1999). These are listed in Table 2.

Table 2. Geochronologic data obtained on Baia de Arieş Unit

Genetic tectonic setting. As a passive continental margin sequence, the Baia de Arieş Unit should have a continental crust basement. We do not know its basement because the Baia de Arieş Unit represents a relative thin geological body located in an Alpine nappe.

Plates and metamorphic events. During the first metamorphic event, the Baia de Arieş Unit probably was attached to a lower plate. Arguments for this reasoning can be: the absence of the granite and migmatite generation, the relative high pressure indicated by the staurolite and kyanite formation. During the second metamorphic event, the Baia de Arieş Unit could be in an upper plate position. A higher temperature (sillimanite appearance, incipient anatexis) combined with a lower pressure (cordierite presence, Ghergari, pers. comm. 2002), pleads in this sense. The second metamorphic event can be correlated with the Paleozoic amalgamation of the Someş, Biharia and Baia de Arieş terranes. The Late Carboniferous - Permian extension was located especially within the Biharia terrane. Partly, the metamorphic retrogression was associated with the Variscan Orogeny, but the isotopic ages does not conserve than Alpine influence. Excepting a Late Jurassic age, the other ⁴⁰Ar/³⁹Ar ages (Dallmeyer et al. 1999) show an Early Cretaceous exhumation and cooling.
We mention that the "Tethysian extension" individualized the Preapulian craton and its penninic margin has been represented by the Baia de Arieş Unit rocks. The ⁴⁰Ar/³⁹Ar data did not registered the "Tethysian extension" yet the marginal position of the Baia de Arieş Unit was important for its role of parautochthonous for the Transylvanides overthrusting. During the Late Jurassic - Early Cretaceous contractional period, the Preapulian craton was situated in an upper plate position regarding the Getic craton. At the end of the contractional period, a new extensional tectonics permitted the emplacement of the Late Cretaceous calc-alkaline "Banatitic" suite.

Structural geology. The Baia de Arieş Unit rocks are strong foliated and folded. The prominent foliation is a transposed one, yet as in many Carpathian situations, it is parallel to the lithons. The folding is well visible in the carbonate rocks. The foliation transposition and folding appear to be associated with the beginning of the second metamorphic event.
During the Early Cretaceous time the Transylvanides overthrust the Baia de Arieş crystalline (the Austrian tectogenesis), and further the Baia de Arieş crystalline has been involved in the pre-Gosau tectogenesis, as the uppermost unit of the Biharia Nappe System. Before the Cretaceous period ending, the Transylvanides together with their basement were thrust and folded again (the Laramian tectogenesis).

The Baia de Arieş Unit represents a passive continental margin suite of unknown protolith age. When it was first time metamorphosed follows to be found. The second metamorphic event can be connected with the probable Devonian amalgamation of the Someş, Biharia and Baia de Arieş Units. It was weaker affected by the late Variscan extension and were not get Variscan isotope ages from its rocks. Instead, the Alpine metamorphic imprint is quite strong on the Baia de Arieş Unit rocks. It was exhumed from middle crustal levels during the Late Jurassic - Early Cretaceous times. Because the Baia de Arieş Nappe body was hot during its emplacement, it provided the metamorphic heat to the Păiuşeni Unit and Vulturese-Belioara Unit, and favored a ductile deformation to the three Units rocks.

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