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

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.