Pre‐Alpine Crust in the Apuseni Mountains, Romania: Insights from Sm‐Nd and U‐Pb Data

Abstract

The Apuseni Mountains in Romania expose pre‐Alpine crustal basement rocks between the Carpathian and Dinaride segments of the Alpine Orogen. Because of a lack of direct constraints, the age and plate affinity of the basement rocks have been largely uncertain. New Sm‐Nd data for key lithotectonic assemblages and U‐Pb zircon data for crosscutting plutons are presented. They show the presence of Early Proterozoic crustal components and help constrain the age and nature of several contrasting pre‐Alpine crustal domains. Basement rocks of the Apuseni Mountains consist of a northern granite‐gneiss terrane (Someş) bounded by an amphibolite/granodiorite belt (Codru), a composite igneous belt overprinted by Alpine shearing (Highiş‐Biharia), and a southern carbonate‐lens mica schist‐gneiss terrane (Baia de Arieş). The northern terrane has yielded TDM model ages ranging from 1.90 to 1.76 Ga and ϵNd(0) values ranging from −15.5 to −12.5. The southern terrane has yielded TDM model ages of 1.82–1.69 Ga and ϵNd(0) values of −11.7 to −7.2. Sm‐Nd data from Paleozoic granitic intrusions are consistent with the mixing of mantle‐derived magma and Proterozoic crust. U‐Pb zircon dates indicate that plutons were emplaced at ∼278 Ma in the northern gneiss terrane, at ∼372 Ma in its amphibolite rim, and at ∼364 and ∼206 Ma in the southern terrane. Low‐strain pods in the Alpine shear zone separating the two gneissic terranes were derived from the Late Cambrian Biharia Igneous Complex and the Early Permian Highiş Igneous Complex. The Codru amphibolite belt and the Biharia Igneous Complex may represent a Paleozoic suture‐magmatic belt pair of the Variscan Orogen. The Highiş Igneous Complex and associated clastic assemblages may record late Variscan transcurrent tectonism. The Apuseni crustal fragment is inferred to be of European‐plate affinity, similar to the Carpathian crust, and its characteristics support the correlation of basement domains across what were thought to be Alpine and Variscan sutures.