Permian volcanic rocks from the Apuseni Mountains (Romania): Geochemistry and tectonic constraints

An Early Permian volcanic assemblage is well exposed in the central-western part of the Apuseni Mountains (Romania). The rocks are represented by rhyolites, basalts and subordinate andesites suggesting a bimodal volcanic activity that is intimately associated with a post-orogenic (Variscan) syn-sedimentary intra-basinal continental molasse sequences. The mafic and mafic-intermediate rocks belong to sub-alkaline tholeiitic series were separated in three groups (I-III) showing a high Th and Pb abundances, depletion in Nb, Ta and Sr, and slightly enriched in LREE patterns (La-N/Yb-N = 1.4-4.4). Isotopically, the rocks of Group I have the initial ratios Sr-87/Sr-86((I)) = 0.709351-0.707112, Nd-143/Nd-144((I)) = 0.512490-0.512588 and high positive epsilon(Nd270) values from 3.9 to 5.80; the rocks of Group II present for the initial ratios values Sr-87/Sr-86((I)) = 0.709434-0.710092, Nd-143/Nd-144((I)) = 0.512231-0.512210 and for epsilon(Nd270) the negative values from -1.17 to -1.56; the rocks of Group III display for the initial ratios the values Sr-87/Sr-86((I)) = 0.710751-0.709448, Nd-143/Nd-144((I)) = 0.512347-0.512411 and for epsilon(Nd270) the positive values from 1.64 to 2.35. The rocks resembling continental tholeiites, suggest a mantle origin and were further affected by fractionation and crustal contamination. In addition, the REE geochemistry (1 > Sm-N/Yb-N <2.5; 0.9> La-N/Sm-N <2.5) suggests that these rocks were generated by high percentage partial melting of a metasomatized mantle in the garnet peridotite facies. The felsic rocks are enriched in Cs, Rb Th and U and depleted in Nb, Ta, Sr, Eu, and Ti. The REE fractionation patterns show a strong negative Eu anomaly (Eu/Eu* = 0.23-0.40). The felsic rocks show the initial ratios the values: Sr-87/Sr-86((I)) = 0.704096-0.707805, Nd-143/Nd-144((I)) = 0.512012-0.512021 and for sNano the negative values from -5.27 to -5.44. They suggest to be generated within the lower crust during the emplacement of mantle-derived magmas that provided necessary heat to crustal partial melting. The mafic-intermediate and felsic rocks from Apuseni Mts represent a specific example of an association of bimodal volcanic rocks within a post-collisional extensional setting. (C) 2013 Elsevier GmbH. All rights reserved.