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

Abstract

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 (LaN/YbN=1.4–4.4). Isotopically, the rocks of Group I have the initial ratios 87Sr/86Sr(i)=0.709351–0.707112, 143Nd/144Nd(i)=0.512490–0.512588 and high positive ɛNd270 values from 3.9 to 5.80; the rocks of Group II present for the initial ratios values 87Sr/86Sr(i)=0.709434–0.710092, 143Nd/144Nd(i)=0.512231–0.512210 and for ɛNd270 the negative values from −1.17 to −1.56; the rocks of Group III display for the initial ratios the values 87Sr/86Sr(i)=0.710751–0.709448, 143Nd/144Nd(i)=0.512347–0.512411 and for ɛNd270 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>SmN/YbN<2.5; 0.9>LaN/SmN<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: 87Sr/86Sr(i)=0.704096–0.707805, 143Nd/144Nd(i)=0.512012–0.512021 and for ɛNd270 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.