Geodynamic implications of Jurassic ophiolites associated with island-arc volcanics, South Apuseni Mountains, western Romania

The South Apuseni Mountains are located in the inner zone of the Carpathian belt. This area is characterized by a complex assemblage of nappes, in which Jurassic igneous associations are well represented. New geological and geochemical data on these igneous associations document the occurrence of Middle Jurassic ophiolites overlain by Late Jurassic calc-alkaline volcanic rocks. The ophiolite sequence is characterized by: (1) an intrusive section mainly represented by small gabbroic bodies showing both layered and isotropic textures, as well as scarce ultramafic cumulates, melagabbros, gabbronorites, and ferrogabbros, (2) a basaltic sheeted dike complex; (3) a volcanic sequence including massive and pillow-lava basalts and rare pillow breecias; and (4) very rare Callovian-Oxfordian radiolarian cherts. Although chemically variable, gabbros show a clear high-Ti magmatic affinity. Basaltic rocks display N-MORB-normalized incompatible-element patterns consistent with the compositions of present-day mid-ocean ridge basalts. Their bigh-Ti magmatic affinity is indicated by the chondrite-normalized BEE patterns, which are rather flat or slightly enriched in light REE. All these features suggest that ophiolites of the South Apuseni Mountains were generated in a mid-ocean ridge setting. Evidence of ductile deformation and synkinematic metamorphism are lacking, suggesting that ophiolites were dismembered in multiple slices at shallow structural levels during the orogenic phases. Calc-alkaline rocks are represented by massive lava flows including basalts, basaltic andesites, andesites, dacites. and rhyolites, as well as by some granitoid complexes intruded into the ophiolitic sequence. Volcanics show highly porphyritic textures, with clinopyroxene. orthopyroxene, hornblende, and plagioclase phenocrysts, and incompatible-element compositions characterized by Ta-Nb, P, and Ti depletion, as well as by Rb-Ba-Th and La-Ce enrichment. These rocks also show marked light BEE enrichments, commonly interpreted to be a consequence of mantle source enrichment by subduction-derived components. Ophiolites represent the remnants of an oceanic basin, whereas cale-alkaline rocks represent a magmatic island-arc setting developed over the previously formed oceanic lithosphere. The geological and petrological data suggest that the South Apuseni Mountains ophiolites can be correlated with the mid-ocean ridge-type ophiolites of the Vardar zone. They are interpreted to have assumed their present location in the inner zone of the Carpathian belt during the tectonic escape of Adria-related microplates during the Late Paleogene-Early Neogene.