Petrological characterization of the upper Miocene Rodna-Bârgău sub-volcanic district (Eastern Carpathians, Romania)

During Neogene, the Eastern Carpathians were the locus of abundant igneous activity with subduction-related geochemical characteristics. Among the various districts, the Rodna-Bârgău sub-volcanic complex represents a potentially key-area due to its peculiar position at the junction point between ALCAPA, Tisza-Dacia and East European plates, the exclusively sub-volcanic nature of its products, compared with the mainly effusive nature of the products from the other districts and the occurrence of a wide range of chemical compositions coupled with xenoliths of various nature, testifying complex low-pressure evolution. Despite this, little attention has been so far devoted to the study of the products of the area, which still remain poorly characterized. The present work is thus aimed to a full petrological investigation of the products of the Rodna-Bârgău district, which will possibly shed some new light on the magmagenetic and geodynamic evolution of the entire Carpathian arc. The preliminary analyses on representative samples allowed the recognition of numerous petrographic types, ranging from basalt/microgabbro to mainly andesite/microdiorite up to dacite/granodiorite and rhyolite/microgranite. Rock compositions show a well defined calcalkaline serial affinity in which, however, a clear distinction between a low-K and a high-K magmatic suites can be observed. Rocks of the first series are amphibole-bearing andesites/microdiorites, dacites and rhyolites/microdiorites with SiO2 = 58.0-74.6, MgO = 3.36-0.09 and K2O = 1.49- 2.93 wt.%, whereas rocks of the high-K suite are microgabbros, amphibole-bearing basaltic andesites/andesites and dacites with SiO2 = 53.4-64.1, MgO = 10.2-2.18 and K2O = 1.29-3.23 wt.%. Intermediate-evolved terms of the two series are remarkably different also on petrographic grounds, given that 1) low-K rocks (K2O = 1.71-3.23 wt.%) display a yellow-pale green tschermakite to Mg-hornblende amphibole (Si = 6.01-6.94, K = 0.03-0.09 a.p.f.u.), whereas high-K ones (K2O = 1.16-1.46 wt.%) feature a green hastingsite-pargasite (Si = 5.82-6.36, K = 0.16-0.34 a.p.f.u.); 2) low-K andesites display rare Na-rich alkali feldspar (Or ~49) in the groundmass, whereas high-K equivalents have higher abundances of a K-richer variety (Or ~70); 3) accessory biotite (Mg# = 0.57-0.61) is present only in high-K andesites and dacites. Incompatible element abundances suggest subduction-modified magma sources and again indicate a clear distinction between the two rock series, with rocks of the high-K suite displaying a stronger enrichment in LILE (e.g., Rb = 34-57 and 56-118 ppm, Ba = 359-631 and 578-819 ppm, in least evolved and intermediate-evolved terms, respectively) and LREE (e.g., LaN = 22.7-31.6 and 30.6-44.4, NdN = 12.1-14.9 and 13.0-20.0) with respect to those of the low-K one (Rb = 27-46 and 38-107 ppm, Ba = 158-288 and 253-619 ppm, LaN = 15.3-22.4 and 17.0-42.6, NdN = 8.1-11.2 and 9.7-13.4, respectively for intermediate and evolved rocks).