Miocene extension and magma generation in the Apuseni Mts. (western Romania): a review

ABSTRACT

The Apuseni Mts. is a key area to study the interplay between intra-continental extensional tectonics, sedimentation, and magmatism. These events occurred from ~14.5 to 7 Ma, covering the collision of the Tisza-Dacia mega-unit with the East European Platform at ~11 Ma and formation of a thin-skinned fold-thrust belt in the East Carpathians. The Zărand-Brad-Zlatna basin (~120 km in length) is the main graben system associated with the magmatic activity. To its south, it is bounded by two younger (<12 Ma) sub-basins: Caraci and Săcărâmb both hosting active magmatism. To the north-east, the smaller Roşia Montană–Baia de Arieş–Bucium sub-basin and, to the south-east, the Deva sub-basin are also associated with magmatism. Several andesite intrusions are also known in the northernmost Apuseni Mts. at Moigrad. Volcanism from ~14.5 to ~12 Ma displays a typical calc-alkaline affinity with high ²⁰⁸Pb/²⁰⁴Pb (38.6–38.8) and ²⁰⁷Pb/²⁰⁴Pb (15.65–15.67), moderate to high ⁸⁷Sr/⁸⁶Sr (0.7047–0.7085) and low to high δ¹⁸O_wr (>7‰) values. The younger igneous rocks (<12 Ma) show high Sr/Y and a variable but significant enrichment in LILE up to ~7 Ma, coupled with low ⁸⁷Sr/⁸⁶Sr (0.7040–0.7045), low ²⁰⁸Pb/²⁰⁴Pb (38.2–38.6) and ²⁰⁷Pb/²⁰⁴Pb (15.59–15.63), and low δ¹⁸O (5.5–5.7‰). The geodynamic evolution of the area started as regional transtensional with rotational tectonics generating horst and graben structures and ended with a transpressional event. This setting provided the conditions for the formation of a large underplated magma chamber at the crust – mantle boundary (MASH zone), in response to hot upwelling convective asthenosphere. Melting of the lower crust and mixing with mantle melts may have played an important part in the magmatic evolution. The spatial and temporal changes in composition were generated in magma at the crust – mantle boundary, with different crustal thicknesses. Further crustal assimilation and minor fractional crystallization in crustal magma chambers were characteristic for each region.

KEYWORDS:

• Apuseni mountains
• miocene
• calc-alkaline magmatism
• high-Sr/Y calc-alkaline
• extension

Highlights:

-This review includes all the petrographic types of the Miocene magmatic rocks in the Apuseni Mts;

Additionally, the local Pleistocene alkali-basalts are shown in the diagram for comparison;

-A time-dependent compositional shift indicates Assimilation-Fractional-Crystallization (AFC) processes in two tectonic stages;

-The Miocene extensional model, during collision/post-collision in the East Carpathians, explains the

magma generation processes in a regional context.

Acknowledgments

We dedicate the manuscript to the memory of Prof. Frank Horváth. Kalin Kouzmanov and Răzvan-Gabriel Popa are thanked for their contributions. Mihai N. Ducea, Peter Luffi, and Antoine J. J. Bracco Gartner are thanked for their suggestions. A grant of the Ministry of National Education, CNCS-UEFISCDI, project number PN-II-ID-PCE-2012-4-0137 and a grant of Ministry of Research and Innovation, CNCS – UEFISCDI, project number PN-III-P4-ID-PCCF-2016-4-0014, within PNCDI III are acknowledged. Z.P. contribution was supported by the European Union and the State of Hungary, co-financed by the European Regional Development Fund in the project of GINOP-2.3.2-15-2016-00009 ‘ICER’. We thank Gabor Tari and three anonymous reviewers for their comments and detailed reviews.

Disclosure statement

The authors declare that they have not known competing financial interests or personal relationships that could have looked to influence the work reported in this paper.

Supplementary material

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Additional information

Funding

This work was supported by the grant of Ministry of Research and Innovation, CNCS – UEFISCDI [PN-III-P4-ID-PCCF-2016-4-0014, within PNCDI III]; grant of the Ministry of National Education, CNCS–UEFISCDI [number PN-II-ID-PCE-2012-4-0137]; European Union and the State of Hungary, co-financed by the European Regional Development Fund [GINOP-2.3.2-15-2016-00009 ‘ICER’].